Details
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Task
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Status: Closed (View Workflow)
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Critical
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Resolution: Fixed
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None
Description
In some cases an index on a column cannot be used by the range optimizer because of data type mismatch.
Example 1
In this example the index on the VARCHAR column cannot be used for integer lookups:
CREATE OR REPLACE TABLE t1 (a VARCHAR(10), KEY(a)); |
DELIMITER $$
|
FOR i IN 1..99 |
DO
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INSERT INTO t1 VALUES (i); |
END FOR; |
$$
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DELIMITER ;
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EXPLAIN SELECT * FROM t1 WHERE a=1; |
+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+
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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 | a | a | 13 | NULL | 99 | Using where; Using index |
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+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+
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1 row in set, 1 warning (0.000 sec)
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Example 2
In this example the index cannot be used because the field gets wrapped into a CONVERT() function during argument character set aggregation, which makes the index not applicable:
CREATE OR REPLACE TABLE t1 (a VARCHAR(10) CHARACTER SET latin1, KEY(a)); |
DELIMITER $$
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FOR i IN 1..99 |
DO
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INSERT INTO t1 VALUES (i); |
END FOR; |
$$
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DELIMITER ;
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EXPLAIN SELECT * FROM t1 WHERE a=_utf8mb3'1' COLLATE utf8mb3_bin; |
+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
|
+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+
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| 1 | SIMPLE | t1 | index | NULL | a | 13 | NULL | 99 | Using where; Using index |
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+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+
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Let's make it easier to analyze
Under terms of this tasks we'll add SQL notes to SELECT and other statements using the range optimizer:
- when an index cannot be used due to data type mismatch
- when an index cannot be used because the column get wrapped into CONVERT()
Additionally, let's print those notes in the Slow Query Log if log_slow_verbosity=query_plan,explain is set.
This will make it easier to catch and analyze sub-optimal queries.
Attachments
Issue Links
- causes
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MDEV-32388 MSAN / Valgrind errors in Item_func_like::get_mm_leaf upon query from partitioned table
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- Closed
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- Closed
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- relates to
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- Closed
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- Closed
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- Closed
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- Open
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- Open
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