優(yōu)化DBLE獨(dú)立子查詢教程
01問(wèn)題
前期開(kāi)發(fā)反饋在使用獨(dú)立子查詢時(shí)，不論子查詢中結(jié)果集有幾個(gè)，語(yǔ)句都會(huì)卡死遲遲得不到返回結(jié)果。但是如果去掉子查詢，直接賦值查詢很快得到返回結(jié)果。聽(tīng)到這個(gè)情況第一反應(yīng)很可能DBLE在獨(dú)立子查詢上沒(méi)有做相關(guān)優(yōu)化，真的是這樣么？下面我們將問(wèn)題復(fù)現(xiàn)以及優(yōu)化方式進(jìn)行展示。
02 演示及優(yōu)化
環(huán)境檢查
DBLE版本：2.19.11.5
MySQL版本：5.7.28
涉及分片表：src_biz_filelist
分片鍵：batch_no
涉及垂直表：src_image_txn_jnl
分片拆分規(guī)則：stringhash
節(jié)點(diǎn)數(shù)量：4
2.1 原始語(yǔ)句及執(zhí)行計(jì)劃
獨(dú)立子查詢語(yǔ)句如下：
select caption,batch_no,image_no from src_biz_filelist where batch_no in (SELECT imageid FROM src_image_txn_jnl WHERE front_seq_num = '001780568097' and txn_dt = '2029-08-20')
執(zhí)行計(jì)劃如下：
mysql> explain select caption,batch_no,image_no from src_biz_filelist where batch_no in (SELECT imageid FROM src_image_txn_jnl WHERE front_seq_num = '001780568097' and txn_dt = '2029-08-20');
+-------------------+-----------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| DATA_NODE | TYPE | SQL/REF |
+-------------------+-----------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| dn1_0 | BASE SQL | select `src_biz_filelist`.`ysyid`,`src_biz_filelist`.`batch_no`,`src_biz_filelist`.`image_no`,`src_biz_filelist`.`caption`,`src_biz_filelist`.`create_time`,`src_biz_filelist`.`update_time`,`src_biz_filelist`.`img_seql`,`src_biz_filelist`.`data_date` from `src_biz_filelist` ORDER BY `src_biz_filelist`.`batch_no` ASC |
| dn2_0 | BASE SQL | select `src_biz_filelist`.`ysyid`,`src_biz_filelist`.`batch_no`,`src_biz_filelist`.`image_no`,`src_biz_filelist`.`caption`,`src_biz_filelist`.`create_time`,`src_biz_filelist`.`update_time`,`src_biz_filelist`.`img_seql`,`src_biz_filelist`.`data_date` from `src_biz_filelist` ORDER BY `src_biz_filelist`.`batch_no` ASC |
| dn3_0 | BASE SQL | select `src_biz_filelist`.`ysyid`,`src_biz_filelist`.`batch_no`,`src_biz_filelist`.`image_no`,`src_biz_filelist`.`caption`,`src_biz_filelist`.`create_time`,`src_biz_filelist`.`update_time`,`src_biz_filelist`.`img_seql`,`src_biz_filelist`.`data_date` from `src_biz_filelist` ORDER BY `src_biz_filelist`.`batch_no` ASC |
| dn4_0 | BASE SQL | select `src_biz_filelist`.`ysyid`,`src_biz_filelist`.`batch_no`,`src_biz_filelist`.`image_no`,`src_biz_filelist`.`caption`,`src_biz_filelist`.`create_time`,`src_biz_filelist`.`update_time`,`src_biz_filelist`.`img_seql`,`src_biz_filelist`.`data_date` from `src_biz_filelist` ORDER BY `src_biz_filelist`.`batch_no` ASC |
| merge_and_order_1 | MERGE_AND_ORDER | dn1_0; dn2_0; dn3_0; dn4_0 |
| shuffle_field_1 | SHUFFLE_FIELD | merge_and_order_1 |
| dn1_1 | BASE SQL | select `autoalias_src_image_txn_jnl`.`autoalias_scalar` from (select distinct `src_image_txn_jnl`.`imageid` as `autoalias_scalar` from `src_image_txn_jnl` where (`src_image_txn_jnl`.`front_seq_num` = '001780568097') AND (`src_image_txn_jnl`.`txn_dt` = '2029-08-20') order by autoalias_scalar ASC) autoalias_src_image_txn_jnl order by `autoalias_src_image_txn_jnl`.`autoalias_scalar` ASC |
| merge_1 | MERGE | dn1_1 |
| join_1 | JOIN | shuffle_field_1; merge_1 |
| shuffle_field_2 | SHUFFLE_FIELD | join_1 |
+-------------------+-----------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
10 rows in set (0.00 sec)
**執(zhí)行計(jì)劃可見(jiàn)，DBLE對(duì)語(yǔ)句進(jìn)行了拆分。首先分別在4個(gè)數(shù)據(jù)節(jié)點(diǎn)全表掃描分片表src_biz_filelist，將各自返回的結(jié)果集在DBLE層合并排序。然后將子查詢結(jié)果去重排序后返回。最后在DBLE層進(jìn)行匹配操作。當(dāng)分片表數(shù)據(jù)量較大時(shí)，全部提取數(shù)據(jù)動(dòng)作，即使不與子查詢中結(jié)果集匹配，效率也不會(huì)很好。
**
按照開(kāi)發(fā)描述改寫子查詢語(yǔ)句賦予具體值，進(jìn)行查看：
mysql> explain select caption,batch_no,image_no from src_biz_filelist where batch_no in ('0250002403989000IMMEDA200630095447WV','1650002432532000IMMEDA200630095233WS');
+-----------+----------+------------------------------------------------------------------------------------------------------------------------------------------------------+
| DATA_NODE | TYPE | SQL/REF |
+-----------+----------+------------------------------------------------------------------------------------------------------------------------------------------------------+
| dn4 | BASE SQL | select caption,batch_no,image_no from src_biz_filelist where batch_no in ('0250002403989000IMMEDA200630095447WV','1650002432532000IMMEDA200630095233WS') |
+-----------+----------+------------------------------------------------------------------------------------------------------------------------------------------------------+
1 rows in set (0.00 sec)
因?yàn)榉制韘rc_biz_filelist的分片鍵為batch_no，語(yǔ)句直接下壓到了數(shù)據(jù)所在的dn4節(jié)點(diǎn)。驗(yàn)證了開(kāi)發(fā)描述的在使用獨(dú)立子查詢時(shí)，不論子查詢中結(jié)果集有幾個(gè)，語(yǔ)句都會(huì)卡死遲遲得不到返回結(jié)果。如果去掉子查詢賦予具體值，很快得到返回結(jié)果。程序進(jìn)行語(yǔ)句拆分傳入具體值查詢，也是我們的優(yōu)化方法之一。那是不是DBLE在獨(dú)立子查詢上真的沒(méi)有做相關(guān)優(yōu)化呢？我們繼續(xù)操作。
**2.2 **獨(dú)立標(biāo)量子查詢語(yǔ)句及執(zhí)行計(jì)劃
如果明確子查詢結(jié)果單個(gè)值情況下，可以改寫使用獨(dú)立標(biāo)量子查詢，執(zhí)行計(jì)劃如下：
mysql> explain select caption,batch_no,image_no from src_biz_filelist where batch_no = (SELECT imageid FROM src_image_txn_jnl WHERE front_seq_num = '001780568097' and txn_dt = '2029-08-20');
+--------------------+-----------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| DATA_NODE | TYPE | SQL/REF |
+--------------------+-----------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| dn1_0 | BASE SQL | select `src_image_txn_jnl`.`imageid` as `autoalias_scalar` from `src_image_txn_jnl` where (`src_image_txn_jnl`.`front_seq_num` = '001780568097') AND (`src_image_txn_jnl`.`txn_dt` = '2029-08-20') limit 0,2 |
| merge_1 | MERGE | dn1_0 |
| scalar_sub_query_1 | SCALAR_SUB_QUERY | merge_1 |
| dn1_1 | BASE SQL(May No Need) | scalar_sub_query_1; select `src_biz_filelist`.`caption`,`src_biz_filelist`.`batch_no`,`src_biz_filelist`.`image_no` from `src_biz_filelist` where `src_biz_filelist`.`batch_no` = '{NEED_TO_REPLACE}' |
| dn2_0 | BASE SQL(May No Need) | scalar_sub_query_1; select `src_biz_filelist`.`caption`,`src_biz_filelist`.`batch_no`,`src_biz_filelist`.`image_no` from `src_biz_filelist` where `src_biz_filelist`.`batch_no` = '{NEED_TO_REPLACE}' |
| dn3_0 | BASE SQL(May No Need) | scalar_sub_query_1; select `src_biz_filelist`.`caption`,`src_biz_filelist`.`batch_no`,`src_biz_filelist`.`image_no` from `src_biz_filelist` where `src_biz_filelist`.`batch_no` = '{NEED_TO_REPLACE}' |
| dn4_0 | BASE SQL(May No Need) | scalar_sub_query_1; select `src_biz_filelist`.`caption`,`src_biz_filelist`.`batch_no`,`src_biz_filelist`.`image_no` from `src_biz_filelist` where `src_biz_filelist`.`batch_no` = '{NEED_TO_REPLACE}' |
| merge_2 | MERGE | dn1_1; dn2_0; dn3_0; dn4_0 |
| shuffle_field_1 | SHUFFLE_FIELD | merge_2 |
+--------------------+-----------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
9 rows in set (0.00 sec)
執(zhí)行計(jì)劃可見(jiàn)：dn1_0部分先將子查詢結(jié)果返回，出現(xiàn)了SCALAR_SUB_QUERY標(biāo)量子查詢的標(biāo)識(shí)，被引用到外部查詢中，語(yǔ)句直接下壓到了全部數(shù)據(jù)節(jié)點(diǎn)。這種情況下依據(jù)分片鍵進(jìn)行等值查詢，效率是OK的。
****03 解決
通過(guò)查看原始語(yǔ)句在DBLE的執(zhí)行計(jì)劃，下壓到MySQL時(shí)，由于DBLE對(duì)語(yǔ)句進(jìn)行了重寫，并沒(méi)有使用到MySQL優(yōu)化器改寫語(yǔ)句為semi join方式。在MySQL中需要使用子查詢完成的任務(wù)，很多時(shí)候可以使用連接方式來(lái)實(shí)現(xiàn)，那么我們?cè)谏蠈泳椭貙憺閖oin語(yǔ)句實(shí)驗(yàn)一下。
查看DBLE文檔server.xml 系統(tǒng)參數(shù)配置部分有一個(gè)useJoinStrategy參數(shù)，該參數(shù)作用是開(kāi)啟之后會(huì)嘗試判斷join兩邊的where來(lái)重新調(diào)整查詢SQL下發(fā)的順序，默認(rèn)關(guān)閉。添加true至server.xml的標(biāo)簽中，并重啟DBLE。
查看改寫之后語(yǔ)句執(zhí)行計(jì)劃：
mysql> explain select t1.caption,t1.batch_no,t1.image_no from src_biz_filelist t1 inner join src_image_txn_jnl t2 on t1.batch_no=t2.imageid WHERE t2.front_seq_num = '001780568097' and t2.txn_dt = '2029-08-20';
+-------------------+-----------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| DATA_NODE | TYPE | SQL/REF |
+-------------------+-----------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| dn1_0 | BASE SQL | select `t2`.`id`,`t2`.`evt_id`,`t2`.`front_seq_num`,`t2`.`org_id`,`t2`.`teller_id`,`t2`.`txn_dt`,`t2`.`txn_tm`,`t2`.`txn_type_cd`,`t2`.`txn_cd`,`t2`.`sub_seq_num`,`t2`.`bcs_seq_num`,`t2`.`user_ccy_cd`,`t2`.`acct_num`,`t2`.`acct_ccy_cd`,`t2`.`acct_amt`,`t2`.`txn_ccy_cd`,`t2`.`txn_amt`,`t2`.`exch_rate`,`t2`.`resp_num`,`t2`.`proc_info`,`t2`.`auth_teller_id`,`t2`.`terminal_num`,`t2`.`cntrpty_acct_num`,`t2`.`cntrpty_acct_name`,`t2`.`vchr_type_encode`,`t2`.`vchr_num`,`t2`.`vchr_dt`,`t2`.`chk_teller_id`,`t2`.`vchr_main_file_desc`,`t2`.`vchr_accessory_desc`,`t2`.`biz_status_cd`,`t2`.`fin_txn_ind`,`t2`.`txn_complt_dt`,`t2`.`ident_chk_chker_id`,`t2`.`ident_chk_result_cd`,`t2`.`veri_seal_chker_id`,`t2`.`veri_seal_result_cd`,`t2`.`tel_che_chker_id`,`t2`.`tel_che_result_cd`,`t2`.`tel_che_comment`,`t2`.`pay_acct_doubt_type_cd`,`t2`.`rcv_acct_doubt_type_cd`,`t2`.`job_center`,`t2`.`imageid`,`t2`.`pay_name`,`t2`.`txn_desc`,`t2`.`origin_txn_seq_num`,`t2`.`txn_start_tm`,`t2`.`txn_memo`,`t2`.`bcs_or_back_txn_num`,`t2`.`bea_intrfc_num`,`t2`.`txn_auth_teller`,`t2`.`send_back_or_back_err_info`,`t2`.`data_date` from `src_image_txn_jnl` `t2` where (`t2`.`front_seq_num` = '001780568097') AND (`t2`.`txn_dt` = '2029-08-20') order by `t2`.`imageid` ASC |
| merge_1 | MERGE | dn1_0 |
| nest_loop_1 | NEST_LOOP | merge_1 |
| dn1_1 | BASE SQL(May No Need) | nest_loop_1's RESULTS; select `t1`.`ysyid`,`t1`.`batch_no`,`t1`.`image_no`,`t1`.`caption`,`t1`.`create_time`,`t1`.`update_time`,`t1`.`img_seql`,`t1`.`data_date` from `src_biz_filelist` `t1` where t1.batch_no IN ('{NEED_TO_REPLACE}') ORDER BY `t1`.`batch_no` ASC |
| dn2_0 | BASE SQL(May No Need) | nest_loop_1's RESULTS; select `t1`.`ysyid`,`t1`.`batch_no`,`t1`.`image_no`,`t1`.`caption`,`t1`.`create_time`,`t1`.`update_time`,`t1`.`img_seql`,`t1`.`data_date` from `src_biz_filelist` `t1` where t1.batch_no IN ('{NEED_TO_REPLACE}') ORDER BY `t1`.`batch_no` ASC |
| dn3_0 | BASE SQL(May No Need) | nest_loop_1's RESULTS; select `t1`.`ysyid`,`t1`.`batch_no`,`t1`.`image_no`,`t1`.`caption`,`t1`.`create_time`,`t1`.`update_time`,`t1`.`img_seql`,`t1`.`data_date` from `src_biz_filelist` `t1` where t1.batch_no IN ('{NEED_TO_REPLACE}') ORDER BY `t1`.`batch_no` ASC |
| dn4_0 | BASE SQL(May No Need) | nest_loop_1's RESULTS; select `t1`.`ysyid`,`t1`.`batch_no`,`t1`.`image_no`,`t1`.`caption`,`t1`.`create_time`,`t1`.`update_time`,`t1`.`img_seql`,`t1`.`data_date` from `src_biz_filelist` `t1` where t1.batch_no IN ('{NEED_TO_REPLACE}') ORDER BY `t1`.`batch_no` ASC |
| merge_and_order_1 | MERGE_AND_ORDER | dn1_1; dn2_0; dn3_0; dn4_0 |
| shuffle_field_1 | SHUFFLE_FIELD | merge_and_order_1 |
| join_1 | JOIN | nest_loop_1; shuffle_field_1 |
| shuffle_field_2 | SHUFFLE_FIELD | join_1 |
+-------------------+-----------------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
11 rows in set (0.00 sec)
執(zhí)行計(jì)劃可見(jiàn)：dn1_0部分先將子查詢結(jié)果返回，出現(xiàn)了NEST_LOOP嵌套循環(huán)標(biāo)識(shí)，嵌套循環(huán)的結(jié)果集被引用到外部查詢中，語(yǔ)句直接下壓到了全部數(shù)據(jù)節(jié)點(diǎn)。
開(kāi)啟MySQL general_log 日志查看語(yǔ)句實(shí)際在MySQL層運(yùn)行情況如下：
2021-05-12T08:58:09.324017+08:00 1080 Query select `t2`.`id`,`t2`.`evt_id`,`t2`.`front_seq_num`,`t2`.`org_id`,`t2`.`teller_id`,`t2`.`txn_dt`,`t2`.`txn_tm`,`t2`.`txn_type_cd`,`t2`.`txn_cd`,`t2`.`sub_seq_num`,`t2`.`bcs_seq_num`,`t2`.`user_ccy_cd`,`t2`.`acct_num`,`t2`.`acct_ccy_cd`,`t2`.`acct_amt`,`t2`.`txn_ccy_cd`,`t2`.`txn_amt`,`t2`.`exch_rate`,`t2`.`resp_num`,`t2`.`proc_info`,`t2`.`auth_teller_id`,`t2`.`terminal_num`,`t2`.`cntrpty_acct_num`,`t2`.`cntrpty_acct_name`,`t2`.`vchr_type_encode`,`t2`.`vchr_num`,`t2`.`vchr_dt`,`t2`.`chk_teller_id`,`t2`.`vchr_main_file_desc`,`t2`.`vchr_accessory_desc`,`t2`.`biz_status_cd`,`t2`.`fin_txn_ind`,`t2`.`txn_complt_dt`,`t2`.`ident_chk_chker_id`,`t2`.`ident_chk_result_cd`,`t2`.`veri_seal_chker_id`,`t2`.`veri_seal_result_cd`,`t2`.`tel_che_chker_id`,`t2`.`tel_che_result_cd`,`t2`.`tel_che_comment`,`t2`.`pay_acct_doubt_type_cd`,`t2`.`rcv_acct_doubt_type_cd`,`t2`.`job_center`,`t2`.`imageid`,`t2`.`pay_name`,`t2`.`txn_desc`,`t2`.`origin_txn_seq_num`,`t2`.`txn_start_tm`,`t2`.`txn_memo`,`t2`.`bcs_or_back_txn_num`,`t2`.`bea_intrfc_num`,`t2`.`txn_auth_teller`,`t2`.`send_back_or_back_err_info`,`t2`.`data_date` from `src_image_txn_jnl` `t2` where (`t2`.`front_seq_num` = '001780568097') AND (`t2`.`txn_dt` = '2029-08-20') order by `t2`.`imageid` ASC
2021-05-12T08:58:09.326400+08:00 1091 Query select `t1`.`ysyid`,`t1`.`batch_no`,`t1`.`image_no`,`t1`.`caption`,`t1`.`create_time`,`t1`.`update_time`,`t1`.`img_seql`,`t1`.`data_date` from `src_biz_filelist` `t1` where t1.batch_no IN ('0250002403989000IMMEDA200630095447WV', '0250002401013000IMMEDA200914092919JX') ORDER BY `t1`.`batch_no` ASC
語(yǔ)句執(zhí)行順序變化，獨(dú)立子查詢被賦予了具體值，并按分片鍵進(jìn)行數(shù)據(jù)路由的特性，下發(fā)到指定分片節(jié)點(diǎn)進(jìn)行查詢。
需要注意的是關(guān)于nestLoop還有兩個(gè)參數(shù)nestLoopConnSize /nestLoopRowsSize 若臨時(shí)表行數(shù)大于這兩個(gè)值乘積，DBLE則會(huì)報(bào)告一個(gè)后端連接錯(cuò)誤。
綜上優(yōu)化方式有以下幾種：
1、程序進(jìn)行語(yǔ)句拆分傳入具體值進(jìn)行查詢。
2、如果明確子查詢結(jié)果單個(gè)值情況下，可以改寫使用獨(dú)立標(biāo)量子查詢。
3、添加 true 參數(shù)，并改寫為連接查詢。
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子查詢（Subquery）的優(yōu)化一直以來(lái)都是 SQL 查詢優(yōu)化中的難點(diǎn)之一。關(guān)聯(lián)子
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GaussDB (DWS) 根據(jù)子查詢在 SQL 語(yǔ)句中的位置把子查詢分成了子查詢、子鏈接兩種形
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