MDEV-35183 ADD FULLTEXT INDEX unnecessarily DROPS FTS COMMON TABLES

- InnoDB fulltext rebuilds the FTS COMMON table while adding the
new fulltext index. This can be optimized by avoiding rebuilding
the FTS COMMON table in case of FTS COMMON TABLE already exists.

Reviewed-by: Marko Mäkelä <marko.makela@mariadb.com>

mysql-test/suite/innodb_fts/r/fulltext.result

@@ -784,3 +784,28 @@ f1	MATCH(f1) AGAINST ("test" IN BOOLEAN MODE)
 test	0.000000001885928302414186
 DROP TABLE t1;
 # End of 10.3 tests
+#
+#  MDEV-35183 ADD FULLTEXT INDEX unnecessarily DROPS FTS
+#		COMMON TABLES
+#
+CREATE TABLE t1 (
+ID INT UNSIGNED AUTO_INCREMENT NOT NULL PRIMARY KEY,
+title VARCHAR(200), book VARCHAR(200),
+FULLTEXT fidx(title)) ENGINE = InnoDB;
+INSERT INTO t1(title) VALUES('database');
+ALTER TABLE t1 DROP INDEX fidx;
+select space into @common_space from information_schema.innodb_sys_tables where name like "test/FTS_%_CONFIG";
+ALTER TABLE t1 ADD FULLTEXT fidx_1(book);
+select space=@common_space from information_schema.innodb_sys_tables where name like "test/FTS_%_CONFIG";
+space=@common_space
+1
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `ID` int(10) unsigned NOT NULL AUTO_INCREMENT,
+  `title` varchar(200) DEFAULT NULL,
+  `book` varchar(200) DEFAULT NULL,
+  PRIMARY KEY (`ID`),
+  FULLTEXT KEY `fidx_1` (`book`)
+) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=latin1 COLLATE=latin1_swedish_ci
+DROP TABLE t1;

mysql-test/suite/innodb_fts/t/fulltext.test

@@ -803,3 +803,19 @@ SELECT f1, MATCH(f1) AGAINST ("test" IN BOOLEAN MODE) FROM t1;
 DROP TABLE t1;
 
 --echo # End of 10.3 tests
+
+--echo #
+--echo #  MDEV-35183 ADD FULLTEXT INDEX unnecessarily DROPS FTS
+--echo #		COMMON TABLES
+--echo #
+CREATE TABLE t1 (
+        ID INT UNSIGNED AUTO_INCREMENT NOT NULL PRIMARY KEY,
+        title VARCHAR(200), book VARCHAR(200),
+        FULLTEXT fidx(title)) ENGINE = InnoDB;
+INSERT INTO t1(title) VALUES('database');
+ALTER TABLE t1 DROP INDEX fidx;
+select space into @common_space from information_schema.innodb_sys_tables where name like "test/FTS_%_CONFIG";
+ALTER TABLE t1 ADD FULLTEXT fidx_1(book);
+select space=@common_space from information_schema.innodb_sys_tables where name like "test/FTS_%_CONFIG";
+SHOW CREATE TABLE t1;
+DROP TABLE t1;

storage/innobase/handler/handler0alter.cc

@@ -6293,6 +6293,7 @@ prepare_inplace_alter_table_dict(
 	dberr_t			error;
 	ulint			num_fts_index;
 	dict_add_v_col_t*	add_v = NULL;
+	bool			fts_instance_created = false;
 	ha_innobase_inplace_ctx*ctx;
 
 	DBUG_ENTER("prepare_inplace_alter_table_dict");
@@ -6539,6 +6540,7 @@ new_clustered_failed:
 			new_table_name, NULL, n_cols + n_v_cols, n_v_cols,
 			flags, flags2);
 
+		fts_instance_created = (ctx->new_table->fts != nullptr);
 		/* The rebuilt indexed_table will use the renamed
 		column names. */
 		ctx->col_names = NULL;
@@ -6732,6 +6734,7 @@ wrong_column_name:
 			ctx->new_table->fts = fts_create(
 				ctx->new_table);
 			ctx->new_table->fts->doc_col = fts_doc_id_col;
+			fts_instance_created = true;
 		}
 
 		/* Check if we need to update mtypes of legacy GIS columns.
@@ -7228,8 +7231,11 @@ op_ok:
 		ctx->trx->commit();
 		trx_start_for_ddl(ctx->trx, op);
 
-		if (!ctx->new_table->fts
+		/* If table rebuild happens or fulltext index are
-		    || ib_vector_size(ctx->new_table->fts->indexes) == 0) {
+		being added when common tables doesn't exist then
+		do create new fulltext common tables else use the
+		existing fulltext common tables */
+		if (fts_instance_created) {
 			error = fts_create_common_tables(
 				ctx->trx, ctx->new_table, true);
 
@@ -7240,6 +7246,9 @@ op_ok:
 			if (error != DB_SUCCESS) {
 				goto error_handling;
 			}
+		}
+
+		if (!ib_vector_size(ctx->new_table->fts->indexes)) {
 
 			ctx->new_table->fts->dict_locked = true;