本文介绍了执行计划管理的工作流程和执行计划调优实例。
对于每一条SQL，优化器都会生成相应执行计划。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）。
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，PolarDB-X会为每个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，此过程称为执行计划管理（Plan Management）。
工作流程
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存在此SQL的缓存，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来查找执行计划，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。当执行计划缓存中的执行计划进入执行计划管理时，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，是否需要执行以判断该执行计划的优化程度。
运维命令
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：清理内存中某个基线。
BASELINE CLEAR_PLAN：清理内存中某个执行计划。
BASELINE DELETE：删除磁盘中某个基线。
BASELINE DELETE_PLAN：删除磁盘中某个执行计划。
执行计划调优实践
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在自动演化时会将查询计划枚举过程中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。
执行
EXPLAIN命令，发现一条SQL的执行计划中使用的是Hash Join，并且在Baseline List的基线中，该SQL仅有这一个执行计划。EXPLAIN SELECT * FROM lineitem JOIN part ON l_partkey=p_partkey WHERE p_name LIKE '%green%';返回执行计划信息如下：
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 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)执行以下命令，查看基线：
BASELINE LIST;返回信息如下：
+-------------+--------------------------------------------------------------------------------+------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------+----------+ | 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：EXPLAIN /*+TDDL:bka_join(lineitem, part)*/ SELECT * FROM lineitem JOIN part ON l_partkey=p_partkey WHERE p_name LIKE '%green%';返回的执行计划信息如下：
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 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)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。
采用执行计划管理的
Baseline Add指令为该SQL增加一个执行计划。此时会同时有两套执行计划存在于该SQL的基线中，查询计划枚举优化器会根据代价选择一个执行计划执行。BASELINE ADD SQL /*+TDDL:bka_join(lineitem, part)*/ SELECT * FROM lineitem JOIN part ON l_partkey=p_partkey WHERE p_name LIKE '%green%';返回信息如下：
+-------------+--------+ | BASELINE_ID | STATUS | +-------------+--------+ | -399023558 | OK | +-------------+--------+ 1 row in set (0.09 sec)执行以下命令，查看基线：
BASELINE LIST;返回信息如下：
+-------------+--------------------------------------------------------------------------------+-------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------+----------+ | 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。此时对这条SQL执行EXPLAIN命令，发现随SQL中p_name LIKE ?条件变化，PolarDB-X会选择不同的执行计划。如果需要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使用指定的执行计划。BASELINE FIX SQL /*+TDDL:bka_join(lineitem, part)*/ SELECT * FROM lineitem JOIN part ON l_partkey=p_partkey WHERE p_name LIKE '%green%';返回信息如下：
+-------------+--------+ | 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)再次执行
EXPLAIN命令，查看执行计划。EXPLAIN SELECT * FROM lineitem JOIN part ON l_partkey=p_partkey WHERE p_name LIKE '%green%';返回执行计划信息如下：
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | 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)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。此时即使不加HINT，任意条件下对这条SQL执行EXPLAIN命令，都一定会采用这个执行计划。
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