Plan selection takes 2+ times longer than before MDEV-28852

The test uses all default server startup options, loads original data set from MDEV-28073 (xpo_full_testcase.sql), runs ANALYZE PERSISTENT FOR ALL on all tables, and executes EXPLAIN for a query structured identically to MDEV-28073 (minus a couple of last JOINs), but the tables and fields in the query are mixed up – that is, different tables and fields from the dataset appear in different JOIN positions and conditions. The exact query will be provided.

The query was executed with EXPLAIN FORMAT=JSON on several different versions/builds for comparison. All builds are non-debug.

MariaDB [test]> EXPLAIN FORMAT=JSON SELECT * FROM (((((((((((((((( 

...

1 row in set (25.947 sec)

MariaDB [test]> EXPLAIN FORMAT=JSON SELECT * FROM (((((((((((((((( 

...

1 row in set (9.275 sec)

MariaDB [test]> EXPLAIN FORMAT=JSON SELECT * FROM (((((((((((((((( 

...

1 row in set (2 min 51.670 sec)

MariaDB [test]> EXPLAIN FORMAT=JSON SELECT * FROM (((((((((((((((( 

...

1 row in set (1 min 2.267 sec)

With optimizer_prune_level=2 (default) and optimizer_prune_level=1 the time on the above build is approximately the same.

So, the fastest is current 10.6 main (maybe it has a fix which isn't yet in 10.10 main). But even 10.10 main is over 2 times faster than preview-10.10-optimizer.

For an additional reference (since preview-10.10-optimizer contains two new features),

MariaDB [test]> EXPLAIN FORMAT=JSON SELECT * FROM (((((((((((((((( 

...

1 row in set (28.092 sec)

This ^ is a build with MDEV-26278, but without MDEV-28852 – that is, a baseline for MDEV-28852. It takes roughly the same time as 10.10 main, so MDEV-26278 isn't a culprit.