Details
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Bug
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Status: Stalled (View Workflow)
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Major
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Resolution: Unresolved
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10.1.11
Description
create table t1 (
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a int,
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filler1 char(128),
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filler2 char(128),
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key(a)
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);
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insert into t1
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select A.a+10*B.a+100*C.a, repeat('abc-',32), repeat('abc-',32)
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from ten A, ten B, ten C;
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EXPLAIN shows the correct estimate, we will read 10 rows:
explain select a from t1 order by a limit 10;
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+------+-------------+-------+-------+---------------+------+---------+------+------+-------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+------+-------------+-------+-------+---------------+------+---------+------+------+-------------+
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| 1 | SIMPLE | t1 | index | NULL | a | 5 | NULL | 10 | Using index |
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+------+-------------+-------+-------+---------------+------+---------+------+------+-------------+
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ANALYZE's rows is different from the EXPLAIN:
analyze select a from t1 order by a limit 10;
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MariaDB [j1]> analyze select a from t1 order by a limit 10;
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+------+-------------+-------+-------+---------------+------+---------+------+------+--------+----------+------------+-------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | r_rows | filtered | r_filtered | Extra |
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+------+-------------+-------+-------+---------------+------+---------+------+------+--------+----------+------------+-------------+
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| 1 | SIMPLE | t1 | index | NULL | a | 5 | NULL | 978 | 10.00 | 100.00 | 100.00 | Using index |
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+------+-------------+-------+-------+---------------+------+---------+------+------+--------+----------+------------+-------------+
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The value is wrong, it ignores the LIMIT clause.
Attachments
Issue Links
Activity
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from mariadb linked issue |
From: Jason Alpers <jasealpers@xxxxxxxxx>
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Date: Fri, 2 Apr 2021 14:04:25 -0500
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Hello,
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We're hitting the problem in MDEV-9504 (https://jira.mariadb.org/browse/MDEV-9504). I see from the commit at https://github.com/MariaDB/server/commit/07b8aefe90ca830d2de068f2966cd2288b158a88
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that this change was backed out due to test failures. I ran through all the mysql-tests with and without the change and haven't been able to pinpoint a difference though.
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The reverting commit indicates the correct value for select_limit is only computed when the query plan changes. Any chance someone could provide more detail on what needs to be changed to avoid regression with this fix?
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If there are sample queries that regress with only the original MDEV-9504 changes, that would be helpful.
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The patch caused failures in buildbot. Fixing them is not easy.
Reverting the patch.