instead of select * inspect schema to get selectable columns

src/library/PostgREST/Query/OpenApi.hs

@@ -5,11 +5,18 @@ Description : Types for reflecting the role privileges on the OpenAPI output.
 module PostgREST.Query.OpenApi
   ( TableAccess (..)
   , TablesAccess
+  , tablesAccessStatement
+  , decodeTablesAccess
   ) where
 
-import qualified Data.HashMap.Strict as HM
+import qualified Data.HashMap.Strict             as HM
+import qualified Hasql.Decoders                  as HD
+import qualified Hasql.DynamicStatements.Statement as SQL
+import qualified Hasql.Statement                 as SQL
 
-import PostgREST.SchemaCache.Identifiers (FieldName, QualifiedIdentifier)
+import qualified PostgREST.Query.SqlFragment as SqlFragment
+
+import PostgREST.SchemaCache.Identifiers (FieldName, QualifiedIdentifier (..))
 
 import Protolude
 
@@ -27,3 +34,27 @@ data TableAccess = TableAccess
   deriving (Show, Eq)
 
 type TablesAccess = HM.HashMap QualifiedIdentifier TableAccess
+
+-- | Statement that returns the privileges the current role has on each
+-- accessible relation of the given schema.
+tablesAccessStatement :: Text -> SQL.Statement () TablesAccess
+tablesAccessStatement schema =
+  SQL.dynamicallyParameterized (SqlFragment.accessibleTables schema) decodeTablesAccess False
+
+decodeTablesAccess :: HD.Result TablesAccess
+decodeTablesAccess =
+  let
+    row = (,) <$> (QualifiedIdentifier <$> column HD.text <*> column HD.text)
+              <*> (TableAccess
+                    <$> arrayColumn HD.text
+                    <*> arrayColumn HD.text
+                    <*> arrayColumn HD.text
+                    <*> column HD.bool)
+  in
+    HM.fromList <$> HD.rowList row
+
+column :: HD.Value a -> HD.Row a
+column = HD.column . HD.nonNullable
+
+arrayColumn :: HD.Value a -> HD.Row [a]
+arrayColumn = column . HD.listArray . HD.nonNullable