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87bf8fb9e9
During v6 indexing changes, we replaced
idx_posts_root_id with idx_posts_root_id_delete_at.
This causes MySQL to trigger the index_merge path
again with PRIMARY and idx_posts_root_id_delete_at
as shown below:
```
mysql> UPDATE Posts SET DeleteAt = 1637998911685, UpdateAt = 1637998911685, Props = JSON_SET(Props, '$.deleteBy', 'buqskqrwmjnhfuqskqrwmjn4ca') Where Id = 'q38uaydtpink5f4wkmcsn8h47o' OR RootId = 'q38uaydtpink5f4wkmcsn8h47o';
Query OK, 9 rows affected (17.29 sec)
Rows matched: 10 Changed: 9 Warnings: 0
mysql> EXPLAIN UPDATE Posts SET DeleteAt = 1637998911685, UpdateAt = 1637998911685, Props = JSON_SET(Props, '$.deleteBy', 'buqskqrwmjnhfuqskqrwmjn4ca') Where Id = 'q38uaydtpink5f4wkmcsn8h47o' OR RootId = 'q38uaydtpink5f4wkmcsn8h47o'\G
*************************** 1. row ***************************
id: 1
select_type: UPDATE
table: Posts
partitions: NULL
type: index_merge
possible_keys: PRIMARY,idx_posts_root_id_delete_at
key: idx_posts_root_id_delete_at,PRIMARY
key_len: 107,106
ref: NULL
rows: 9
filtered: 100.00
Extra: Using sort_union(idx_posts_root_id_delete_at,PRIMARY); Using where; Using temporary
1 row in set, 1 warning (0.00 sec)
```
To fix the temporary sort, we order by Id
```
mysql> UPDATE Posts SET DeleteAt = 1637998911686, UpdateAt = 1637998911686, Props = JSON_SET(Props, '$.deleteBy', 'buqskqrwmjnhfuqskqrwmjn4ca') Where Id = 'q38uaydtpink5f4wkmcsn8h47o' OR RootId = 'q38uaydtpink5f4wkmcsn8h47o' ORDER BY Id;
Query OK, 9 rows affected (0.01 sec)
Rows matched: 9 Changed: 9 Warnings: 0
mysql> EXPLAIN UPDATE Posts SET DeleteAt = 1637998911686, UpdateAt = 1637998911686, Props = JSON_SET(Props, '$.deleteBy', 'buqskqrwmjnhfuqskqrwmjn4ca') Where Id = 'q38uaydtpink5f4wkmcsn8h47o' OR RootId = 'q38uaydtpink5f4wkmcsn8h47o' ORDER BY Id\G
*************************** 1. row ***************************
id: 1
select_type: UPDATE
table: Posts
partitions: NULL
type: index_merge
possible_keys: PRIMARY,idx_posts_root_id_delete_at
key: idx_posts_root_id_delete_at,PRIMARY
key_len: 107,106
ref: NULL
rows: 9
filtered: 100.00
Extra: Using sort_union(idx_posts_root_id_delete_at,PRIMARY); Using where; Using filesort
1 row in set, 1 warning (0.00 sec)
```
Postgres uses Bitmap Heap scan which does
a Bitmap OR of the index tuples and _then_
fetches the rows from the heap. This is a
much better and sophisticated way. Sadly, MySQL
will fetch the rows from the indexes first,
and then do an OR, which is why index_merge_intersection
is so bad.
See: https://developers.mattermost.com/blog/mysql-index-merge/
for more info.
```release-note
NONE
```
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