Fix behavioural differences due to return type and handle constant string literal inputs differently for T-SQL COALESCE function (#345)

This commit contains the following changes :

    Whenever the caller is T-SQL COALESCE, then call the select_common_type_hook with 'TSQL_COALESCE' context.
    If the input is a constant string literal, first we convert the UNKNOWN node to VARCHAR node and then coerce it to the common type. This change also handles empty or white space strings according to the way T-SQL handles.

Issues Resolved

[BABEL-726]

Extension PR : babelfish-for-postgresql/babelfish_extensions#2507

Signed-off-by: Sai Rohan Basa [email protected]

Author: basasairohan

src/backend/parser/parse_expr.c

@@ -100,6 +100,7 @@ static Node *make_nulltest_from_distinct(ParseState *pstate,
 static List *ExpandChecksumStar(ParseState *pstate, FuncCall *fn, int location);
 
 lookup_param_hook_type lookup_param_hook = NULL;
+handle_constant_literals_hook_type handle_constant_literals_hook = NULL;
 /*
  * transformExpr -
  *	  Analyze and transform expressions. Type checking and type casting is
@@ -2316,6 +2317,8 @@ transformCoalesceExpr(ParseState *pstate, CoalesceExpr *c)
 
 	if (sql_dialect == SQL_DIALECT_TSQL && select_common_type_hook && c->tsql_is_null)
 		newc->coalescetype = select_common_type(pstate, newargs, "ISNULL", NULL);
+	else if(sql_dialect == SQL_DIALECT_TSQL && select_common_type_hook)
+		newc->coalescetype = select_common_type(pstate, newargs, "TSQL_COALESCE", NULL);
 	else
 		newc->coalescetype = select_common_type(pstate, newargs, "COALESCE", NULL);
 	/* coalescecollid will be set by parse_collate.c */
@@ -2326,6 +2329,15 @@ transformCoalesceExpr(ParseState *pstate, CoalesceExpr *c)
 		Node	   *e = (Node *) lfirst(args);
 		Node	   *newe;
 
+		/*
+		 *	T-SQL treats constant string literals as VARCHAR. Hence,
+		 *	coercing into VARCHAR before coercing it to the common type.
+		 */
+		if (sql_dialect == SQL_DIALECT_TSQL && !c->tsql_is_null && handle_constant_literals_hook)
+		{
+			e = (*handle_constant_literals_hook)(pstate, e);
+		}
+
 		newe = coerce_to_common_type(pstate, e,
 									 newc->coalescetype,
 									 "COALESCE");

src/include/parser/parse_coerce.h

@@ -135,4 +135,7 @@ typedef Oid (*select_common_type_hook_type) (ParseState *pstate, List *exprs, co
 extern PGDLLEXPORT select_common_type_hook_type select_common_type_hook;
 typedef int32 (*select_common_typmod_hook_type) (ParseState *pstate, List *exprs, Oid common_type);
 
+typedef Node *(*handle_constant_literals_hook_type) (ParseState *pstate, Node *e);
+extern PGDLLEXPORT handle_constant_literals_hook_type handle_constant_literals_hook;
+
 #endif							/* PARSE_COERCE_H */