在PG 13中,explain命令增加了WAL選項,可檢視命令執行過程中產生的WAL日誌大小,對於寫敏感的應用來說,可透過該命令觀察WAL日誌的統計資訊.

*新增的WAL選項*
```
[local:/data/run/pg13]:5130 pg13@masterdb=# \help explain
Command:     EXPLAIN
Description: show the execution plan of a statement
Syntax:
EXPLAIN [ ( option [, ...] ) ] statement
EXPLAIN [ ANALYZE ] [ VERBOSE ] statement

where option can be one of:

ANALYZE [ boolean ]
    VERBOSE [ boolean ]
    COSTS [ boolean ]
    SETTINGS [ boolean ]
    BUFFERS [ boolean ]
    WAL [ boolean ]
    TIMING [ boolean ]
    SUMMARY [ boolean ]
    FORMAT { TEXT | XML | JSON | YAML }

URL: https://www.postgresql.org/docs/devel/sql-explain.html

[local:/data/run/pg13]:5130 pg13@masterdb=# 
```

*試用體驗*
執行checkpoint後執行SQL,WAL日誌會寫full page
```
[local:/data/run/pg13]:5130 pg13@masterdb=# checkpoint;
CHECKPOINT
[local:/data/run/pg13]:5130 pg13@masterdb=# explain (analyze,wal) update tmp set id = 1;
                                                      QUERY PLAN                               
                       
-----------------------------------------------------------------------------------------------
-----------------------
 Update on tmp  (cost=0.00..23275.00 rows=1000000 width=10) (actual time=1636.839..1636.839 row
s=0 loops=1)
   WAL:  records=1999840  full page writes=8851  bytes=172345779
   ->  Seq Scan on tmp  (cost=0.00..23275.00 rows=1000000 width=10) (actual time=4.686..100.211
 rows=1000000 loops=1)
 Planning Time: 0.149 ms
 Execution Time: 1636.923 ms
(5 rows)

```

再次執行SQL,沒有寫full page,日誌大小相應的減少
```
[local:/data/run/pg13]:5130 pg13@masterdb=# explain (analyze,wal) update tmp set id = 1;
                                                     QUERY PLAN                                
                      
-----------------------------------------------------------------------------------------------
----------------------
 Update on tmp  (cost=0.00..23275.00 rows=1000000 width=10) (actual time=3661.261..3661.261 row
s=0 loops=1)
   WAL:  records=1999719  bytes=131982016
   ->  Seq Scan on tmp  (cost=0.00..23275.00 rows=1000000 width=10) (actual time=3.920..98.921 
rows=1000000 loops=1)
 Planning Time: 0.088 ms
 Execution Time: 3661.310 ms
(5 rows)

```
節省23%的空間,38KB大小
```
[local:/data/run/pg13]:5130 pg13@masterdb=# select (172345779-131982016)/172345779::float;
      ?column?       
---------------------
 0.23420221391090756
(1 row)

[local:/data/run/pg13]:5130 pg13@masterdb=# select (172345779-131982016)/1024/1024;
 ?column? 
----------
       38
(1 row)

```

**參考資料**
[New in pg13: WAL monitoring](https://rjuju.github.io/postgresql/2020/04/07/new-in-pg13-WAL-monitoring.html)

PostgreSQL DBA(172) - PG 13（WAL activity in EXPLAIN）

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