PostgreSQL 12為explain命令提供了settings選項,可檢視影響執行計劃的設定修改.

測試資料:
```
testdb=# drop table if exists t_settings;
NOTICE:  table "t_settings" does not exist, skipping
DROP TABLE
testdb=# create table t_settings(id int,c1 varchar(20));
CREATE TABLE
testdb=# 
testdb=# insert into t_settings select x,'c1'||x from generate_series(1,100000) as x;
INSERT 0 100000
testdb=# create index idx_t_settings_id on t_settings(id);
CREATE INDEX
```

*PG 11*
```
testdb=# explain (settings on) select * from t_settings where id = 1;
ERROR:  unrecognized EXPLAIN option "settings"
LINE 1: explain (settings on) select * from t_settings where id = 1;
                 ^
```
PG 11不支援該特性.

*PG 12*
PG 12新增了該特性
```
[local]:5432 pg12@testdb=# explain (settings on) select * from t_settings where id = 1;
                                    QUERY PLAN                                     
-----------------------------------------------------------------------------------
 Bitmap Heap Scan on t_settings  (cost=12.17..570.66 rows=500 width=62)
   Recheck Cond: (id = 1)
   ->  Bitmap Index Scan on idx_t_settings_id  (cost=0.00..12.04 rows=500 width=0)
         Index Cond: (id = 1)
(4 rows)

Time: 5.403 ms
```

修改引數,檢視執行計劃
```
[local]:5432 pg12@testdb=# set enable_indexscan=off;
SET
Time: 0.555 ms
[local]:5432 pg12@testdb=# explain (settings on) select * from t_settings where id = 1;
                                   QUERY PLAN                                   
--------------------------------------------------------------------------------
 Bitmap Heap Scan on t_settings  (cost=4.30..8.31 rows=1 width=11)
   Recheck Cond: (id = 1)
   ->  Bitmap Index Scan on idx_t_settings_id  (cost=0.00..4.30 rows=1 width=0)
         Index Cond: (id = 1)
 Settings: enable_indexscan = 'off'
(5 rows)

Time: 0.759 ms
```
注意執行計劃中的" Settings: enable_indexscan = 'off' ",把影響執行計劃的引數修改列印出來,這是一個pretty cool特性,增強了易用性.

**參考資料**
[Waiting for PostgreSQL 12 – Add SETTINGS option to EXPLAIN, to print modified settings](https://www.depesz.com/2019/04/30/waiting-for-postgresql-12-add-settings-option-to-explain-to-print-modified-settings/)

PostgreSQL DBA(70) - PG 12 Add SETTINGS option to EXPLAIN

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