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本文介紹如何管理執行計劃，將重復或者復雜查詢的執行計劃長久地保存下來。

背景信息

對于每一條SQL，優化器都會生成相應執行計劃。但是很多情況下，應用請求的SQL都是重復的（僅參數不同），參數化之后的SQL完全相同。這時，可以按照參數化之后的SQL構造一個緩存，將除了參數以外的各種信息（比如執行計劃）緩存起來，稱為執行計劃緩存（Plan Cache）。

另一方面，對于較復雜的查詢（例如涉及到多個表的Join），為了使其執行計劃能保持相對穩定，不因為版本升級等原因發生變化。執行計劃管理（Plan Management）為每個SQL記錄一組執行計劃，該執行計劃會被持久化地保存，即使版本升級也會保留。

工作流程概覽

當PolarDB-X 1.0收到一條查詢SQL時，會經歷以下流程：

對查詢SQL進行參數化處理，將所有參數替換為占位符?。
以參數化的SQL作為Key，查找執行計劃緩存中是否有緩存；如果沒有，則調用優化器進行優化。
如果該SQL是簡單查詢，則直接執行，跳過執行計劃管理相關步驟。
如果該SQL是復雜查詢，則使用基線（Baseline）中固化的執行計劃；如果有多個，則選擇代價最低的那個。

執行計劃緩存

PolarDB-X 1.0默認開啟執行計劃緩存功能。EXPLAIN結果中的HitCache表示當前SQL是否命中執行計劃緩存。開啟執行計劃緩存后，PolarDB-X 1.0會對SQL做參數化處理，參數化會將SQL中的常量用占位符?替換，并構建出相應的參數列表。在執行計劃中也可以看到LogicalView算子的SQL中含有?。

執行計劃管理

對于復雜SQL，經過執行計劃緩存之后，還會經過執行計劃管理流程。

執行計劃緩存和執行計劃管理都是采用參數化后的SQL作為Key來執行計劃。執行計劃緩存中會緩存所有SQL的執行計劃，而執行計劃管理僅對復雜查詢SQL進行處理。

由于受到具體參數的影響，SQL模版和最優的執行計劃并非一一對應的。

在執行計劃管理中，每一條SQL對應一個基線，每個基線中包含一個或多個執行計劃。實際使用中，會根據當時的參數選擇其中代價最小的執行計劃來執行。

當執行計劃緩存中的執行計劃走進執行計劃管理時，SPM會操作一個流程判斷該執行計劃是否是已知的，是已知的話，是否代價是最小的；不是已知的話，是否需要執行一下以判斷該執行計劃的優化程度。

運維指令

PolarDB-X 1.0提供了豐富的指令集用于管理執行計劃，語法如下：

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>：將SQL以HINT修復過后的執行計劃記錄固定下來。
BASELINE LOAD：將系統表中指定的基線信息刷新到內存并使其生效。
BASELINE LOAD_PLAN：將系統表中指定的執行計劃信息刷新到內存并使其生效。
BASELINE LIST：列出當前所有的基線信息。BASELINE PERSIST：將指定的基線落盤。
BASELINE PERSIST_PLAN：將指定的執行計劃落盤。
BASELINE CLEAR：內存中清理某個基線。
BASELINE CLEAR_PLAN：內存中清理某個執行計劃。
BASELINE DELETE：磁盤中刪除某個基線。
BASELINE DELETE_PLAN：磁盤中刪除某個執行計劃。

執行計劃調優實戰

數據發生變化或PolarDB-X 1.0優化器引擎升級后，針對同一條SQL，有可能會出現更好的執行計劃。SPM在自動演化時會將CBO優化自動發現的更優執行計劃加入到SQL的基線中。除此以外，您也可以通過SPM的指令主動優化執行計劃。

例如以下的SQL：

SELECT *
FROM lineitem JOIN part ON l_partkey=p_partkey
WHERE p_name LIKE '%green%';

正常EXPLAIN發現該SQL生成的執行計劃使用的是Hash Join，并且在Baseline List的基線中，該SQL僅有這一個執行計劃：

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)
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)

假如這個SQL在某些條件下采用BKA Join（Lookup Join）會有更好的性能，那么首先需要想辦法利用HINT引導PolarDB-X 1.0生成符合預期的執行計劃。BKA Join的HINT格式為：

/*+TDDL:BKA_JOIN(lineitem, part)*/

通過EXPLAIN [HINT] [SQL]觀察出來的執行計劃是否符合預期：

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)

注意此時由于Hint的干預，Join的算法已修正為BKA Join。但是這并不會對基線造成變動，如果想以后每次遇到這條SQL都使用上面的計劃，還需要將其加入到基線中。

可以采用執行計劃管理的Baseline Add指令為該SQL增加一個執行計劃。這樣就會同時有兩套執行計劃存在于該SQL的基線中，CBO優化器會根據代價選擇一個執行計劃執行。

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)
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)

通過以上Baseline List指令展示出來的結果，可以看到基于BKA_JOIN的執行計劃已增加到該SQL的基線中。此時EXPLAIN這條SQL，發現隨SQL中p_name LIKE ? 條件變化，PolarDB-X 1.0會選擇不同的執行計劃。如果想讓PolarDB-X 1.0固定使用上述的執行計劃（而非在兩個中挑選一個），可以采用Baseline Fix指令強制PolarDB-X 1.0走指定的執行計劃。

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)
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指令執行完后，可以看到BKA Join執行計劃的Fix狀態位已被置為1。此時就算不加HINT，任意條件下Explain這條SQL，都一定會采用這個執行計劃。

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)

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背景信息

工作流程概覽

執行計劃緩存

執行計劃管理

執行計劃調優實戰