[BABEL] Resolved string literal type as sys.varchar in select_common_type for babelfish

In select_common_type() currently string literal has type unknownoid but
for babelfish, string literal should be of type sys.varchar, this is
causing incorrect results in babelfish in common type selection.
Fixed this by updating type of string literal as sys.varchar and also migrated babelfish related changes from select_common_type to babelfish pre-hook, select_common_type_hook.

Task: BABEL-6127
Signed-off-by: Rohit Bhagat <rohitbgt@amazon.com>

cr: https://code.amazon.com/reviews/CR-227519704

Author: rohit01010

src/backend/parser/parse_coerce.c

@@ -1430,7 +1430,6 @@ select_common_type(ParseState *pstate, List *exprs, const char *context,
 	TYPCATEGORY pcategory;
 	bool		pispreferred;
 	ListCell   *lc;
-	const char *dump_restore = GetConfigOption("babelfishpg_tsql.dump_restore", true, false);
 
 	if (select_common_type_hook)
 	{
@@ -1495,9 +1494,7 @@ select_common_type(ParseState *pstate, List *exprs, const char *context,
 				pcategory = ncategory;
 				pispreferred = nispreferred;
 			}
-			else if (ncategory != pcategory &&
-				sql_dialect != SQL_DIALECT_TSQL && /* T-SQL allows to select common datatype between different categories */
-				(!dump_restore || (dump_restore && strcmp(dump_restore, "on") != 0))) /* allow common datatype between different categories while restoring babelfish database */
+			else if (ncategory != pcategory)
 			{
 				/*
 				 * both types in different categories? then not much hope...
@@ -1527,55 +1524,9 @@ select_common_type(ParseState *pstate, List *exprs, const char *context,
 				pcategory = ncategory;
 				pispreferred = nispreferred;
 			}
-			else if ((sql_dialect == SQL_DIALECT_TSQL ||
-					 (dump_restore && strcmp(dump_restore, "on") == 0)) &&
-					 determine_datatype_precedence_hook != NULL &&
-					 !pispreferred &&
-					 can_coerce_type(1, &ptype, &ntype, COERCION_IMPLICIT) &&
-					 can_coerce_type(1, &ntype, &ptype, COERCION_IMPLICIT) &&
-					 determine_datatype_precedence_hook(ntype, ptype))
-			{
-				/*
-				 * T-SQL allows implicit casting on both-side.
-				 * common datatype should be decided by datatype precedence rule.
-				 */
-				pexpr = nexpr;
-				ptype = ntype;
-				pcategory = ncategory;
-				pispreferred = nispreferred;
-			}
-		} else if (sql_dialect == SQL_DIALECT_TSQL && ntype == ptype)
-		{
-			/*
-			 * For the columns which have the same base type, we choose the
-			 * expression with higher precedence type in T-SQL.
-			 * For example, smallmoney UNION money, the base type of
-			 * them are both fixeddecimal. But we shouldn't use smallmoney as
-			 * the result type, it could loss precision.
-			 * Here we don't need to update other variables since they are the
-			 * same.
-			 */
-			if (is_tsql_base_datatype_hook &&
-				(*is_tsql_base_datatype_hook)(exprType(nexpr)) &&
-				determine_datatype_precedence_hook &&
-				determine_datatype_precedence_hook(exprType(nexpr),
-												   exprType(pexpr)))
-				pexpr = nexpr;
 		}
 	}
 
-	/*
-	 * If the preferred type is not the result type of corresponding
-	 * expression, it means the result type is a domain type in Postgres. Some
-	 * base data types in T-SQL are implemented as domain types in Babelfish.
-	 * From SQL Server's perspective, we should try to retain those types as
-	 * result types.
-	 */
-	if (sql_dialect == SQL_DIALECT_TSQL && ptype != exprType(pexpr) &&
-		is_tsql_base_datatype_hook &&
-		(*is_tsql_base_datatype_hook)(exprType(pexpr)))
-		ptype = exprType(pexpr);
-
 	/*
 	 * If all the inputs were UNKNOWN type --- ie, unknown-type literals ---
 	 * then resolve as type TEXT.  This situation comes up with constructs
@@ -1587,23 +1538,6 @@ select_common_type(ParseState *pstate, List *exprs, const char *context,
 	 * literals while they are still literals, so a decision has to be made
 	 * now.
 	 */
-	if (ptype == UNKNOWNOID && sql_dialect == SQL_DIALECT_TSQL)
-	{
-		bool all_nullconst = true;
-		foreach(lc, exprs)
-		{
-			Node* expr = (Node *) lfirst(lc);
-			if (exprType(expr) != UNKNOWNOID || !IsA(expr, Const) || !((Const *) expr)->constisnull)
-			{
-				all_nullconst = false;
-				break;
-			}
-		}
-
-		if (all_nullconst)
-			ptype = INT4OID; /* in T-SQL, it is should be decided to INT4 */
-	}
-
 	if (ptype == UNKNOWNOID)
 		ptype = TEXTOID;