Add aggregate functions (#2925)

The aggregate functions SUM(), MAX(), MIN(), AVG(), and COUNT() are now supported.

src/PostgREST/ApiRequest/QueryParams.hs

@@ -31,8 +31,8 @@ import Data.Tree                     (Tree (..))
 import Text.Parsec.Error             (errorMessages,
                                       showErrorMessages)
 import Text.ParserCombinators.Parsec (GenParser, ParseError, Parser,
-                                      anyChar, between, char, digit,
-                                      eof, errorPos, letter,
+                                      anyChar, between, char, choice,
+                                      digit, eof, errorPos, letter,
                                       lookAhead, many1, noneOf,
                                       notFollowedBy, oneOf,
                                       optionMaybe, sepBy, sepBy1,
@@ -43,7 +43,8 @@ import PostgREST.RangeQuery              (NonnegRange, allRange,
                                           rangeOffset, restrictRange)
 import PostgREST.SchemaCache.Identifiers (FieldName)
 
-import PostgREST.ApiRequest.Types (EmbedParam (..), EmbedPath, Field,
+import PostgREST.ApiRequest.Types (AggregateFunction (..),
+                                   EmbedParam (..), EmbedPath, Field,
                                    Filter (..), FtsOperator (..),
                                    Hint, JoinType (..),
                                    JsonOperand (..),
@@ -58,7 +59,7 @@ import PostgREST.ApiRequest.Types (EmbedParam (..), EmbedPath, Field,
                                    SimpleOperator (..), SingleVal,
                                    TrileanVal (..))
 
-import Protolude hiding (try)
+import Protolude hiding (Sum, try)
 
 data QueryParams =
   QueryParams
@@ -99,7 +100,7 @@ data QueryParams =
 -- 'select' is a reserved parameter that selects the fields to be returned:
 --
 -- >>> qsSelect <$> parse False "select=name,location"
--- Right [Node {rootLabel = SelectField {selField = ("name",[]), selCast = Nothing, selAlias = Nothing}, subForest = []},Node {rootLabel = SelectField {selField = ("location",[]), selCast = Nothing, selAlias = Nothing}, subForest = []}]
+-- Right [Node {rootLabel = SelectField {selField = ("name",[]), selAggregateFunction = Nothing, selAggregateCast = Nothing, selCast = Nothing, selAlias = Nothing}, subForest = []},Node {rootLabel = SelectField {selField = ("location",[]), selAggregateFunction = Nothing, selAggregateCast = Nothing, selCast = Nothing, selAlias = Nothing}, subForest = []}]
 --
 -- Filters are parameters whose value contains an operator, separated by a '.' from its value:
 --
@@ -282,16 +283,16 @@ pTreePath = do
 -- Parse select= into a Forest of SelectItems
 --
 -- >>> P.parse pFieldForest "" "id"
--- Right [Node {rootLabel = SelectField {selField = ("id",[]), selCast = Nothing, selAlias = Nothing}, subForest = []}]
+-- Right [Node {rootLabel = SelectField {selField = ("id",[]), selAggregateFunction = Nothing, selAggregateCast = Nothing, selCast = Nothing, selAlias = Nothing}, subForest = []}]
 --
 -- >>> P.parse pFieldForest "" "client(id)"
--- Right [Node {rootLabel = SelectRelation {selRelation = "client", selAlias = Nothing, selHint = Nothing, selJoinType = Nothing}, subForest = [Node {rootLabel = SelectField {selField = ("id",[]), selCast = Nothing, selAlias = Nothing}, subForest = []}]}]
+-- Right [Node {rootLabel = SelectRelation {selRelation = "client", selAlias = Nothing, selHint = Nothing, selJoinType = Nothing}, subForest = [Node {rootLabel = SelectField {selField = ("id",[]), selAggregateFunction = Nothing, selAggregateCast = Nothing, selCast = Nothing, selAlias = Nothing}, subForest = []}]}]
 --
 -- >>> P.parse pFieldForest "" "*,client(*,nested(*))"
--- Right [Node {rootLabel = SelectField {selField = ("*",[]), selCast = Nothing, selAlias = Nothing}, subForest = []},Node {rootLabel = SelectRelation {selRelation = "client", selAlias = Nothing, selHint = Nothing, selJoinType = Nothing}, subForest = [Node {rootLabel = SelectField {selField = ("*",[]), selCast = Nothing, selAlias = Nothing}, subForest = []},Node {rootLabel = SelectRelation {selRelation = "nested", selAlias = Nothing, selHint = Nothing, selJoinType = Nothing}, subForest = [Node {rootLabel = SelectField {selField = ("*",[]), selCast = Nothing, selAlias = Nothing}, subForest = []}]}]}]
+-- Right [Node {rootLabel = SelectField {selField = ("*",[]), selAggregateFunction = Nothing, selAggregateCast = Nothing, selCast = Nothing, selAlias = Nothing}, subForest = []},Node {rootLabel = SelectRelation {selRelation = "client", selAlias = Nothing, selHint = Nothing, selJoinType = Nothing}, subForest = [Node {rootLabel = SelectField {selField = ("*",[]), selAggregateFunction = Nothing, selAggregateCast = Nothing, selCast = Nothing, selAlias = Nothing}, subForest = []},Node {rootLabel = SelectRelation {selRelation = "nested", selAlias = Nothing, selHint = Nothing, selJoinType = Nothing}, subForest = [Node {rootLabel = SelectField {selField = ("*",[]), selAggregateFunction = Nothing, selAggregateCast = Nothing, selCast = Nothing, selAlias = Nothing}, subForest = []}]}]}]
 --
 -- >>> P.parse pFieldForest "" "*,...client(*),other(*)"
--- Right [Node {rootLabel = SelectField {selField = ("*",[]), selCast = Nothing, selAlias = Nothing}, subForest = []},Node {rootLabel = SpreadRelation {selRelation = "client", selHint = Nothing, selJoinType = Nothing}, subForest = [Node {rootLabel = SelectField {selField = ("*",[]), selCast = Nothing, selAlias = Nothing}, subForest = []}]},Node {rootLabel = SelectRelation {selRelation = "other", selAlias = Nothing, selHint = Nothing, selJoinType = Nothing}, subForest = [Node {rootLabel = SelectField {selField = ("*",[]), selCast = Nothing, selAlias = Nothing}, subForest = []}]}]
+-- Right [Node {rootLabel = SelectField {selField = ("*",[]), selAggregateFunction = Nothing, selAggregateCast = Nothing, selCast = Nothing, selAlias = Nothing}, subForest = []},Node {rootLabel = SpreadRelation {selRelation = "client", selHint = Nothing, selJoinType = Nothing}, subForest = [Node {rootLabel = SelectField {selField = ("*",[]), selAggregateFunction = Nothing, selAggregateCast = Nothing, selCast = Nothing, selAlias = Nothing}, subForest = []}]},Node {rootLabel = SelectRelation {selRelation = "other", selAlias = Nothing, selHint = Nothing, selJoinType = Nothing}, subForest = [Node {rootLabel = SelectField {selField = ("*",[]), selAggregateFunction = Nothing, selAggregateCast = Nothing, selCast = Nothing, selAlias = Nothing}, subForest = []}]}]
 --
 -- >>> P.parse pFieldForest "" ""
 -- Right []
@@ -299,7 +300,7 @@ pTreePath = do
 -- >>> P.parse pFieldForest "" "id,clients(name[])"
 -- Left (line 1, column 16):
 -- unexpected '['
--- expecting letter, digit, "-", "->>", "->", "::", ")", "," or end of input
+-- expecting letter, digit, "-", "->>", "->", "::", ".", ")", "," or end of input
 --
 -- >>> P.parse pFieldForest "" "data->>-78xy"
 -- Left (line 1, column 11):
@@ -452,35 +453,37 @@ pRelationSelect :: Parser SelectItem
 pRelationSelect = lexeme $ do
     alias <- optionMaybe ( try(pFieldName <* aliasSeparator) )
     name <- pFieldName
+    guard (name /= "count")
     (hint, jType) <- pEmbedParams
     try (void $ lookAhead (string "("))
     return $ SelectRelation name alias hint jType
 
+
 -- |
 -- Parse regular fields in select
 --
 -- >>> P.parse pFieldSelect "" "name"
--- Right (SelectField {selField = ("name",[]), selCast = Nothing, selAlias = Nothing})
+-- Right (SelectField {selField = ("name",[]), selAggregateFunction = Nothing, selAggregateCast = Nothing, selCast = Nothing, selAlias = Nothing})
 --
 -- >>> P.parse pFieldSelect "" "name->jsonpath"
--- Right (SelectField {selField = ("name",[JArrow {jOp = JKey {jVal = "jsonpath"}}]), selCast = Nothing, selAlias = Nothing})
+-- Right (SelectField {selField = ("name",[JArrow {jOp = JKey {jVal = "jsonpath"}}]), selAggregateFunction = Nothing, selAggregateCast = Nothing, selCast = Nothing, selAlias = Nothing})
 --
 -- >>> P.parse pFieldSelect "" "name::cast"
--- Right (SelectField {selField = ("name",[]), selCast = Just "cast", selAlias = Nothing})
+-- Right (SelectField {selField = ("name",[]), selAggregateFunction = Nothing, selAggregateCast = Nothing, selCast = Just "cast", selAlias = Nothing})
 --
 -- >>> P.parse pFieldSelect "" "alias:name"
--- Right (SelectField {selField = ("name",[]), selCast = Nothing, selAlias = Just "alias"})
+-- Right (SelectField {selField = ("name",[]), selAggregateFunction = Nothing, selAggregateCast = Nothing, selCast = Nothing, selAlias = Just "alias"})
 --
 -- >>> P.parse pFieldSelect "" "alias:name->jsonpath::cast"
--- Right (SelectField {selField = ("name",[JArrow {jOp = JKey {jVal = "jsonpath"}}]), selCast = Just "cast", selAlias = Just "alias"})
+-- Right (SelectField {selField = ("name",[JArrow {jOp = JKey {jVal = "jsonpath"}}]), selAggregateFunction = Nothing, selAggregateCast = Nothing, selCast = Just "cast", selAlias = Just "alias"})
 --
 -- >>> P.parse pFieldSelect "" "*"
--- Right (SelectField {selField = ("*",[]), selCast = Nothing, selAlias = Nothing})
+-- Right (SelectField {selField = ("*",[]), selAggregateFunction = Nothing, selAggregateCast = Nothing, selCast = Nothing, selAlias = Nothing})
 --
 -- >>> P.parse pFieldSelect "" "name!hint"
 -- Left (line 1, column 5):
 -- unexpected '!'
--- expecting letter, digit, "-", "->>", "->", "::", ")", "," or end of input
+-- expecting letter, digit, "-", "->>", "->", "::", ".", ")", "," or end of input
 --
 -- >>> P.parse pFieldSelect "" "*!hint"
 -- Left (line 1, column 2):
@@ -495,18 +498,36 @@ pFieldSelect :: Parser SelectItem
 pFieldSelect = lexeme $ try (do
     s <- pStar
     pEnd
-    return $ SelectField (s, []) Nothing Nothing)
-  <|> do
-    alias <- optionMaybe ( try(pFieldName <* aliasSeparator) )
-    fld <- pField
-    cast' <- optionMaybe (string "::" *> pIdentifier)
+    return $ SelectField (s, []) Nothing Nothing Nothing Nothing)
+  <|> try (do
+    alias    <- optionMaybe ( try(pFieldName <* aliasSeparator) )
+    _        <- string "count()"
+    aggCast' <- optionMaybe (string "::" *> pIdentifier)
     pEnd
-    return $ SelectField fld (toS <$> cast') alias
+    return $ SelectField ("*", []) (Just Count) (toS <$> aggCast') Nothing alias)
+  <|> do
+    alias    <- optionMaybe ( try(pFieldName <* aliasSeparator) )
+    fld      <- pField
+    cast'    <- optionMaybe (string "::" *> pIdentifier)
+    agg      <- optionMaybe (try (char '.' *> pAggregation <* string "()"))
+    aggCast' <- optionMaybe (string "::" *> pIdentifier)
+    pEnd
+    return $ SelectField fld agg (toS <$> aggCast') (toS <$> cast') alias
   where
     pEnd = try (void $ lookAhead (string ")")) <|>
            try (void $ lookAhead (string ",")) <|>
            try eof
     pStar = string "*" $> "*"
+    pAggregation = choice
+      [ string "sum"   $> Sum
+      , string "avg"   $> Avg
+      , string "count" $> Count
+      -- Using 'try' for "min" and "max" to allow backtracking.
+      -- This is necessary because both start with the same character 'm',
+      -- and without 'try', a partial match on "max" would prevent "min" from being tried.
+      , try (string "max") $> Max
+      , try (string "min") $> Min
+      ]
 
 
 -- |

src/PostgREST/ApiRequest/Types.hs

@@ -1,6 +1,7 @@
 {-# LANGUAGE DuplicateRecordFields #-}
 module PostgREST.ApiRequest.Types
-  ( Alias
+  ( AggregateFunction(..)
+  , Alias
   , Cast
   , Depth
   , EmbedParam(..)
@@ -42,12 +43,14 @@ import PostgREST.SchemaCache.Routine      (Routine (..))
 
 import Protolude
 
--- | The value in `/tbl?select=alias:field::cast`
+-- | The value in `/tbl?select=alias:field.aggregateFunction()::cast`
 data SelectItem
   = SelectField
-    { selField :: Field
-    , selCast  :: Maybe Cast
-    , selAlias :: Maybe Alias
+    { selField             :: Field
+    , selAggregateFunction :: Maybe AggregateFunction
+    , selAggregateCast     :: Maybe Cast
+    , selCast              :: Maybe Cast
+    , selAlias             :: Maybe Alias
     }
 -- | The value in `/tbl?select=alias:another_tbl(*)`
   | SelectRelation
@@ -65,7 +68,8 @@ data SelectItem
   deriving (Eq, Show)
 
 data ApiRequestError
-  = AmbiguousRelBetween Text Text [Relationship]
+  = AggregatesNotAllowed
+  | AmbiguousRelBetween Text Text [Relationship]
   | AmbiguousRpc [Routine]
   | BinaryFieldError MediaType
   | MediaTypeError [ByteString]
@@ -135,6 +139,9 @@ type Cast = Text
 type Alias = Text
 type Hint = Text
 
+data AggregateFunction = Sum | Avg | Max | Min | Count
+  deriving (Show, Eq)
+
 data EmbedParam
   -- | Disambiguates an embedding operation when there's multiple relationships
   -- between two tables. Can be the name of a foreign key constraint, column

src/PostgREST/Config.hs

@@ -69,6 +69,7 @@ import Protolude hiding (Proxy, toList)
 
 data AppConfig = AppConfig
   { configAppSettings              :: [(Text, Text)]
+  , configDbAggregates             :: Bool
   , configDbAnonRole               :: Maybe BS.ByteString
   , configDbChannel                :: Text
   , configDbChannelEnabled         :: Bool
@@ -138,7 +139,8 @@ toText conf =
   where
     -- apply conf to all pgrst settings
     pgrstSettings = (\(k, v) -> (k, v conf)) <$>
-      [("db-anon-role",              q . T.decodeUtf8 . fromMaybe "" . configDbAnonRole)
+      [("db-aggregates-enabled",         T.toLower . show . configDbAggregates)
+      ,("db-anon-role",              q . T.decodeUtf8 . fromMaybe "" . configDbAnonRole)
       ,("db-channel",                q . configDbChannel)
       ,("db-channel-enabled",            T.toLower . show . configDbChannelEnabled)
       ,("db-extra-search-path",      q . T.intercalate "," . configDbExtraSearchPath)
@@ -232,6 +234,7 @@ parser :: Maybe FilePath -> Environment -> [(Text, Text)] -> RoleSettings -> Rol
 parser optPath env dbSettings roleSettings roleIsolationLvl =
   AppConfig
     <$> parseAppSettings "app.settings"
+    <*> (fromMaybe False <$> optBool "db-aggregates-enabled")
     <*> (fmap encodeUtf8 <$> optString "db-anon-role")
     <*> (fromMaybe "pgrst" <$> optString "db-channel")
     <*> (fromMaybe True <$> optBool "db-channel-enabled")

src/PostgREST/Config/Database.hs

@@ -45,7 +45,8 @@ prefix = "pgrst."
 dbSettingsNames :: [Text]
 dbSettingsNames =
   (prefix <>) <$>
-  ["db_anon_role"
+  ["db_aggregates_enabled"
+  ,"db_anon_role"
   ,"db_pre_config"
   ,"db_extra_search_path"
   ,"db_max_rows"

src/PostgREST/Error.hs

@@ -61,6 +61,7 @@ class (JSON.ToJSON a) => PgrstError a where
     responseLBS (status err) (baseHeader : headers err) $ errorPayload err
 
 instance PgrstError ApiRequestError where
+  status AggregatesNotAllowed{}  = HTTP.status400
   status AmbiguousRelBetween{}   = HTTP.status300
   status AmbiguousRpc{}          = HTTP.status300
   status BinaryFieldError{}      = HTTP.status406
@@ -198,6 +199,9 @@ instance JSON.ToJSON ApiRequestError where
     (Just $ JSON.String $ T.decodeUtf8 ("Invalid preferences: " <> BS.intercalate ", " prefs))
     Nothing
 
+  toJSON AggregatesNotAllowed = toJsonPgrstError
+    ApiRequestErrorCode23 "Use of aggregate functions is not allowed" Nothing Nothing
+
   toJSON (NoRelBetween parent child embedHint schema allRels) = toJsonPgrstError
     SchemaCacheErrorCode00
     ("Could not find a relationship between '" <> parent <> "' and '" <> child <> "' in the schema cache")
@@ -604,6 +608,7 @@ data ErrorCode
   | ApiRequestErrorCode20
   | ApiRequestErrorCode21
   | ApiRequestErrorCode22
+  | ApiRequestErrorCode23
   -- Schema Cache errors
   | SchemaCacheErrorCode00
   | SchemaCacheErrorCode01
@@ -652,6 +657,7 @@ buildErrorCode code = "PGRST" <> case code of
   ApiRequestErrorCode20  -> "120"
   ApiRequestErrorCode21  -> "121"
   ApiRequestErrorCode22  -> "122"
+  ApiRequestErrorCode23  -> "123"
 
   SchemaCacheErrorCode00 -> "200"
   SchemaCacheErrorCode01 -> "201"

src/PostgREST/Plan.hs

@@ -34,7 +34,7 @@ import qualified Data.Set                      as S
 import qualified PostgREST.SchemaCache.Routine as Routine
 
 import Data.Either.Combinators (mapLeft, mapRight)
-import Data.List               (delete)
+import Data.List               (delete, lookup)
 import Data.Tree               (Tree (..))
 
 import PostgREST.ApiRequest                  (Action (..),
@@ -296,18 +296,21 @@ resolveQueryInputField ctx field = withTextParse ctx $ resolveTypeOrUnknown ctx
 -- | Adds filters, order, limits on its respective nodes.
 -- | Adds joins conditions obtained from resource embedding.
 readPlan :: QualifiedIdentifier -> AppConfig -> SchemaCache -> ApiRequest -> Either Error ReadPlanTree
-readPlan qi@QualifiedIdentifier{..} AppConfig{configDbMaxRows} SchemaCache{dbTables, dbRelationships, dbRepresentations} apiRequest  =
+readPlan qi@QualifiedIdentifier{..} AppConfig{configDbMaxRows, configDbAggregates} SchemaCache{dbTables, dbRelationships, dbRepresentations} apiRequest  =
   let
     -- JSON output format hardcoded for now. In the future we might want to support other output mappings such as CSV.
     ctx = ResolverContext dbTables dbRepresentations qi "json"
   in
     mapLeft ApiRequestError $
     treeRestrictRange configDbMaxRows (iAction apiRequest) =<<
+    validateAggFunctions configDbAggregates =<<
+    hoistSpreadAggFunctions =<<
+    addRelSelects =<<
     addNullEmbedFilters =<<
     validateSpreadEmbeds =<<
     addRelatedOrders =<<
-    addDataRepresentationAliases =<<
-    expandStarsForDataRepresentations ctx =<<
+    addAliases =<<
+    expandStars ctx =<<
     addRels qiSchema (iAction apiRequest) dbRelationships Nothing =<<
     addLogicTrees ctx apiRequest =<<
     addRanges apiRequest =<<
@@ -320,7 +323,7 @@ initReadRequest ctx@ResolverContext{qi=QualifiedIdentifier{..}} =
   foldr (treeEntry rootDepth) $ Node defReadPlan{from=qi ctx, relName=qiName, depth=rootDepth} []
   where
     rootDepth = 0
-    defReadPlan = ReadPlan [] (QualifiedIdentifier mempty mempty) Nothing [] [] allRange mempty Nothing [] Nothing mempty Nothing Nothing False rootDepth
+    defReadPlan = ReadPlan [] (QualifiedIdentifier mempty mempty) Nothing [] [] allRange mempty Nothing [] Nothing mempty Nothing Nothing False [] rootDepth
     treeEntry :: Depth -> Tree SelectItem -> ReadPlanTree -> ReadPlanTree
     treeEntry depth (Node si fldForest) (Node q rForest) =
       let nxtDepth = succ depth in
@@ -336,49 +339,86 @@ initReadRequest ctx@ResolverContext{qi=QualifiedIdentifier{..}} =
             (Node defReadPlan{from=QualifiedIdentifier qiSchema selRelation, relName=selRelation, relHint=selHint, relJoinType=selJoinType, depth=nxtDepth, relIsSpread=True} [])
             fldForest:rForest
         SelectField{..} ->
-          Node q{select=(resolveOutputField ctx{qi=from q} selField, selCast, selAlias):select q} rForest
+          Node q{select=CoercibleSelectField (resolveOutputField ctx{qi=from q} selField) selAggregateFunction selAggregateCast selCast selAlias:select q} rForest
 
--- | Preserve the original field name if data representation is used to coerce the value.
-addDataRepresentationAliases :: ReadPlanTree -> Either ApiRequestError ReadPlanTree
-addDataRepresentationAliases rPlanTree = Right $ fmap (\rPlan@ReadPlan{select=sel} -> rPlan{select=map aliasSelectItem sel}) rPlanTree
+-- If an alias is explicitly specified, it is always respected. However, an alias may be
+-- determined automatically in the case of a select term with a JSON path, or in the case
+-- of domain representations.
+addAliases :: ReadPlanTree -> Either ApiRequestError ReadPlanTree
+addAliases = Right . fmap addAliasToPlan
   where
-    aliasSelectItem :: (CoercibleField, Maybe Cast, Maybe Alias) -> (CoercibleField, Maybe Cast, Maybe Alias)
-    -- If there already is an alias, don't overwrite it.
-    aliasSelectItem (fld@(CoercibleField{cfName=fieldName, cfTransform=(Just _)}), Nothing, Nothing) = (fld, Nothing, Just fieldName)
-    aliasSelectItem fld = fld
+    addAliasToPlan rp@ReadPlan{select=sel} = rp{select=map aliasSelectField sel}
+
+    aliasSelectField :: CoercibleSelectField -> CoercibleSelectField
+    aliasSelectField field@CoercibleSelectField{csField=fieldDetails, csAggFunction=aggFun, csAlias=alias}
+      | isJust alias || isJust aggFun = field
+      | isJsonKeyPath fieldDetails, Just key <- lastJsonKey fieldDetails = field { csAlias = Just key }
+      | isTransformPath fieldDetails = field { csAlias = Just (cfName fieldDetails) }
+      | otherwise = field
+
+    isJsonKeyPath CoercibleField{cfJsonPath=(_: _)} = True
+    isJsonKeyPath _                                 = False
+
+    isTransformPath CoercibleField{cfTransform=(Just _), cfName=_} = True
+    isTransformPath _ = False
+
+    lastJsonKey CoercibleField{cfName=fieldName, cfJsonPath=jsonPath} =
+      case jOp <$> lastMay jsonPath of
+            Just (JKey key) -> Just key
+            Just (JIdx _)   -> Just $ fromMaybe fieldName lastKey
+              -- We get the lastKey because on:
+              -- `select=data->1->mycol->>2`, we need to show the result as [ {"mycol": ..}, {"mycol": ..} ]
+              -- `select=data->3`, we need to show the result as [ {"data": ..}, {"data": ..} ]
+              where lastKey = jVal <$> find (\case JKey{} -> True; _ -> False) (jOp <$> reverse jsonPath)
+            Nothing -> Nothing
 
 knownColumnsInContext :: ResolverContext -> [Column]
 knownColumnsInContext ResolverContext{..} =
   fromMaybe [] $ HM.lookup qi tables >>=
   Just . tableColumnsList
 
--- | Expand "select *" into explicit field names of the table, if necessary to apply data representations.
-expandStarsForDataRepresentations :: ResolverContext -> ReadPlanTree -> Either ApiRequestError ReadPlanTree
-expandStarsForDataRepresentations ctx@ResolverContext{qi} rPlanTree = Right $ fmap expandStars rPlanTree
+-- | Expand "select *" into explicit field names of the table in the following situations:
+-- * When there are data representations present.
+-- * When there is an aggregate function in a given ReadPlan or its parent.
+expandStars :: ResolverContext -> ReadPlanTree -> Either ApiRequestError ReadPlanTree
+expandStars ctx rPlanTree = Right $ expandStarsForReadPlan False rPlanTree
   where
-    expandStars :: ReadPlan -> ReadPlan
+    expandStarsForReadPlan :: Bool -> ReadPlanTree -> ReadPlanTree
+    expandStarsForReadPlan hasAgg (Node rp@ReadPlan{select, from=fromQI, fromAlias=alias} children) =
+      let
+        newHasAgg = hasAgg || any (isJust . csAggFunction) select
+        newCtx = adjustContext ctx fromQI alias
+        newRPlan = expandStarsForTable newCtx newHasAgg rp
+      in Node newRPlan (map (expandStarsForReadPlan newHasAgg) children)
+
+    -- Choose the appropriate context based on whether we're dealing with "pgrst_source"
+    adjustContext :: ResolverContext -> QualifiedIdentifier -> Maybe Text -> ResolverContext
     -- When the schema is "" and the table is the source CTE, we assume the true source table is given in the from
     -- alias and belongs to the request schema. See the bit in `addRels` with `newFrom = ...`.
-    expandStars rPlan@ReadPlan{from=(QualifiedIdentifier "" "pgrst_source"), fromAlias=(Just tblAlias)} =
-      expandStarsForTable ctx{qi=qi{qiName=tblAlias}} rPlan
-    expandStars rPlan@ReadPlan{from=fromTable} =
-      expandStarsForTable ctx{qi=fromTable} rPlan
+    adjustContext context@ResolverContext{qi=ctxQI} (QualifiedIdentifier "" "pgrst_source") (Just a) = context{qi=ctxQI{qiName=a}}
+    adjustContext context fromQI _ = context{qi=fromQI}
 
-expandStarsForTable :: ResolverContext -> ReadPlan -> ReadPlan
-expandStarsForTable ctx@ResolverContext{representations, outputType} rplan@ReadPlan{select=selectItems} =
-  -- If we have a '*' select AND the target table has at least one data representation, expand.
-  if ("*" `elem` map (\(field, _, _) -> cfName field) selectItems) && any hasOutputRep knownColumns
-    then rplan{select=concatMap (expandStarSelectItem knownColumns) selectItems}
-    else rplan
+expandStarsForTable :: ResolverContext -> Bool -> ReadPlan -> ReadPlan
+expandStarsForTable ctx@ResolverContext{representations, outputType} hasAgg rp@ReadPlan{select=selectFields}
+  -- We expand if either of the below are true:
+  -- * We have a '*' select AND there is an aggregate function in this ReadPlan's sub-tree.
+  -- * We have a '*' select AND the target table has at least one data representation.
+  -- We ignore any '*' selects that have an aggregate function attached (i.e for COUNT(*)).
+  | hasStarSelect && (hasAgg || hasDataRepresentation) = rp{select = concatMap (expandStarSelectField knownColumns) selectFields}
+  | otherwise = rp
   where
+    hasStarSelect = "*" `elem` map (cfName . csField) filteredSelectFields
+    filteredSelectFields = filter (isNothing . csAggFunction) selectFields
+    hasDataRepresentation = any hasOutputRep knownColumns
     knownColumns = knownColumnsInContext ctx
 
     hasOutputRep :: Column -> Bool
     hasOutputRep col = HM.member (colNominalType col, outputType) representations
 
-    expandStarSelectItem :: [Column] -> (CoercibleField, Maybe Cast, Maybe Alias) -> [(CoercibleField, Maybe Cast, Maybe Alias)]
-    expandStarSelectItem columns (CoercibleField{cfName="*", cfJsonPath=[]}, b, c) = map (\col -> (withOutputFormat ctx $ resolveColumnField col, b, c)) columns
-    expandStarSelectItem _ selectItem = [selectItem]
+    expandStarSelectField :: [Column] -> CoercibleSelectField -> [CoercibleSelectField]
+    expandStarSelectField columns sel@CoercibleSelectField{csField=CoercibleField{cfName="*", cfJsonPath=[]}, csAggFunction=Nothing} =
+      map (\col -> sel { csField = withOutputFormat ctx $ resolveColumnField col }) columns
+    expandStarSelectField _ selectField = [selectField]
 
 -- | Enforces the `max-rows` config on the result
 treeRestrictRange :: Maybe Integer -> Action -> ReadPlanTree -> Either ApiRequestError ReadPlanTree
@@ -535,6 +575,123 @@ findRel schema allRels origin target hint =
             )
       ) $ fromMaybe mempty $ HM.lookup (QualifiedIdentifier schema origin, schema) allRels
 
+
+addRelSelects :: ReadPlanTree -> Either ApiRequestError ReadPlanTree
+addRelSelects node@(Node rp forest)
+  | null forest = Right node
+  | otherwise   =
+    let newForest     = rights $ addRelSelects <$> forest
+        newRelSelects = mapMaybe generateRelSelectField newForest
+    in Right $ Node rp { relSelect = newRelSelects } newForest
+
+generateRelSelectField :: ReadPlanTree -> Maybe RelSelectField
+generateRelSelectField (Node rp@ReadPlan{relToParent=Just _, relAggAlias, relIsSpread = True} _) =
+  Just $ Spread { rsSpreadSel = generateSpreadSelectFields rp, rsAggAlias = relAggAlias }
+generateRelSelectField (Node ReadPlan{relToParent=Just rel, select, relName, relAlias, relAggAlias, relIsSpread = False} forest) =
+  Just $ JsonEmbed { rsEmbedMode, rsSelName, rsAggAlias = relAggAlias, rsEmptyEmbed }
+  where
+    rsSelName = fromMaybe relName relAlias
+    rsEmbedMode = if relIsToOne rel then JsonObject else JsonArray
+    rsEmptyEmbed = null select && null forest
+generateRelSelectField _ = Nothing
+
+generateSpreadSelectFields :: ReadPlan -> [SpreadSelectField]
+generateSpreadSelectFields ReadPlan{select, relSelect} =
+  -- We combine the select and relSelect fields into a single list of SpreadSelectField.
+  selectSpread ++ relSelectSpread
+  where
+    selectSpread = map selectToSpread select
+    selectToSpread :: CoercibleSelectField -> SpreadSelectField
+    selectToSpread CoercibleSelectField{csField = CoercibleField{cfName}, csAlias} =
+      SpreadSelectField { ssSelName = fromMaybe cfName csAlias, ssSelAggFunction = Nothing, ssSelAggCast = Nothing, ssSelAlias = Nothing }
+
+    relSelectSpread = concatMap relSelectToSpread relSelect
+    relSelectToSpread :: RelSelectField -> [SpreadSelectField]
+    relSelectToSpread (JsonEmbed{rsSelName}) =
+      [SpreadSelectField { ssSelName = rsSelName, ssSelAggFunction = Nothing, ssSelAggCast = Nothing, ssSelAlias = Nothing }]
+    relSelectToSpread (Spread{rsSpreadSel}) =
+      rsSpreadSel
+
+-- When aggregates are present in a ReadPlan that will be spread, we "hoist"
+-- to the highest level possible so that their semantics make sense. For instance,
+-- imagine the user performs the following request:
+-- `GET /projects?select=client_id,...project_invoices(invoice_total.sum())`
+--
+-- In this case, it is sensible that we would expect to receive the sum of the
+-- `invoice_total`, grouped by the `client_id`. Without hoisting, the sum would
+-- be performed in the sub-query for the joined table `project_invoices`, thus
+-- making it essentially a no-op. With hoisting, we hoist the aggregate function
+-- so that the aggregate function is performed in a more sensible context.
+--
+-- We will try to hoist the aggregate function to the highest possible level,
+-- which means that we hoist until we reach the root node, or until we reach a
+-- ReadPlan that will be embedded a JSON object or JSON array.
+
+-- This type alias represents an aggregate that is to be hoisted to the next
+-- level up. The first tuple of `Alias` and `FieldName` contain the alias for
+-- the joined table and the original field name for the hoisted field.
+--
+-- The second tuple contains the aggregate function to be applied, the cast, and
+-- the alias, if it was supplied by the user or otherwise determined.
+type HoistedAgg = ((Alias, FieldName), (AggregateFunction, Maybe Cast, Maybe Alias))
+
+hoistSpreadAggFunctions :: ReadPlanTree -> Either ApiRequestError ReadPlanTree
+hoistSpreadAggFunctions tree = Right $ fst $ applySpreadAggHoistingToNode tree
+
+applySpreadAggHoistingToNode :: ReadPlanTree -> (ReadPlanTree, [HoistedAgg])
+applySpreadAggHoistingToNode (Node rp@ReadPlan{relAggAlias, relToParent, relIsSpread} children) =
+  let (newChildren, childAggLists) = unzip $ map applySpreadAggHoistingToNode children
+      allChildAggLists = concat childAggLists
+      (newSelects, aggList) = if depth rp == 0 || (isJust relToParent && not relIsSpread)
+                                then (select rp, [])
+                                else hoistFromSelectFields relAggAlias (select rp)
+
+      newRelSelects = if null children
+                      then relSelect rp
+                      else map (hoistIntoRelSelectFields allChildAggLists) $ relSelect rp
+  in  (Node rp { select = newSelects, relSelect = newRelSelects } newChildren, aggList)
+
+-- Hoist aggregate functions from the select list of a ReadPlan, and return the
+-- updated select list and the list of hoisted aggregates.
+hoistFromSelectFields :: Alias -> [CoercibleSelectField] -> ([CoercibleSelectField], [HoistedAgg])
+hoistFromSelectFields aggAlias fields =
+    let (newFields, maybeAggs) = foldr processField ([], []) fields
+    in (newFields, catMaybes maybeAggs)
+  where
+    processField field (newFields, aggList) =
+      let (modifiedField, maybeAgg) = modifyField field
+      in (modifiedField : newFields, maybeAgg : aggList)
+
+    modifyField field =
+      case csAggFunction field of
+        Just aggFunc ->
+          ( field { csAggFunction = Nothing, csAggCast = Nothing },
+            Just ((aggAlias, determineFieldName field), (aggFunc, csAggCast field, csAlias field)))
+        Nothing -> (field, Nothing)
+
+    determineFieldName field = fromMaybe (cfName $ csField field) (csAlias field)
+
+-- Taking the hoisted aggregates, modify the rel selects to apply the aggregates,
+-- and any applicable casts or aliases.
+hoistIntoRelSelectFields :: [HoistedAgg] -> RelSelectField -> RelSelectField
+hoistIntoRelSelectFields aggList r@(Spread {rsSpreadSel = spreadSelects, rsAggAlias = aggAlias}) =
+    r { rsSpreadSel = map updateSelect spreadSelects }
+  where
+    updateSelect s =
+        case lookup (aggAlias, ssSelName s) aggList of
+            Just (aggFunc, aggCast, fldAlias) ->
+                s { ssSelAggFunction = Just aggFunc,
+                    ssSelAggCast     = aggCast,
+                    ssSelAlias       = fldAlias }
+            Nothing -> s
+hoistIntoRelSelectFields _ r = r
+
+validateAggFunctions :: Bool -> ReadPlanTree -> Either ApiRequestError ReadPlanTree
+validateAggFunctions aggFunctionsAllowed (Node rp@ReadPlan {select} forest)
+  | aggFunctionsAllowed = Node rp <$> traverse (validateAggFunctions aggFunctionsAllowed) forest
+  | any (isJust . csAggFunction) select = Left AggregatesNotAllowed
+  | otherwise = Node rp <$> traverse (validateAggFunctions aggFunctionsAllowed) forest
+
 addFilters :: ResolverContext -> ApiRequest -> ReadPlanTree -> Either ApiRequestError ReadPlanTree
 addFilters ctx ApiRequest{..} rReq =
   foldr addFilterToNode (Right rReq) flts
@@ -608,7 +765,8 @@ addRelatedOrders (Node rp@ReadPlan{order,from} forest) = do
 --         relName = "projects",
 --         relToParent = Nothing,
 --         relJoinConds = [],
---         relAlias = Nothing, relAggAlias = "clients_projects_1", relHint = Nothing, relJoinType = Nothing, relIsSpread = False, depth = 1
+--         relAlias = Nothing, relAggAlias = "clients_projects_1", relHint = Nothing, relJoinType = Nothing, relIsSpread = False, depth = 1,
+--         relSelect = []
 --       },
 --       subForest = []
 --     }
@@ -633,7 +791,8 @@ addRelatedOrders (Node rp@ReadPlan{order,from} forest) = do
 --         )
 --       ],
 --       order = [], range_ = fullRange, relName = "clients", relToParent = Nothing, relJoinConds = [], relAlias = Nothing, relAggAlias = "", relHint = Nothing,
---       relJoinType = Nothing, relIsSpread = False, depth = 0
+--       relJoinType = Nothing, relIsSpread = False, depth = 0,
+--       relSelect = []
 --     },
 --     subForest = subForst
 --   }
@@ -789,7 +948,7 @@ inferColsEmbedNeeds (Node ReadPlan{select} forest) pkCols
   | "*" `elem` fldNames = ["*"]
   | otherwise           = returnings
   where
-    fldNames = cfName . (\(f, _, _) -> f) <$> select
+    fldNames = cfName . csField <$> select
     -- Without fkCols, when a mutatePlan to
     -- /projects?select=name,clients(name) occurs, the RETURNING SQL part would
     -- be `RETURNING name`(see QueryBuilder).  This would make the embedding

src/PostgREST/Plan/ReadPlan.hs

@@ -6,11 +6,12 @@ module PostgREST.Plan.ReadPlan
 
 import Data.Tree (Tree (..))
 
-import PostgREST.ApiRequest.Types         (Alias, Cast, Depth, Hint,
+import PostgREST.ApiRequest.Types         (Alias, Depth, Hint,
                                            JoinType, NodeName)
-import PostgREST.Plan.Types               (CoercibleField (..),
-                                           CoercibleLogicTree,
-                                           CoercibleOrderTerm)
+import PostgREST.Plan.Types               (CoercibleLogicTree,
+                                           CoercibleOrderTerm,
+                                           CoercibleSelectField (..),
+                                           RelSelectField (..))
 import PostgREST.RangeQuery               (NonnegRange)
 import PostgREST.SchemaCache.Identifiers  (FieldName,
                                            QualifiedIdentifier)
@@ -28,7 +29,7 @@ data JoinCondition =
   deriving (Eq, Show)
 
 data ReadPlan = ReadPlan
-  { select       :: [(CoercibleField, Maybe Cast, Maybe Alias)]
+  { select       :: [CoercibleSelectField]
   , from         :: QualifiedIdentifier
   , fromAlias    :: Maybe Alias
   , where_       :: [CoercibleLogicTree]
@@ -42,6 +43,7 @@ data ReadPlan = ReadPlan
   , relHint      :: Maybe Hint
   , relJoinType  :: Maybe JoinType
   , relIsSpread  :: Bool
+  , relSelect    :: [RelSelectField]
   , depth        :: Depth
   -- ^ used for aliasing
   }

src/PostgREST/Plan/Types.hs

@@ -1,13 +1,18 @@
 module PostgREST.Plan.Types
   ( CoercibleField(..)
+  , CoercibleSelectField(..)
   , unknownField
   , CoercibleLogicTree(..)
   , CoercibleFilter(..)
   , TransformerProc
   , CoercibleOrderTerm(..)
+  , RelSelectField(..)
+  , RelJsonEmbedMode(..)
+  , SpreadSelectField(..)
   ) where
 
-import PostgREST.ApiRequest.Types (Field, JsonPath, LogicOperator,
+import PostgREST.ApiRequest.Types (AggregateFunction, Alias, Cast,
+                                   Field, JsonPath, LogicOperator,
                                    OpExpr, OrderDirection, OrderNulls)
 
 import PostgREST.SchemaCache.Identifiers (FieldName)
@@ -65,3 +70,37 @@ data CoercibleOrderTerm
     , coNullOrder :: Maybe OrderNulls
     }
   deriving (Eq, Show)
+
+data CoercibleSelectField = CoercibleSelectField
+  { csField       :: CoercibleField
+  , csAggFunction :: Maybe AggregateFunction
+  , csAggCast     :: Maybe Cast
+  , csCast        :: Maybe Cast
+  , csAlias       :: Maybe Alias
+  }
+  deriving (Eq, Show)
+
+data RelJsonEmbedMode = JsonObject | JsonArray
+  deriving (Show, Eq)
+
+data RelSelectField
+  = JsonEmbed
+      { rsSelName    :: FieldName
+      , rsAggAlias   :: Alias
+      , rsEmbedMode  :: RelJsonEmbedMode
+      , rsEmptyEmbed :: Bool
+      }
+  | Spread
+      { rsSpreadSel :: [SpreadSelectField]
+      , rsAggAlias  :: Alias
+      }
+  deriving (Eq, Show)
+
+data SpreadSelectField =
+  SpreadSelectField
+  { ssSelName        :: FieldName
+  , ssSelAggFunction :: Maybe AggregateFunction
+  , ssSelAggCast     :: Maybe Cast
+  , ssSelAlias       :: Maybe Alias
+  }
+  deriving (Eq, Show)

src/PostgREST/Query/QueryBuilder.hs

@@ -19,7 +19,8 @@ module PostgREST.Query.QueryBuilder
 import qualified Data.ByteString.Char8           as BS
 import qualified Hasql.DynamicStatements.Snippet as SQL
 
-import Data.Tree (Tree (..))
+import Data.Maybe (fromJust)
+import Data.Tree  (Tree (..))
 
 import PostgREST.ApiRequest.Preferences   (PreferResolution (..))
 import PostgREST.Config.PgVersion         (PgVersion, pgVersion110,
@@ -27,8 +28,7 @@ import PostgREST.Config.PgVersion         (PgVersion, pgVersion110,
 import PostgREST.SchemaCache.Identifiers  (QualifiedIdentifier (..))
 import PostgREST.SchemaCache.Relationship (Cardinality (..),
                                            Junction (..),
-                                           Relationship (..),
-                                           relIsToOne)
+                                           Relationship (..))
 import PostgREST.SchemaCache.Routine      (RoutineParam (..))
 
 import PostgREST.ApiRequest.Types
@@ -42,45 +42,70 @@ import PostgREST.RangeQuery        (allRange)
 import Protolude
 
 readPlanToQuery :: ReadPlanTree -> SQL.Snippet
-readPlanToQuery (Node ReadPlan{select,from=mainQi,fromAlias,where_=logicForest,order, range_=readRange, relToParent, relJoinConds} forest) =
+readPlanToQuery node@(Node ReadPlan{select,from=mainQi,fromAlias,where_=logicForest,order, range_=readRange, relToParent, relJoinConds, relSelect} forest) =
   "SELECT " <>
-  intercalateSnippet ", " ((pgFmtSelectItem qi <$> (if null select && null forest then defSelect else select)) ++ selects) <> " " <>
+  intercalateSnippet ", " ((pgFmtSelectItem qi <$> (if null select && null forest then defSelect else select)) ++ joinsSelects) <> " " <>
   fromFrag <> " " <>
   intercalateSnippet " " joins <> " " <>
   (if null logicForest && null relJoinConds
     then mempty
     else "WHERE " <> intercalateSnippet " AND " (map (pgFmtLogicTree qi) logicForest ++ map pgFmtJoinCondition relJoinConds)) <> " " <>
+  groupF qi select relSelect <> " " <>
   orderF qi order <> " " <>
   limitOffsetF readRange
   where
     fromFrag = fromF relToParent mainQi fromAlias
     qi = getQualifiedIdentifier relToParent mainQi fromAlias
-    defSelect = [(unknownField "*" [], Nothing, Nothing)] -- gets all the columns in case of an empty select, ignoring/obtaining these columns is done at the aggregation stage
-    (selects, joins) = foldr getSelectsJoins ([],[]) forest
+    -- gets all the columns in case of an empty select, ignoring/obtaining these columns is done at the aggregation stage
+    defSelect = [CoercibleSelectField (unknownField "*" []) Nothing Nothing Nothing Nothing]
+    joins = getJoins node
+    joinsSelects = getJoinSelects node
 
-getSelectsJoins :: ReadPlanTree -> ([SQL.Snippet], [SQL.Snippet]) -> ([SQL.Snippet], [SQL.Snippet])
-getSelectsJoins (Node ReadPlan{relToParent=Nothing} _) _ = ([], [])
-getSelectsJoins rr@(Node ReadPlan{select, relName, relToParent=Just rel, relAggAlias, relAlias, relJoinType, relIsSpread} forest) (selects,joins) =
+getJoinSelects :: ReadPlanTree -> [SQL.Snippet]
+getJoinSelects (Node ReadPlan{relSelect} _) =
+  mapMaybe relSelectToSnippet relSelect
+  where
+    relSelectToSnippet :: RelSelectField -> Maybe SQL.Snippet
+    relSelectToSnippet fld =
+      let aggAlias = pgFmtIdent $ rsAggAlias fld
+      in
+        case fld of
+          JsonEmbed{rsEmptyEmbed = True} ->
+            Nothing
+          JsonEmbed{rsSelName, rsEmbedMode = JsonObject} ->
+            Just $ "row_to_json(" <> aggAlias <> ".*)::jsonb AS " <> pgFmtIdent rsSelName
+          JsonEmbed{rsSelName, rsEmbedMode = JsonArray} ->
+            Just $ "COALESCE( " <> aggAlias <> "." <> aggAlias <> ", '[]') AS " <> pgFmtIdent rsSelName
+          Spread{rsSpreadSel, rsAggAlias} ->
+            Just $ intercalateSnippet ", " (pgFmtSpreadSelectItem rsAggAlias <$> rsSpreadSel)
+
+getJoins :: ReadPlanTree -> [SQL.Snippet]
+getJoins (Node _ []) = []
+getJoins (Node ReadPlan{relSelect} forest) =
+  map (\fld ->
+         let alias = rsAggAlias fld
+             matchingNode = fromJust $ find (\(Node ReadPlan{relAggAlias} _) -> alias == relAggAlias) forest
+         in getJoin fld matchingNode
+      ) relSelect
+
+getJoin :: RelSelectField -> ReadPlanTree -> SQL.Snippet
+getJoin fld node@(Node ReadPlan{relJoinType} _) =
   let
-    subquery = readPlanToQuery rr
-    aliasOrName = pgFmtIdent $ fromMaybe relName relAlias
-    aggAlias = pgFmtIdent relAggAlias
     correlatedSubquery sub al cond =
       (if relJoinType == Just JTInner then "INNER" else "LEFT") <> " JOIN LATERAL ( " <> sub <> " ) AS " <> al <> " ON " <> cond
-    (sel, joi) = if relIsToOne rel
-      then
-        ( if relIsSpread
-            then aggAlias <> ".*"
-            else "row_to_json(" <> aggAlias <> ".*) AS " <> aliasOrName
-        , correlatedSubquery subquery aggAlias "TRUE")
-      else
-        ( "COALESCE( " <> aggAlias <> "." <> aggAlias <> ", '[]') AS " <> aliasOrName
-        , correlatedSubquery (
-            "SELECT json_agg(" <> aggAlias <> ") AS " <> aggAlias <>
-            "FROM (" <> subquery <> " ) AS " <> aggAlias
-          ) aggAlias $ if relJoinType == Just JTInner then aggAlias <> " IS NOT NULL" else "TRUE")
+    subquery = readPlanToQuery node
+    aggAlias = pgFmtIdent $ rsAggAlias fld
   in
-  (if null select && null forest then selects else sel:selects, joi:joins)
+    case fld of
+      JsonEmbed{rsEmbedMode = JsonObject} ->
+        correlatedSubquery subquery aggAlias "TRUE"
+      Spread{} ->
+        correlatedSubquery subquery aggAlias "TRUE"
+      JsonEmbed{rsEmbedMode = JsonArray} ->
+        let
+          subq = "SELECT json_agg(" <> aggAlias <> ")::jsonb AS " <> aggAlias <> " FROM (" <> subquery <> " ) AS " <> aggAlias
+          condition = if relJoinType == Just JTInner then aggAlias <> " IS NOT NULL" else "TRUE"
+        in correlatedSubquery subq aggAlias condition
 
 mutatePlanToQuery :: MutatePlan -> SQL.Snippet
 mutatePlanToQuery (Insert mainQi iCols body onConflct putConditions returnings _ applyDefaults) =

src/PostgREST/Query/SqlFragment.hs

@@ -9,6 +9,7 @@ module PostgREST.Query.SqlFragment
   ( noLocationF
   , handlerF
   , countF
+  , groupF
   , fromQi
   , limitOffsetF
   , locationF
@@ -21,6 +22,7 @@ module PostgREST.Query.SqlFragment
   , pgFmtLogicTree
   , pgFmtOrderTerm
   , pgFmtSelectItem
+  , pgFmtSpreadSelectItem
   , fromJsonBodyF
   , responseHeadersF
   , responseStatusF
@@ -54,7 +56,8 @@ import Control.Arrow ((***))
 import Data.Foldable                 (foldr1)
 import Text.InterpolatedString.Perl6 (qc)
 
-import PostgREST.ApiRequest.Types        (Alias, Cast,
+import PostgREST.ApiRequest.Types        (AggregateFunction (..),
+                                          Alias, Cast,
                                           FtsOperator (..),
                                           JsonOperand (..),
                                           JsonOperation (..),
@@ -75,6 +78,9 @@ import PostgREST.Plan.Types              (CoercibleField (..),
                                           CoercibleFilter (..),
                                           CoercibleLogicTree (..),
                                           CoercibleOrderTerm (..),
+                                          CoercibleSelectField (..),
+                                          RelSelectField (..),
+                                          SpreadSelectField (..),
                                           unknownField)
 import PostgREST.RangeQuery              (NonnegRange, allRange,
                                           rangeLimit, rangeOffset)
@@ -86,7 +92,7 @@ import PostgREST.SchemaCache.Routine     (MediaHandler (..),
                                           funcReturnsSetOfScalar,
                                           funcReturnsSingleComposite)
 
-import Protolude hiding (cast)
+import Protolude hiding (Sum, cast)
 
 sourceCTEName :: Text
 sourceCTEName = "pgrst_source"
@@ -258,12 +264,34 @@ pgFmtCoerceNamed :: CoercibleField -> SQL.Snippet
 pgFmtCoerceNamed CoercibleField{cfName=fn, cfTransform=(Just formatterProc)} = pgFmtCallUnary formatterProc (pgFmtIdent fn) <> " AS " <> pgFmtIdent fn
 pgFmtCoerceNamed CoercibleField{cfName=fn} = pgFmtIdent fn
 
-pgFmtSelectItem :: QualifiedIdentifier -> (CoercibleField, Maybe Cast, Maybe Alias) -> SQL.Snippet
-pgFmtSelectItem table (fld, Nothing, alias) = pgFmtTableCoerce table fld <> pgFmtAs (cfName fld) (cfJsonPath fld) alias
+pgFmtSelectItem :: QualifiedIdentifier -> CoercibleSelectField -> SQL.Snippet
+pgFmtSelectItem table CoercibleSelectField{csField=fld, csAggFunction=agg, csAggCast=aggCast, csCast=cast, csAlias=alias} =
+  pgFmtApplyAggregate agg aggCast (pgFmtApplyCast cast (pgFmtTableCoerce table fld)) <> pgFmtAs alias
+
+pgFmtSpreadSelectItem :: Alias -> SpreadSelectField -> SQL.Snippet
+pgFmtSpreadSelectItem aggAlias SpreadSelectField{ssSelName, ssSelAggFunction, ssSelAggCast, ssSelAlias} =
+  pgFmtApplyAggregate ssSelAggFunction ssSelAggCast fullSelName <> pgFmtAs ssSelAlias
+  where
+    fullSelName = case ssSelName of
+      "*" -> pgFmtIdent aggAlias <> ".*"
+      _   -> pgFmtIdent aggAlias <> "." <> pgFmtIdent ssSelName
+
+pgFmtApplyAggregate :: Maybe AggregateFunction -> Maybe Cast -> SQL.Snippet -> SQL.Snippet
+pgFmtApplyAggregate Nothing _ snippet = snippet
+pgFmtApplyAggregate (Just agg) aggCast snippet =
+  pgFmtApplyCast aggCast aggregatedSnippet
+  where
+    convertAggFunction :: AggregateFunction -> SQL.Snippet
+    -- Convert from e.g. Sum (the data type) to SUM
+    convertAggFunction = SQL.sql . BS.map toUpper . BS.pack . show
+    aggregatedSnippet = convertAggFunction agg <> "(" <> snippet <> ")"
+
+pgFmtApplyCast :: Maybe Cast -> SQL.Snippet -> SQL.Snippet
+pgFmtApplyCast Nothing snippet = snippet
 -- Ideally we'd quote the cast with "pgFmtIdent cast". However, that would invalidate common casts such as "int", "bigint", etc.
 -- Try doing: `select 1::"bigint"` - it'll err, using "int8" will work though. There's some parser magic that pg does that's invalidated when quoting.
 -- Not quoting should be fine, we validate the input on Parsers.
-pgFmtSelectItem table (fld, Just cast, alias) = "CAST (" <> pgFmtTableCoerce table fld <> " AS " <> SQL.sql (encodeUtf8 cast) <> " )" <> pgFmtAs (cfName fld) (cfJsonPath fld) alias
+pgFmtApplyCast (Just cast) snippet = "CAST( " <> snippet <> " AS " <> SQL.sql (encodeUtf8 cast) <> " )"
 
 -- TODO: At this stage there shouldn't be a Maybe since ApiRequest should ensure that an INSERT/UPDATE has a body
 fromJsonBodyF :: Maybe LBS.ByteString -> [CoercibleField] -> Bool -> Bool -> Bool -> SQL.Snippet
@@ -395,17 +423,40 @@ pgFmtJsonPath = \case
     pgFmtJsonOperand (JKey k) = unknownLiteral k
     pgFmtJsonOperand (JIdx i) = unknownLiteral i <> "::int"
 
-pgFmtAs :: FieldName -> JsonPath -> Maybe Alias -> SQL.Snippet
-pgFmtAs _ [] Nothing = mempty
-pgFmtAs fName jp Nothing = case jOp <$> lastMay jp of
-  Just (JKey key) -> " AS " <> pgFmtIdent key
-  Just (JIdx _)   -> " AS " <> pgFmtIdent (fromMaybe fName lastKey)
-    -- We get the lastKey because on:
-    -- `select=data->1->mycol->>2`, we need to show the result as [ {"mycol": ..}, {"mycol": ..} ]
-    -- `select=data->3`, we need to show the result as [ {"data": ..}, {"data": ..} ]
-    where lastKey = jVal <$> find (\case JKey{} -> True; _ -> False) (jOp <$> reverse jp)
-  Nothing -> mempty
-pgFmtAs _ _ (Just alias) = " AS " <> pgFmtIdent alias
+pgFmtAs :: Maybe Alias -> SQL.Snippet
+pgFmtAs Nothing      = mempty
+pgFmtAs (Just alias) = " AS " <> pgFmtIdent alias
+
+groupF :: QualifiedIdentifier -> [CoercibleSelectField] -> [RelSelectField] -> SQL.Snippet
+groupF qi select relSelect
+  | (noSelectsAreAggregated && noRelSelectsAreAggregated) || null groupTerms = mempty
+  | otherwise = " GROUP BY " <> intercalateSnippet ", " groupTerms
+  where
+    noSelectsAreAggregated = null $ [s | s@(CoercibleSelectField { csAggFunction = Just _ }) <- select]
+    noRelSelectsAreAggregated = all (\case Spread sels _ -> all (isNothing . ssSelAggFunction) sels; _ -> True) relSelect
+    groupTermsFromSelect = mapMaybe (pgFmtGroup qi) select
+    groupTermsFromRelSelect = mapMaybe groupTermFromRelSelectField relSelect
+    groupTerms = groupTermsFromSelect ++ groupTermsFromRelSelect
+
+groupTermFromRelSelectField :: RelSelectField -> Maybe SQL.Snippet
+groupTermFromRelSelectField (JsonEmbed { rsSelName }) =
+  Just $ pgFmtIdent rsSelName
+groupTermFromRelSelectField (Spread { rsSpreadSel, rsAggAlias }) =
+  if null groupTerms
+  then Nothing
+  else
+    Just $ intercalateSnippet ", " groupTerms
+  where
+    processField :: SpreadSelectField -> Maybe SQL.Snippet
+    processField SpreadSelectField{ssSelAggFunction = Just _} = Nothing
+    processField SpreadSelectField{ssSelName, ssSelAlias} =
+      Just $ pgFmtIdent rsAggAlias <> "." <> pgFmtIdent (fromMaybe ssSelName ssSelAlias)
+    groupTerms = mapMaybe processField rsSpreadSel
+
+pgFmtGroup :: QualifiedIdentifier -> CoercibleSelectField -> Maybe SQL.Snippet
+pgFmtGroup _  CoercibleSelectField{csAggFunction=Just _} = Nothing
+pgFmtGroup _  CoercibleSelectField{csAlias=Just alias, csAggFunction=Nothing} = Just $ pgFmtIdent alias
+pgFmtGroup qi CoercibleSelectField{csField=fld, csAlias=Nothing, csAggFunction=Nothing} = Just $ pgFmtField qi fld
 
 countF :: SQL.Snippet -> Bool -> (SQL.Snippet, SQL.Snippet)
 countF countQuery shouldCount =