feat: Allow filtering by IS DISTINCT FROM using the isdistinct operator

src/PostgREST/ApiRequest/QueryParams.hs

@@ -589,11 +589,13 @@ pOpExpr pSVal = do
   OpExpr boolExpr <$> pOperation
   where
     pOperation :: Parser Operation
-    pOperation = pIn <|> pIs <|> try pFts <|> try pOp <?> "operator (eq, gt, ...)"
+    pOperation = pIn <|> pIs <|> pIsDist <|> try pFts <|> try pOp <?> "operator (eq, gt, ...)"
 
     pIn = In <$> (try (string "in" *> pDelimiter) *> pListVal)
     pIs = Is <$> (try (string "is" *> pDelimiter) *> pTriVal)
 
+    pIsDist = IsDistinctFrom <$> (try (string "isdistinct" *> pDelimiter) *> pSVal)
+
     pOp = do
       opStr <- try (P.manyTill anyChar (try pDelimiter))
       op <- parseMaybe ("unknown single value operator " <> opStr) . operator $ toS opStr

src/PostgREST/ApiRequest/Types.hs

@@ -191,6 +191,7 @@ data Operation
   = Op SimpleOperator SingleVal
   | In ListVal
   | Is TrileanVal
+  | IsDistinctFrom SingleVal
   | Fts FtsOperator (Maybe Language) SingleVal
   deriving (Eq)
 

src/PostgREST/Query/SqlFragment.hs

@@ -305,6 +305,8 @@ pgFmtFilter table (Filter fld (OpExpr hasNot oper)) = notOp <> " " <> case oper
      TriNull    -> "NULL"
      TriUnknown -> "UNKNOWN"
 
+   IsDistinctFrom val -> pgFmtField table fld <> " IS DISTINCT FROM " <> unknownLiteral val
+
    -- We don't use "IN", we use "= ANY". IN has the following disadvantages:
    -- + No way to use an empty value on IN: "col IN ()" is invalid syntax. With ANY we can do "= ANY('{}')"
    -- + Can invalidate prepared statements: multiple parameters on an IN($1, $2, $3) will lead to using different prepared statements and not take advantage of caching.