为什么 MySQL 8.0 比 5.7 慢 1000 倍以上来执行这个 SELECT 查询

Question

I'm able to have both versions 5.7 and 8.0 of MySQL running on the same hardware (Windows 10 with NVMe SSD). The following query is over 1000 times faster to execute under MySQL 5.7 than MySQL 8.0:

SELECT `oacode`,ST_asWKB(`shape`) as `shape` FROM 
    (SELECT * FROM oa_bdy WHERE seatname='Barnsley Central') AS `vtable`
    WHERE MBRIntersects(`shape`,ST_GeomFromText('POLYGON ((426519 410113, 426519 4156675,
    432073 415667, 432073 410113, 426519 410113))', 27700));

It takes 0.016 secs to execute on MySQL 5.7, but 19.6 secs on MySQL 8.0.

The tables use MyISAM, and the oa_bdy table has 232,296 rows, but only 290 of those match the inner WHERE. One difference is that 'EXPLAIN' on the query produces different results from the two versions.

MySQL	id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
v5.7	1	SIMPLE	oa_bdy	(NULL)	range	shape	shape	34	(NULL)	1406	10.00	Using where
v8.0	1	SIMPLE	oa_bdy	(NULL)	ALL	(NULL)	(NULL)	(NULL)	(NULL)	232296	10.00	Using where

I don't know enough about the MySQL internals to resolve this, but the performance difference is quite significant and detrimental.

Does anyone have any suggestions?

Thanks.

Update (7-Feb-2021). Thanks for the comment about spatial index. The schema for both versions was the same (one was a copy of the other) and the schema does contain a spatial index.

CREATE TABLE `oa_bdy` (
   `shape` geometry NOT NULL,
   `oacode` varchar(9) NOT NULL,
   `seatname` varchar(43) DEFAULT NULL,
   PRIMARY KEY (`oacode`),
   SPATIAL KEY `shape` (`shape`)
 ) ENGINE=MyISAM DEFAULT CHARSET=utf8

One problem seems to be that there is a new behaviour in MySQL 8 which will ignore the spatial index (for query optimization purposes) unless the column has a defined SRID (rather than just the individual cells). When the schema under 8.0 is changed to

CREATE TABLE `oa_bdy` (
   `shape` geometry NOT NULL SRID 27700,
   `oacode` varchar(9) NOT NULL,
   `seatname` varchar(43) DEFAULT NULL,
   PRIMARY KEY (`oacode`),
   SPATIAL KEY `shape` (`shape`)
 ) ENGINE=MyISAM DEFAULT CHARSET=utf8

then the EXPLAIN under 8.0 matches that of 5.7, and the run-time under 8.0 drops to 0.391 secs, which is still 24 times slower than MySQL 5.7.

Is there any way of getting the run-times under MySQL 8.0 to be similar to those of MySQL 5.7 for this sort of query?

Answer 1

You definitely have an index on your older database that isn't there on the newer one. Due to the lack of index on the new database, the only way MySQL can satisfy your query is to compare every row in the table to your search criterion. With the right index, the number of rows needing comparing is much smaller.

Do SHOW CREATE TABLE oa_bdy on both 5.7 and 8.0, and compare them. The older one will surely mention one or more indexes.

You can create an index to help with the query you showed us by doing this....

CREATE SPATIAL INDEX x_oa_bdy_shape ON oa_bdy(shape);