Updated code to assign stmt conditionally, free up memory context, added

comments and code refactoring

Signed-off-by: Shard Gupta <shardga@amazon.com>

Author: shardgupta

contrib/babelfishpg_tsql/src/cursor.c

@@ -1450,20 +1450,9 @@ execute_sp_cursoropen_common(int *stmt_handle, int *cursor_handle, const char *s
 	MemoryContext oldcontext;
 	MemoryContext savedPortalCxt;
 	PLtsql_stmt_execsql *parse_result;
-	PLtsql_function *function;
+	PLtsql_function *func;
 	char *stmt_copy;
 
-	/*
-	 * Parse the function's text
-	 */
-	if(stmt)
-	{
-		stmt_copy = pstrdup(stmt);
-		function = pltsql_compile_inline(stmt_copy, NULL);
-		parse_result = (PLtsql_stmt_execsql *) lsecond(function->action->body);
-		stmt = parse_result->sqlstmt->query;
-	}
-
 	/*
 	 * Connect to SPI manager. should be handled in the same way with
 	 * pltsql_inline_handler()
@@ -1482,6 +1471,47 @@ execute_sp_cursoropen_common(int *stmt_handle, int *cursor_handle, const char *s
 
 	if (prepare)
 	{
+		/*
+		 * This entire block is to parse the statement by antlr and use the resultant
+		 * statement to sent to Postgres cursor execution. This is necessary in some use case,
+		 * for example when we have PostgreSQL reversed keywords in query which is valid in TSQL.
+		 * Antlr parser will add the quotes at necessary places in query so that Postgres engine
+		 * can resolve this query correctly.
+		 */
+	    if (stmt)
+		{
+			// Copy the original statement, because we don't want to use the resultant query in every case.
+			stmt_copy = pstrdup(stmt);
+			// Send to antlr parser
+		    func = pltsql_compile_inline(stmt_copy, NULL);
+			// Increase function use count, so that we don't deallocate function memory accidently
+			func->use_count++;
+
+			/*
+			 * Check the node list of type PLtsql_stmt_type. Cursor only support single statement.
+			 * If there are more than 1 statement we will through the error. func->action-body
+			 * returned by pltsql_compile_inline contains two default nodes, PLTSQL_STMT_INIT being first
+			 * and PLTSQL_STMT_RETURN being last. So total number of nodes should be 3 for cursor. Actual
+			 * query statement will be at second position of type PLTSQL_STMT_EXECSQL.
+
+			 * This is defensive code, where we only reassign the stmt variable to parsed query,
+			 * if the cmd_type is PLTSQL_STMT_EXECSQL. There might be other types of cmd_type like
+			 * PLTSQL_STMT_EXECSQL (for procedures), for them we will keep the old behavior.
+			 */
+		    if((( (PLtsql_stmt *) lsecond(func->action->body))->cmd_type ==
+				PLTSQL_STMT_EXECSQL) && list_length(func->action->body) == 3)
+			{
+		    	parse_result = (PLtsql_stmt_execsql *) lsecond(func->action->body);
+		    	stmt = pstrdup(parse_result->sqlstmt->query);
+			}
+
+			//Function is not need anymore, so decrease the usage count and free function memory.
+			func->use_count--;
+			Assert(func->use_count == 0);
+			pltsql_free_function_memory(func);
+		}
+
+
 		/* prepare plan and insert a cursor entry */
 		plan = SPI_prepare_cursor(stmt, nBindParams, boundParamsOidList, cursor_options);
 		if (plan == NULL)