背景信息
对于每一条SQL，优化器都会生成相应执行计划。但是很多情况下，应用请求的SQL都是重复的（仅参数不同），参数化之后的SQL完全相同。这时，可以按照参数化之后的SQL构造一个缓存，将除了参数以外的各种信息（比如执行计划）缓存起来，称为执行计划缓存（Plan Cache）。
另一方面，对于较复杂的查询（例如涉及到多个表的Join），为了使其执行计划能保持相对稳定，不因为版本升级等原因发生变化。执行计划管理（Plan Management）为每个SQL记录一组执行计划，该执行计划会被持久化地保存，即使版本升级也会保留。
工作流程概览
当PolarDB-X收到一条查询SQL时，会经历以下流程：
- 对查询SQL进行参数化处理，将所有参数替换为占位符
?。 - 以参数化的SQL作为Key，查找执行计划缓存中是否有缓存；如果没有，则调用优化器进行优化。
- 如果该SQL是简单查询，则直接执行，跳过执行计划管理相关步骤。
- 如果该SQL是复杂查询，则使用基线（Baseline）中固化的执行计划；如果有多个，则选择代价最低的那个。
执行计划缓存
PolarDB-X默认开启执行计划缓存功能。EXPLAIN结果中的HitCache表示当前SQL是否命中执行计划缓存。开启执行计划缓存后，PolarDB-X会对SQL做参数化处理，参数化会将SQL中的常量用占位符?替换，并构建出相应的参数列表。在执行计划中也可以看到LogicalView算子的SQL中含有?。
执行计划管理
对于复杂SQL，经过执行计划缓存之后，还会经过执行计划管理流程。
执行计划缓存和执行计划管理都是采用参数化后的SQL作为Key来执行计划。执行计划缓存中会缓存所有SQL的执行计划，而执行计划管理仅对复杂查询SQL进行处理。
由于受到具体参数的影响，SQL模版和最优的执行计划并非一一对应的。
在执行计划管理中，每一条SQL对应一个基线，每个基线中包含一个或多个执行计划。实际使用中，会根据当时的参数选择其中代价最小的执行计划来执行。
计划选择
当执行计划缓存中的执行计划走进执行计划管理时，SPM会操作一个流程判断该执行计划是否是已知的，是已知的话，是否代价是最小的；不是已知的话，是否需要执行一下以判断该执行计划的优化程度。
运维指令
PolarDB-X提供了丰富的指令集用于管理执行计划，语法如下：
BASELINE (LOAD|PERSIST|CLEAR|VALIDATE|LIST|DELETE) [Signed Integer,Signed Integer....] BASELINE (ADD|FIX) SQL (HINT Select Statemtnt)
- BASELINE (ADD|FIX) SQL <HINT> <Select Statement>：将SQL以HINT修复过后的执行计划记录固定下来。
- BASELINE LOAD：将系统表中指定的基线信息刷新到内存并使其生效。
- BASELINE LOAD_PLAN：将系统表中指定的执行计划信息刷新到内存并使其生效。
- BASELINE LIST：列出当前所有的基线信息。BASELINE PERSIST：将指定的基线落盘。
- BASELINE PERSIST_PLAN：将指定的执行计划落盘。
- BASELINE CLEAR：内存中清理某个基线。
- BASELINE CLEAR_PLAN：内存中清理某个执行计划。
- BASELINE DELETE：磁盘中删除某个基线。
- BASELINE DELETE_PLAN：磁盘中删除某个执行计划。
执行计划调优实战
数据发生变化或PolarDB-X优化器引擎升级后，针对同一条SQL，有可能会出现更好的执行计划。SPM在自动演化时会将CBO优化自动发现的更优执行计划加入到SQL的基线中。除此以外，您也可以通过SPM的指令主动优化执行计划。
例如以下的SQL：
SELECT * FROM lineitem JOIN part ON l_partkey=p_partkey WHERE p_name LIKE '%green%';
正常EXPLAIN发现该SQL生成的执行计划使用的是Hash Join，并且在Baseline List的基线中，该SQL仅有这一个执行计划：
mysql> explain select * from lineitem join part on l_partkey=p_partkey where p_name like '%geen%'; +----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | LOGICAL PLAN | +----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Gather(parallel=true) | | ParallelHashJoin(condition="l_partkey = p_partkey", type="inner") | | LogicalView(tables="[00-03].lineitem", shardCount=4, sql="SELECT `l_orderkey`, `l_partkey`, `l_suppkey`, `l_linenumber`, `l_quantity`, `l_extendedprice`, `l_discount`, `l_tax`, `l_returnflag`, `l_linestatus`, `l_shipdate`, `l_commitdate`, `l_receiptdate`, `l_shipinstruct`, `l_shipmode`, `l_comment` FROM `lineitem` AS `lineitem`", parallel=true) | | LogicalView(tables="[00-03].part", shardCount=4, sql="SELECT `p_partkey`, `p_name`, `p_mfgr`, `p_brand`, `p_type`, `p_size`, `p_container`, `p_retailprice`, `p_comment` FROM `part` AS `part` WHERE (`p_name` LIKE ?)", parallel=true) | | HitCache:true | | | | | +----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ 7 rows in set (0.06 sec) mysql> baseline list; +-------------+--------------------------------------------------------------------------------+------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------+----------+ | BASELINE_ID | PARAMETERIZED_SQL | PLAN_ID | EXTERNALIZED_PLAN | FIXED | ACCEPTED | +-------------+--------------------------------------------------------------------------------+------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------+----------+ | -399023558 | SELECT * FROM lineitem JOIN part ON l_partkey = p_partkey WHERE p_name LIKE ? | -935671684 | Gather(parallel=true) ParallelHashJoin(condition="l_partkey = p_partkey", type="inner") LogicalView(tables="[00-03].lineitem", shardCount=4, sql="SELECT `l_orderkey`, `l_partkey`, `l_suppkey`, `l_linenumber`, `l_quantity`, `l_extendedprice`, `l_discount`, `l_tax`, `l_returnflag`, `l_linestatus`, `l_shipdate`, `l_commitdate`, `l_receiptdate`, `l_shipinstruct`, `l_shipmode`, `l_comment` FROM `lineitem` AS `lineitem`", parallel=true) LogicalView(tables="[00-03].part", shardCount=4, sql="SELECT `p_partkey`, `p_name`, `p_mfgr`, `p_brand`, `p_type`, `p_size`, `p_container`, `p_retailprice`, `p_comment` FROM `part` AS `part` WHERE (`p_name` LIKE ?)", parallel=true) | 0 | 1 | +-------------+--------------------------------------------------------------------------------+------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------+----------+ 1 row in set (0.02 sec)
假如这个SQL在某些条件下采用BKA Join（Lookup Join）会有更好的性能，那么首先需要想办法利用HINT引导PolarDB-X生成符合预期的执行计划。BKA Join的HINT格式为：
/*+TDDL:BKA_JOIN(lineitem, part)*/
通过EXPLAIN [HINT] [SQL]观察出来的执行计划是否符合预期：
mysql> explain /*+TDDL:bka_join(lineitem, part)*/ select * from lineitem join part on l_partkey=p_partkey where p_name like '%geen%'; +----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | LOGICAL PLAN | +----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Gather(parallel=true) | | ParallelBKAJoin(condition="l_partkey = p_partkey", type="inner") | | LogicalView(tables="[00-03].lineitem", shardCount=4, sql="SELECT `l_orderkey`, `l_partkey`, `l_suppkey`, `l_linenumber`, `l_quantity`, `l_extendedprice`, `l_discount`, `l_tax`, `l_returnflag`, `l_linestatus`, `l_shipdate`, `l_commitdate`, `l_receiptdate`, `l_shipinstruct`, `l_shipmode`, `l_comment` FROM `lineitem` AS `lineitem`", parallel=true) | | Gather(concurrent=true) | | LogicalView(tables="[00-03].part", shardCount=4, sql="SELECT `p_partkey`, `p_name`, `p_mfgr`, `p_brand`, `p_type`, `p_size`, `p_container`, `p_retailprice`, `p_comment` FROM `part` AS `part` WHERE (`p_name` LIKE ?)") | | HitCache:false | | | | | +----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ 8 rows in set (0.14 sec)
注意此时由于Hint的干预，Join的算法已修正为BKA Join。但是这并不会对基线造成变动，如果想以后每次遇到这条SQL都使用上面的计划，还需要将其加入到基线中。
可以采用执行计划管理的Baseline Add指令为该SQL增加一个执行计划。这样就会同时有两套执行计划存在于该SQL的基线中，CBO优化器会根据代价选择一个执行计划执行。
mysql> baseline add sql /*+TDDL:bka_join(lineitem, part)*/ select * from lineitem join part on l_partkey=p_partkey where p_name like '%geen%'; +-------------+--------+ | BASELINE_ID | STATUS | +-------------+--------+ | -399023558 | OK | +-------------+--------+ 1 row in set (0.09 sec) mysql> baseline list; +-------------+--------------------------------------------------------------------------------+-------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------+----------+ | BASELINE_ID | PARAMETERIZED_SQL | PLAN_ID | EXTERNALIZED_PLAN | FIXED | ACCEPTED | +-------------+--------------------------------------------------------------------------------+-------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------+----------+ | -399023558 | SELECT * FROM lineitem JOIN part ON l_partkey = p_partkey WHERE p_name LIKE ? | -1024543942 | Gather(parallel=true) ParallelBKAJoin(condition="l_partkey = p_partkey", type="inner") LogicalView(tables="[00-03].lineitem", shardCount=4, sql="SELECT `l_orderkey`, `l_partkey`, `l_suppkey`, `l_linenumber`, `l_quantity`, `l_extendedprice`, `l_discount`, `l_tax`, `l_returnflag`, `l_linestatus`, `l_shipdate`, `l_commitdate`, `l_receiptdate`, `l_shipinstruct`, `l_shipmode`, `l_comment` FROM `lineitem` AS `lineitem`", parallel=true) Gather(concurrent=true) LogicalView(tables="[00-03].part", shardCount=4, sql="SELECT `p_partkey`, `p_name`, `p_mfgr`, `p_brand`, `p_type`, `p_size`, `p_container`, `p_retailprice`, `p_comment` FROM `part` AS `part` WHERE (`p_name` LIKE ?)") | 0 | 1 | | -399023558 | SELECT * FROM lineitem JOIN part ON l_partkey = p_partkey WHERE p_name LIKE ? | -935671684 | Gather(parallel=true) ParallelHashJoin(condition="l_partkey = p_partkey", type="inner") LogicalView(tables="[00-03].lineitem", shardCount=4, sql="SELECT `l_orderkey`, `l_partkey`, `l_suppkey`, `l_linenumber`, `l_quantity`, `l_extendedprice`, `l_discount`, `l_tax`, `l_returnflag`, `l_linestatus`, `l_shipdate`, `l_commitdate`, `l_receiptdate`, `l_shipinstruct`, `l_shipmode`, `l_comment` FROM `lineitem` AS `lineitem`", parallel=true) LogicalView(tables="[00-03].part", shardCount=4, sql="SELECT `p_partkey`, `p_name`, `p_mfgr`, `p_brand`, `p_type`, `p_size`, `p_container`, `p_retailprice`, `p_comment` FROM `part` AS `part` WHERE (`p_name` LIKE ?)", parallel=true) | 0 | 1 | +-------------+--------------------------------------------------------------------------------+-------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------+----------+ 2 rows in set (0.03 sec)
通过以上Baseline List指令展示出来的结果，可以看到基于BKA_JOIN的执行计划已增加到该 SQL的基线中。此时EXPLAIN这条SQL，发现随SQL中p_name LIKE ? 条件变化，PolarDB-X会选择不同的执行计划。如果想让PolarDB-X固定使用上述的执行计划（而非在两个中挑选一个），可以采用Baseline Fix指令强制PolarDB-X走指定的执行计划。
mysql> baseline fix sql /*+TDDL:bka_join(lineitem, part)*/ select * from lineitem join part on l_partkey=p_partkey where p_name like '%geen%'; +-------------+--------+ | BASELINE_ID | STATUS | +-------------+--------+ | -399023558 | OK | +-------------+--------+ 1 row in set (0.07 sec) mysql> baseline list\G *************************** 1. row *************************** BASELINE_ID: -399023558 PARAMETERIZED_SQL: SELECT * FROM lineitem JOIN part ON l_partkey = p_partkey WHERE p_name LIKE ? PLAN_ID: -1024543942 EXTERNALIZED_PLAN: Gather(parallel=true) ParallelBKAJoin(condition="l_partkey = p_partkey", type="inner") LogicalView(tables="[00-03].lineitem", shardCount=4, sql="SELECT `l_orderkey`, `l_partkey`, `l_suppkey`, `l_linenumber`, `l_quantity`, `l_extendedprice`, `l_discount`, `l_tax`, `l_returnflag`, `l_linestatus`, `l_shipdate`, `l_commitdate`, `l_receiptdate`, `l_shipinstruct`, `l_shipmode`, `l_comment` FROM `lineitem` AS `lineitem`", parallel=true) Gather(concurrent=true) LogicalView(tables="[00-03].part", shardCount=4, sql="SELECT `p_partkey`, `p_name`, `p_mfgr`, `p_brand`, `p_type`, `p_size`, `p_container`, `p_retailprice`, `p_comment` FROM `part` AS `part` WHERE (`p_name` LIKE ?)") FIXED: 1 ACCEPTED: 1 *************************** 2. row *************************** BASELINE_ID: -399023558 PARAMETERIZED_SQL: SELECT * FROM lineitem JOIN part ON l_partkey = p_partkey WHERE p_name LIKE ? PLAN_ID: -935671684 EXTERNALIZED_PLAN: Gather(parallel=true) ParallelHashJoin(condition="l_partkey = p_partkey", type="inner") LogicalView(tables="[00-03].lineitem", shardCount=4, sql="SELECT `l_orderkey`, `l_partkey`, `l_suppkey`, `l_linenumber`, `l_quantity`, `l_extendedprice`, `l_discount`, `l_tax`, `l_returnflag`, `l_linestatus`, `l_shipdate`, `l_commitdate`, `l_receiptdate`, `l_shipinstruct`, `l_shipmode`, `l_comment` FROM `lineitem` AS `lineitem`", parallel=true) LogicalView(tables="[00-03].part", shardCount=4, sql="SELECT `p_partkey`, `p_name`, `p_mfgr`, `p_brand`, `p_type`, `p_size`, `p_container`, `p_retailprice`, `p_comment` FROM `part` AS `part` WHERE (`p_name` LIKE ?)", parallel=true) FIXED: 0 ACCEPTED: 1 2 rows in set (0.01 sec)
Baseline Fix指令执行完后，可以看到BKA Join执行计划的Fix状态位已被置为1。此时就算不加HINT，任意条件下Explain这条SQL，都一定会采用这个执行计划。
mysql> explain select * from lineitem join part on l_partkey=p_partkey where p_name like '%green%'; +----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | LOGICAL PLAN | +----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Gather(parallel=true) | | ParallelBKAJoin(condition="l_partkey = p_partkey", type="inner") | | LogicalView(tables="[00-03].lineitem", shardCount=4, sql="SELECT `l_orderkey`, `l_partkey`, `l_suppkey`, `l_linenumber`, `l_quantity`, `l_extendedprice`, `l_discount`, `l_tax`, `l_returnflag`, `l_linestatus`, `l_shipdate`, `l_commitdate`, `l_receiptdate`, `l_shipinstruct`, `l_shipmode`, `l_comment` FROM `lineitem` AS `lineitem`", parallel=true) | | Gather(concurrent=true) | | LogicalView(tables="[00-03].part", shardCount=4, sql="SELECT `p_partkey`, `p_name`, `p_mfgr`, `p_brand`, `p_type`, `p_size`, `p_container`, `p_retailprice`, `p_comment` FROM `part` AS `part` WHERE (`p_name` LIKE ?)") | | HitCache:true | +----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ 8 rows in set (0.01 sec)
