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作者微信:gp_22389860

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btr_estimate_n_rows_in_range_low 函數(shù)用于非唯一訪問(wèn)的行數(shù)評(píng)估也就是在執(zhí)行計(jì)劃生成階段會(huì)去訪問(wèn)實(shí)際的數(shù)據(jù)，盡可能的得到正確的執(zhí)行計(jì)劃，這個(gè)過(guò)程叫做dive。其和參數(shù)eq_range_index_dive_limit有關(guān)，如果參數(shù)設(shè)置1那么將禁用dive功能，使用統(tǒng)計(jì)數(shù)據(jù)進(jìn)行執(zhí)行計(jì)劃的生成。但某些時(shí)候MySQL 5.7根據(jù)統(tǒng)計(jì)數(shù)據(jù)并不能得到正確的執(zhí)行計(jì)劃，因此才有了dive，但是dive會(huì)產(chǎn)生額外的代價(jià)，這個(gè)過(guò)程是需要實(shí)際訪問(wèn)數(shù)據(jù)塊的。

常用的比如非唯一索引的數(shù)據(jù)訪問(wèn)，還有> set eq_range_index_dive_limit=100;

Query OK, 0 rows affected (0.00 sec)

mysql> desc select * from testdvi3 where sex='M';

+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------------+

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------------+

| 1 | SIMPLE | testdvi3 | NULL | ALL | sex | NULL | NULL | NULL | 30 | 96.67 | Using where |

+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------------+

1 row in set, 1 warning (2.74 sec)

mysql> desc select * from testdvi3 where sex='W';

+----+-------------+----------+------------+------+---------------+------+---------+-------+------+----------+-------+

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

+----+-------------+----------+------------+------+---------------+------+---------+-------+------+----------+-------+

| 1 | SIMPLE | testdvi3 | NULL | ref | sex | sex | 9 | const | 1 | 100.00 | NULL |

+----+-------------+----------+------------+------+---------------+------+---------+-------+------+----------+-------+

1 row in set, 1 warning (2.00 sec)

mysql> set eq_range_index_dive_limit=1;

Query OK, 0 rows affected (0.00 sec)

mysql> desc select * from testdvi3 where sex='W';

+----+-------------+----------+------------+------+---------------+------+---------+-------+------+----------+-------+

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

+----+-------------+----------+------------+------+---------------+------+---------+-------+------+----------+-------+

| 1 | SIMPLE | testdvi3 | NULL | ref | sex | sex | 9 | const | 15 | 100.00 | NULL |

+----+-------------+----------+------------+------+---------------+------+---------+-------+------+----------+-------+

1 row in set, 1 warning (0.00 sec)

mysql> desc select * from testdvi3 where sex='M';

+----+-------------+----------+------------+------+---------------+------+---------+-------+------+----------+-------+

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

+----+-------------+----------+------------+------+---------------+------+---------+-------+------+----------+-------+

| 1 | SIMPLE | testdvi3 | NULL | ref | sex | sex | 9 | const | 15 | 100.00 | NULL |

+----+-------------+----------+------------+------+---------------+------+---------+-------+------+----------+-------+

```

來(lái)自 “ ITPUB博客 ” ，鏈接：http://blog.itpub.net/7728585/viewspace-2707569/，如需轉(zhuǎn)載，請(qǐng)注明出處，否則將追究法律責(zé)任。

超強(qiáng)干貨來(lái)襲 云風(fēng)專訪：近40年碼齡，通宵達(dá)旦的技術(shù)人生

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