1berry/MariaDB-server
1
0
Fork 0
Code Issues Packages Projects Releases Wiki Activity

MDEV-31479: Inconsistency between MRR and SQL layer costs can cause poor query plan

Browse Source 
(Same as
TODO-3938: best_access_path shows negative costs for mrr=on)

best_access_path() assumes that quick select cost includes
(quick->rows/TIME_FOR_COMPARE) as a cost of checking the attached
part of the WHERE condition.

It calls adjust_quick_cost() to subtract addition from quick's cost.

The problem was that DS-MRR cost formula didn't include this cost.
For very large tables, adjust_quick_cost() would produce a negative
cost which would cause assert in debug build or bad query plan choice
in release builds.

Approved-by: Monty <monty@mariadb.org>
This commit is contained in:
Sergei Petrunia 2023-06-14 13:55:35 +03:00
parent 841e905f20
commit 0e2e70c4c1
1 changed files with 3 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
sql/multi_range_read.cc
View File

@ -2006,6 +2006,9 @@ bool DsMrr_impl::get_disk_sweep_mrr_cost(uint keynr, ha_rows rows, uint flags,
/* Total cost of all index accesses */
index_read_cost= primary_file->keyread_time(keynr, 1, rows);
cost->add_io(index_read_cost, 1 /* Random seeks */);
cost->cpu_cost+= (rows2double(rows) / TIME_FOR_COMPARE +
MULTI_RANGE_READ_SETUP_COST);
return FALSE;
}