added new grammer, parse node and hook for top N Percent (#624)

Description
This PR implements comprehensive support for SELECT TOP N PERCENT syntax in Babelfish, addressing JIRA-BABEL-1358. The implementation adds percentage-based row limiting functionality to align with SQL Server behavior.

Problem
Babelfish previously only supported SELECT TOP N (fixed count) but lacked support for SELECT TOP N PERCENT (percentage-based limiting), which is a commonly used SQL Server feature for selecting a percentage of rows from result sets.

Related PR -
Extension Link - babelfish-for-postgresql/babelfish_extensions#4111

Issue: BABEL-1358
Signed-off-by: Herambh Shah <herambhs@amazon.com>

Author: herambh-shah

src/backend/parser/analyze.c

@@ -92,8 +92,9 @@ pre_transform_setop_sort_clause_hook_type pre_transform_setop_sort_clause_hook =
 /* Hook to transform TSQL pivot clause in select stmt */
 transform_pivot_clause_hook_type transform_pivot_clause_hook = NULL;
 
-/* Hook to transform TSQL unpivot clauses in select stmt */
-transform_unpivot_clause_hook_type transform_unpivot_clause_hook = NULL;
+/* Hook to transform TSQL unpivot, top N percent clauses in select stmt */
+transform_tsql_select_stmt_hook_type transform_tsql_select_stmt_hook = NULL;
+
 
 static Query *transformOptionalSelectInto(ParseState *pstate, Node *parseTree);
 static Query *transformDeleteStmt(ParseState *pstate, DeleteStmt *stmt);
@@ -1442,10 +1443,10 @@ transformSelectStmt(ParseState *pstate, SelectStmt *stmt)
 
 	qry->commandType = CMD_SELECT;
 
-	/* Unpack and process TSQL UNPIVOT nodes in stmt->fromClause, if present */
-	if(transform_unpivot_clause_hook)
+	/* Unpack and process TSQL UNPIVOT nodes in stmt->fromClause, Process Top N Percent, if present */
+	if(transform_tsql_select_stmt_hook)
 	{
-		(*transform_unpivot_clause_hook)(pstate, stmt);
+		(*transform_tsql_select_stmt_hook)(pstate, stmt);
 	}
 
 	/* process the WITH clause independently of all else */

src/include/nodes/nodes.h

@@ -441,6 +441,7 @@ typedef enum LimitOption
 	LIMIT_OPTION_COUNT,			/* FETCH FIRST... ONLY */
 	LIMIT_OPTION_WITH_TIES,		/* FETCH FIRST... WITH TIES */
 	LIMIT_OPTION_DEFAULT,		/* No limit present */
+	LIMIT_OPTION_PERCENT,		/* FETCH FIRST... PERCENT FOR TSQL */
 } LimitOption;
 
 #endif							/* NODES_H */

src/include/parser/analyze.h

@@ -69,8 +69,9 @@ typedef void (*transform_pivot_clause_hook_type)(ParseState *pstate, SelectStmt
 extern PGDLLEXPORT transform_pivot_clause_hook_type transform_pivot_clause_hook;
 
 /* Hook for transform unpivot clause in tsql select stmt */
-typedef void (*transform_unpivot_clause_hook_type)(ParseState *pstate, SelectStmt *stmt);
-extern PGDLLEXPORT transform_unpivot_clause_hook_type transform_unpivot_clause_hook;
+typedef void (*transform_tsql_select_stmt_hook_type)(ParseState *pstate, SelectStmt *stmt);
+extern PGDLLEXPORT transform_tsql_select_stmt_hook_type transform_tsql_select_stmt_hook;
+
 
 extern Query *parse_analyze_fixedparams(RawStmt *parseTree, const char *sourceText,
 										const Oid *paramTypes, int numParams, QueryEnvironment *queryEnv);