Details
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Bug
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Status: Open (View Workflow)
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Major
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Resolution: Unresolved
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10.1, 10.2, 10.3, 10.4, 10.5
Description
Consider an example:
create table i ( |
id int, |
sku int, |
col1 int, |
key(id) |
);
|
insert into i select seq, seq, seq from seq_1_to_10000; |
create table s ( |
sku int, |
col1 int, |
key(sku) |
);
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insert into s select seq, seq from seq_1_to_20000; |
explain
|
SELECT |
DISTINCT i.col1 |
FROM i JOIN s |
WHERE |
i.sku = s.sku
|
ORDER BY |
i.id DESC LIMIT 1; |
The query plan for this is to use the index over (i.id) to satisfy the ORDER BY ... LIMIT (and shortcut the execution early):
+------+-------------+-------+-------+---------------+------+---------+----------+------+------------------------------+
|
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
|
+------+-------------+-------+-------+---------------+------+---------+----------+------+------------------------------+
|
| 1 | SIMPLE | i | index | NULL | id | 5 | NULL | 1 | Using where; Using temporary |
|
| 1 | SIMPLE | s | ref | sku | sku | 5 | j5.i.sku | 1 | Using index; Distinct |
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+------+-------------+-------+-------+---------------+------+---------+----------+------+------------------------------+
|
The problem is, handling DISTINCT actually prevents the short-cutting.
MariaDB's duplicate-removal algorithm enumerates the whole join output.
analyze format=json
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SELECT DISTINCT i.col1 FROM i JOIN s WHERE i.sku = s.sku
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ORDER BY i.id DESC LIMIT 1;
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ANALYZE FORMAT=JSON shows:
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{
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"query_block": {
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"select_id": 1,
|
"r_loops": 1,
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"r_total_time_ms": 650.5171852,
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"temporary_table": {
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"table": {
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"table_name": "i",
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"access_type": "index",
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"key": "id",
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"key_length": "5",
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"used_key_parts": ["id"],
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"r_loops": 1,
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"rows": 10266,
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"r_rows": 10000,
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"r_table_time_ms": 399.2319244,
|
"r_other_time_ms": 10.25677143,
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"filtered": 100,
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"r_filtered": 100,
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"attached_condition": "i.sku is not null"
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},
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"table": {
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"table_name": "s",
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"access_type": "ref",
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"possible_keys": ["sku"],
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"key": "sku",
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"key_length": "5",
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"used_key_parts": ["sku"],
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"ref": ["j5.i.sku"],
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"r_loops": 10000,
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"rows": 1,
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"r_rows": 1,
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"r_table_time_ms": 227.2785918,
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"r_other_time_ms": 13.72163759,
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"filtered": 100,
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"r_filtered": 100,
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"using_index": true,
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"distinct": true
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}
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}
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}
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}
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Note the "r_rows": 10000. It has computed the whole join output.
Let's run the same query without DISTINCT:
analyze format=json
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SELECT i.col1 FROM i JOIN s WHERE i.sku = s.sku
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ORDER BY i.id DESC LIMIT 1;
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{
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"query_block": {
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"select_id": 1,
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"r_loops": 1,
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"r_total_time_ms": 0.626986227,
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"table": {
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"table_name": "i",
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"access_type": "index",
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"key": "id",
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"key_length": "5",
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"used_key_parts": ["id"],
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"r_loops": 1,
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"rows": 10266,
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"r_rows": 1,
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"r_table_time_ms": 0.314612616,
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"r_other_time_ms": 0.023000019,
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"filtered": 100,
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"r_filtered": 100,
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"attached_condition": "i.sku is not null"
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},
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"table": {
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"table_name": "s",
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"access_type": "ref",
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"possible_keys": ["sku"],
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"key": "sku",
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"key_length": "5",
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"used_key_parts": ["sku"],
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"ref": ["j5.i.sku"],
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"r_loops": 1,
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"rows": 1,
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"r_rows": 1,
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"r_table_time_ms": 0.136636567,
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"r_other_time_ms": 0.125355838,
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"filtered": 100,
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"r_filtered": 100,
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"using_index": true
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}
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}
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}
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