摘要：通常跑批加工场景下，都是大数量做关联操作，通常不建议使用索引。有些时候因为计划误判导致使用索引的可能会导致严重的性能问题。本文从一个典型的索引导致性能的场景重发，剖析此类问题的特征，定位方法和解决方法
本文分享自华为云社区《GaussDB(DWS)性能调优：indexscan导致的性能问题识别与优化 #【玩转PB级数仓GaussDB(DWS)】》，作者： 譡里个檔 。
通常跑批加工场景下，都是大数量做关联操作，通常不建议使用索引。有些时候因为计划误判导致使用索引的可能会导致严重的性能问题。本文从一个典型的索引导致性能的场景重发，剖析此类问题的特征，定位方法和解决方法。
1）在某局点POC测试时发现某SQL语句比较慢，原始SQL如下
WITH /** etl_116583_7960703_994644 **/ LOADABLE as (select "boq_rel_type_id","to_pu_id","to_version","cycle_id", "part_offset_flag","to_boq_id","descr","from_contract_id", "from_version","from_pu_id","ss_id","to_contract_id", "from_boq_id","enable_flag","last_update_date" from (SELECT /*+ PARALLEL(4)*/ BOQ_REL.FROM_BOQ_ID, BOQ_REL.TO_BOQ_ID, BOQ_REL.FROM_PU_ID, BOQ_REL.TO_PU_ID, BOQ_REL.PART_OFFSET_FLAG, BOQ_REL.DESCR, BOQ_REL.SS_ID, BOQ_REL.CYCLE_ID, NVL(BOQ_REL.FROM_VERSION, 'SNULL') FROM_VERSION, NVL(BOQ_REL.TO_VERSION, 'SNULL') TO_VERSION, BOQ_REL.LAST_UPDATE_DATE, FROM_CON.CONTRACT_ID AS FROM_CONTRACT_ID, TO_CON.CONTRACT_ID AS TO_CONTRACT_ID, CLA.CLASS_ID AS BOQ_REL_TYPE_ID, BOQ_REL.ENABLE_FLAG FROM (SELECT A.FROM_BOQ_ID, A.TO_BOQ_ID, A.FROM_PU_ID, A.TO_PU_ID, A.FROM_CONTRACT_NUMBER, A.TO_CONTRACT_NUMBER, A.BOQ_REL_TYPE_CODE, A.PART_OFFSET_FLAG, A.DESCR, A.SS_ID, A.FROM_VERSION, A.TO_VERSION, A.LAST_UPDATE_DATE, A.CYCLE_ID, A.ENABLE_FLAG, DECODE(A.SS_ID, 2820, 2600, A.SS_ID) SS_ID_TMP, ROW_NUMBER() OVER(PARTITION BY FROM_BOQ_ID, TO_BOQ_ID, FROM_PU_ID, TO_PU_ID, FROM_CONTRACT_NUMBER, TO_CONTRACT_NUMBER, BOQ_REL_TYPE_CODE, FROM_VERSION, TO_VERSION ORDER BY DECODE(A.SS_ID, 2820, 1, 2600, 2, 3)) RN FROM LDB_MD_BOQ_REL A) BOQ_REL, (SELECT CONTRACT_ID, HW_CONTRACT_NUM, SS_ID FROM DWI_MD_CONTRACT WHERE CONTRACT_ID IS NOT NULL AND END_TIME = TO_DATE('4712-12-31', 'YYYY-MM-DD')) FROM_CON, (SELECT CONTRACT_ID, HW_CONTRACT_NUM, SS_ID FROM DWI_MD_CONTRACT WHERE CONTRACT_ID IS NOT NULL AND END_TIME = TO_DATE('4712-12-31', 'YYYY-MM-DD')) TO_CON, (SELECT CLASS_ID, CODE, CLASS_TYPE_ID, SS_ID FROM DWI_MD_CLASS WHERE CLASS_TYPE_ID = 193) CLA WHERE BOQ_REL.RN = 1 AND BOQ_REL.FROM_CONTRACT_NUMBER = FROM_CON.HW_CONTRACT_NUM AND BOQ_REL.SS_ID = FROM_CON.SS_ID AND BOQ_REL.TO_CONTRACT_NUMBER = TO_CON.HW_CONTRACT_NUM AND BOQ_REL.SS_ID = TO_CON.SS_ID AND BOQ_REL.BOQ_REL_TYPE_CODE = CLA.CODE AND BOQ_REL.SS_ID_TMP = CLA.SS_ID ) t ), BEFORE_TARGET as (select "from_contract_id","from_pu_id","ss_id","from_boq_id","from_version","to_version", "crt_cycle_id","to_pu_id","to_boq_id","del_flag","last_upd_cycle_id","last_update_date", "descr","enable_flag","crt_job_instance_id","dq_improve_flag","upd_job_instance_id", "to_contract_id","part_offset_flag","boq_rel_type_id" from (SELECT /*+PARALLEL(4)*/ FROM_BOQ_ID, TO_BOQ_ID, FROM_PU_ID, TO_PU_ID, FROM_CONTRACT_ID, TO_CONTRACT_ID, BOQ_REL_TYPE_ID, PART_OFFSET_FLAG, DESCR, SS_ID, CRT_CYCLE_ID, LAST_UPD_CYCLE_ID, DEL_FLAG, DQ_IMPROVE_FLAG, CRT_JOB_INSTANCE_ID, UPD_JOB_INSTANCE_ID, NVL(FROM_VERSION, 'SNULL') FROM_VERSION, NVL(TO_VERSION, 'SNULL') TO_VERSION, LAST_UPDATE_DATE, ENABLE_FLAG FROM DWI_MD_BOQ_REL ) t ), CDC as (select LOADABLE."ss_id",LOADABLE."from_version",LOADABLE."from_boq_id", LOADABLE."part_offset_flag",LOADABLE."from_pu_id", case when BEFORE_TARGET.BOQ_REL_TYPE_ID is null and BEFORE_TARGET.FROM_BOQ_ID is null and BEFORE_TARGET.FROM_CONTRACT_ID is null and BEFORE_TARGET.FROM_PU_ID is null and BEFORE_TARGET.FROM_VERSION is null and BEFORE_TARGET.TO_BOQ_ID is null and BEFORE_TARGET.TO_CONTRACT_ID is null and BEFORE_TARGET.TO_PU_ID is null and BEFORE_TARGET.TO_VERSION is null then 1 else 3 end as "change_code", LOADABLE."to_version",LOADABLE."boq_rel_type_id", LOADABLE."from_contract_id",LOADABLE."to_contract_id", LOADABLE."descr",LOADABLE."last_update_date", LOADABLE."to_pu_id",LOADABLE."enable_flag",LOADABLE."cycle_id", LOADABLE."to_boq_id" from LOADABLE left join BEFORE_TARGET on LOADABLE.BOQ_REL_TYPE_ID = BEFORE_TARGET.BOQ_REL_TYPE_ID and LOADABLE.FROM_BOQ_ID = BEFORE_TARGET.FROM_BOQ_ID and LOADABLE.FROM_CONTRACT_ID = BEFORE_TARGET.FROM_CONTRACT_ID and LOADABLE.FROM_PU_ID = BEFORE_TARGET.FROM_PU_ID and LOADABLE.FROM_VERSION = BEFORE_TARGET.FROM_VERSION and LOADABLE.TO_BOQ_ID = BEFORE_TARGET.TO_BOQ_ID and LOADABLE.TO_CONTRACT_ID = BEFORE_TARGET.TO_CONTRACT_ID and LOADABLE.TO_PU_ID = BEFORE_TARGET.TO_PU_ID and LOADABLE.TO_VERSION = BEFORE_TARGET.TO_VERSION ), TFM_FILTER_DATA_TARGET_OUTPUT_U as (select CDC."to_pu_id",CDC."boq_rel_type_id",CDC."ss_id", Current_Timestamp() as "dw_last_update_date",CDC."to_version", CDC."from_version",20230104000000 as "last_upd_cycle_id", CDC."from_contract_id",CDC."last_update_date",CDC."descr", 'N' as "del_flag",CDC."from_boq_id",CDC."to_boq_id", CDC."enable_flag",CDC."from_pu_id",-1 as "upd_job_instance_id", 'N' as "dq_improve_flag",CDC."to_contract_id", CDC."part_offset_flag" from CDC where CDC.change_code=3 ) update DWI_MD_BOQ_REL TARGET_U set "dq_improve_flag" = TFM_FILTER_DATA_TARGET_OUTPUT_U."dq_improve_flag", "dw_last_update_date" = TFM_FILTER_DATA_TARGET_OUTPUT_U."dw_last_update_date", "upd_job_instance_id" = TFM_FILTER_DATA_TARGET_OUTPUT_U."upd_job_instance_id", "descr" = TFM_FILTER_DATA_TARGET_OUTPUT_U."descr", "part_offset_flag" = TFM_FILTER_DATA_TARGET_OUTPUT_U."part_offset_flag", "last_update_date" = TFM_FILTER_DATA_TARGET_OUTPUT_U."last_update_date", "del_flag" = TFM_FILTER_DATA_TARGET_OUTPUT_U."del_flag", "last_upd_cycle_id" = TFM_FILTER_DATA_TARGET_OUTPUT_U."last_upd_cycle_id", "enable_flag" = TFM_FILTER_DATA_TARGET_OUTPUT_U."enable_flag", "ss_id" = TFM_FILTER_DATA_TARGET_OUTPUT_U."ss_id" from TFM_FILTER_DATA_TARGET_OUTPUT_U where TARGET_U."boq_rel_type_id" = TFM_FILTER_DATA_TARGET_OUTPUT_U."boq_rel_type_id" and TARGET_U."to_version" = TFM_FILTER_DATA_TARGET_OUTPUT_U."to_version" and TARGET_U."to_version" = TFM_FILTER_DATA_TARGET_OUTPUT_U."to_version" and TARGET_U."to_pu_id" = TFM_FILTER_DATA_TARGET_OUTPUT_U."to_pu_id" and TARGET_U."to_pu_id" = TFM_FILTER_DATA_TARGET_OUTPUT_U."to_pu_id" and TARGET_U."to_contract_id" = TFM_FILTER_DATA_TARGET_OUTPUT_U."to_contract_id" and TARGET_U."to_contract_id" = TFM_FILTER_DATA_TARGET_OUTPUT_U."to_contract_id" and TARGET_U."to_boq_id" = TFM_FILTER_DATA_TARGET_OUTPUT_U."to_boq_id" and TARGET_U."to_boq_id" = TFM_FILTER_DATA_TARGET_OUTPUT_U."to_boq_id" and TARGET_U."from_version" = TFM_FILTER_DATA_TARGET_OUTPUT_U."from_version" and TARGET_U."from_version" = TFM_FILTER_DATA_TARGET_OUTPUT_U."from_version" and TARGET_U."from_pu_id" = TFM_FILTER_DATA_TARGET_OUTPUT_U."from_pu_id" and TARGET_U."from_pu_id" = TFM_FILTER_DATA_TARGET_OUTPUT_U."from_pu_id" and TARGET_U."from_contract_id" = TFM_FILTER_DATA_TARGET_OUTPUT_U."from_contract_id" and TARGET_U."from_contract_id" = TFM_FILTER_DATA_TARGET_OUTPUT_U."from_contract_id" and TARGET_U."from_boq_id" = TFM_FILTER_DATA_TARGET_OUTPUT_U."from_boq_id" and TARGET_U."from_boq_id" = TFM_FILTER_DATA_TARGET_OUTPUT_U."from_boq_id" and TARGET_U."boq_rel_type_id" = TFM_FILTER_DATA_TARGET_OUTPUT_U."boq_rel_type_id" ;
2）查询此query的topSQL信息的warning字段，发现SQL自诊断信息中有索引相关告警信息。
3）查询此query的topSQL信息(如下图)，分析历史执行信息，发现id=20的CStore Index Scan算子的耗时为90796.980ms，SQL执行总时长137135.658ms。CStore Index Scan算子的耗时占比为 66%
4）找到原始SQL语句，对查询语句中出现的表dwimd.dwi_md_contract 进行hint，强制其走顺序扫描，避免走indexscan（全量语句见附件）
5）对语句进行explain verbose，查看计划，发现计划符合预期（即表dwimd.dwi_md_contract走tablescan，对于列存表计划上显式为CStore Scan）
6）对语句执行EXPLAIN ANALYZE操作(即实际执行语句)，查看实际执行时间如下，发现SQL语句性能提升近10倍。全量的执行信息见附件
