Bug #31228694 FTS QUERY WITH LIMIT HIT AN ASSERT

Problem:
1. The server terminates abnormally when phrase search doesn't
filter out doc_ids correctly. This problem has been fixed in bug
2. Wrong query result: It's a regression from the bug #22709692 fix.
This fix optimize full-text search query with limit clause.
when FTS expression involves only union operation, we fetch only
number of doc_ids specified with the limit clause.
Fulltext phrase search is not an union operation and we consider
phrase search with plugin parser a union operation.
In phrase search with limit clause, we fetch limited doc_ids for
each token and if any of the selected doc_id does not contain all
tokens in correct order then we do not include that row_id in the
result set.
Therefore phrase search gets fewer number of rows than the qualified
rows exist in the table.

Fix:
Added a condition that phrase search with plugin parser is not a
union operation.

RB: 24925
Reviewed by : Annamalai Gurusami <annamalai.gurusami@oracle.com>

This is a cherry-pick of
mysql/mysql-server@5549920
without a test case, because the test case depends on an n-gram
tokenizer that will be missing from MariaDB until MDEV-10267 is added.

storage/innobase/fts/fts0ast.cc

@@ -1,6 +1,6 @@
 /*****************************************************************************
 
-Copyright (c) 2007, 2018, Oracle and/or its affiliates. All Rights Reserved.
+Copyright (c) 2007, 2020, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2018, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
@@ -557,15 +557,17 @@ fts_ast_node_check_union(
 	fts_ast_node_t*	node)
 {
 	if (node->type == FTS_AST_LIST
-	    || node->type == FTS_AST_SUBEXP_LIST
-	    || node->type == FTS_AST_PARSER_PHRASE_LIST) {
+	    || node->type == FTS_AST_SUBEXP_LIST) {
 
 		for (node = node->list.head; node; node = node->next) {
 			if (!fts_ast_node_check_union(node)) {
 				return(false);
 			}
 		}
 
+	} else if (node->type == FTS_AST_PARSER_PHRASE_LIST) {
+		/* Phrase search for plugin parser */
+		return(false);
 	} else if (node->type == FTS_AST_OPER
 		   && (node->oper == FTS_IGNORE
 		       || node->oper == FTS_EXIST)) {