add: config db-timezone-enabled for optional querying of timezones

To avoid repeated querying of `pg_timezone_names` every time schema
cache is reset, `Prefer: timezone` can be disabled by setting
`db-timezone-enabled = false`.

Signed-off-by: Taimoor Zaeem <taimoorzaeem@gmail.com>

src/PostgREST/Config.hs

@@ -94,6 +94,7 @@ data AppConfig = AppConfig
   , configDbSchemas                :: NonEmpty Text
   , configDbConfig                 :: Bool
   , configDbPreConfig              :: Maybe QualifiedIdentifier
+  , configDbTimezoneEnabled        :: Bool
   , configDbTxAllowOverride        :: Bool
   , configDbTxRollbackAll          :: Bool
   , configDbUri                    :: Text
@@ -181,6 +182,7 @@ toText conf =
       ,("db-schemas",                q . T.intercalate "," . toList . configDbSchemas)
       ,("db-config",                     T.toLower . show . configDbConfig)
       ,("db-pre-config",             q . maybe mempty dumpQi . configDbPreConfig)
+      ,("db-timezone-enabled",           T.toLower . show . configDbTimezoneEnabled)
       ,("db-tx-end",                 q . showTxEnd)
       ,("db-uri",                    q . configDbUri)
       ,("jwt-aud",                   q . fromMaybe mempty . configJwtAudience)
@@ -290,6 +292,7 @@ parser optPath env dbSettings roleSettings roleIsolationLvl =
     <*> parseDbSchemas "db-schemas" "db-schema"
     <*> (fromMaybe True <$> optBool "db-config")
     <*> (fmap toQi <$> optString "db-pre-config")
+    <*> (fromMaybe True <$> optBool "db-timezone-enabled")
     <*> parseTxEnd "db-tx-end" snd
     <*> parseTxEnd "db-tx-end" fst
     <*> (fromMaybe "postgresql://" <$> optString "db-uri")
@@ -715,6 +718,9 @@ exampleConfigFile = S.unlines
   , "## The name of which database schema to expose to REST clients"
   , "db-schemas = \"public\""
   , ""
+  , "## Enable quering pg_timezone_names from db"
+  , "# db-timezone-enabled = true"
+  , ""
   , "## How to terminate database transactions"
   , "## Possible values are:"
   , "## commit (default)"

src/PostgREST/Config/Database.hs

@@ -56,6 +56,7 @@ dbSettingsNames =
   ,"db_prepared_statements"
   ,"db_root_spec"
   ,"db_schemas"
+  ,"db_timezone_enabled"
   ,"db_tx_end"
   ,"db_hoisted_tx_settings"
   ,"jwt_aud"

src/PostgREST/SchemaCache.hs

@@ -159,7 +159,9 @@ querySchemaCache conf@AppConfig{..} = do
   cRels   <- SQL.statement mempty $ allComputedRels prepared
   reps    <- SQL.statement conf $ dataRepresentations prepared
   mHdlers <- SQL.statement conf $ mediaHandlers prepared
-  tzones  <- SQL.statement mempty $ timezones prepared
+  tzones  <- if configDbTimezoneEnabled
+    then SQL.statement mempty $ timezones prepared
+    else pure S.empty
   _       <-
     let sleepCall = SQL.Statement "select pg_sleep($1 / 1000.0)" (param HE.int4) HD.noResult prepared in
     for_ configInternalSCQuerySleep (`SQL.statement` sleepCall) -- only used for testing
@@ -1109,7 +1111,13 @@ decodeMediaHandlers =
 timezones :: Bool -> SQL.Statement () TimezoneNames
 timezones = SQL.Statement sql HE.noParams decodeTimezones
   where
-    sql = "SELECT name FROM pg_timezone_names"
+    sql = encodeUtf8 $ unlines
+      -- This CTE wrapper is only added for clarifying the query under pg_stat_statements
+      ["WITH pgrst_timezones AS ("
+      , "  SELECT name FROM pg_timezone_names"
+      , ")"
+      , "SELECT * FROM pgrst_timezones"
+      ]
     decodeTimezones :: HD.Result TimezoneNames
     decodeTimezones = S.fromList <$> HD.rowList (column HD.text)