MySQL5.7中如何進(jìn)行優(yōu)化union all

MySQL5.7中如何進(jìn)行優(yōu)化union all，相信很多沒有經(jīng)驗(yàn)的人對(duì)此束手無(wú)策，為此本文總結(jié)了問題出現(xiàn)的原因和解決方法，通過這篇文章希望你能解決這個(gè)問題。

 MySQL5.6中，使用union all相當(dāng)于創(chuàng)建一張臨時(shí)表，這在執(zhí)行大的聯(lián)合查詢時(shí)候會(huì)增加I/O開銷，降低查詢速度。
 例如執(zhí)行以下SQL語(yǔ)句：
 (select id from accessLog order by id) union all (select id from access_test order by id);
 在MySQL5.6環(huán)境：

點(diǎn)擊(此處)折疊或打開

mysql> select version();

| version() |

| 5.6.14-log |

1 row in set (0.00 sec)

mysql> explain (select id from accessLog order by id) union all (select id from access_test order by id);

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

| 1 | PRIMARY | accessLog | index | NULL | loginuserId | 9 | NULL | 535513 | Using index |

| 2 | UNION | access_test | index | NULL | idx_loginuid | 9 | NULL | 477248 | Using index |

| NULL | UNION RESULT | <union1,2> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |

 可以看到執(zhí)行計(jì)劃中提現(xiàn)到了創(chuàng)建的臨時(shí)表。
 在MySQL5.7環(huán)境：
點(diǎn)擊(此處)折疊或打開

mysql> select version();

| version() |

| 5.7.18-log |

1 row in set (0.00 sec)

mysql> explain (select id from accessLog order by id) union all (select id from access_test order by id);

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

| 1 | PRIMARY | accessLog | NULL | index | NULL | loginuserId | 9 | NULL | 586090 | 100.00 | Using index |

| 2 | UNION | access_test | NULL | ALL | NULL | NULL | NULL | NULL | 571023 | 100.00 | NULL |

  整個(gè)查詢過程沒有創(chuàng)建臨時(shí)表，按照順序，accessLog表的查詢結(jié)果首先傳輸?shù)娇蛻舳?，然后access_test表的查詢結(jié)果再傳輸?shù)娇蛻舳恕?br/> 注意：此項(xiàng)優(yōu)化對(duì)union和在最外層用order by無(wú)效，如下：
點(diǎn)擊(此處)折疊或打開

mysql> select version();

| version() |

| 5.7.18-log |

1 row in set (0.00 sec)

mysql> explain (select id from accessLog order by id) union all (select id from access_test order by id) order by id;

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

| 1 | PRIMARY | accessLog | NULL | index | NULL | loginuserId | 9 | NULL | 586090 | 100.00 | Using index |

| 2 | UNION | access_test | NULL | ALL | NULL | NULL | NULL | NULL | 571023 | 100.00 | NULL |

| NULL | UNION RESULT | <union1,2> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary; Using filesort |

看完上述內(nèi)容，你們掌握MySQL5.7中如何進(jìn)行優(yōu)化union all的方法了嗎？如果還想學(xué)到更多技能或想了解更多相關(guān)內(nèi)容，歡迎關(guān)注億速云行業(yè)資訊頻道，感謝各位的閱讀！