#25377: COUNT(*) not COUNT('*')
----------------------------------------------+--------------------
Reporter: adamchainz | Owner: nobody
Type: Cleanup/optimization | Status: new
Component: Database layer (models, ORM) | Version: 1.8
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 1
Easy pickings: 0 | UI/UX: 0
----------------------------------------------+--------------------
Performance regression on 1.8 expression refactor. Count stopped
outputting `COUNT(*)` and started outputting `COUNT('*')` due to using a
`Value`. Possibly unnoticed until now because it doesn't affect the most
popular database options much - I can't measure a performance drop for
this on MySQL / InnoDb.
However I have managed to measure a performance drop on MariaDB + Aria
(=MySQL fork + MyISAM fork):
{{{
adamj@localhost [3]> select count(*) from count_test;
+----------+
| count(*) |
+----------+
| 10000 |
+----------+
1 row in set (0.00 sec)
adamj@localhost [10]> select benchmark(100 * 1000 * 1000, (select count(*)
from count_test));
+-----------------------------------------------------------------+
| benchmark(100 * 1000 * 1000, (select count(*) from count_test)) |
+-----------------------------------------------------------------+
| 0 |
+-----------------------------------------------------------------+
1 row in set (0.86 sec)
adamj@localhost [11]> select benchmark(100 * 1000 * 1000, (select
count('*') from count_test));
+-------------------------------------------------------------------+
| benchmark(100 * 1000 * 1000, (select count('*') from count_test)) |
+-------------------------------------------------------------------+
| 0 |
+-------------------------------------------------------------------+
1 row in set (1.23 sec)
}}}
This is because MyISAM / Aria store the count in a metadata variable but
once you introduce the expression it figures it has to do a table scan.
The situation would of course only get worse with more rows, 10000 is
tiny.
--
Ticket URL: <https://code.djangoproject.com/ticket/25377>
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