PostgreSQL unlogged表

PostgreSQL有一種介於正常表和臨時表之間的類型表，稱之為unlogged表，在該表新建的索引也屬於unlogged，該表在寫入數據時候並不將數據寫入到持久的write-ahead log文件中，在數據庫異常關機或者異常崩潰后該表的數據會被truncate掉，但是在寫入性能上會比正常表快幾倍。

分別測試正常表和unlogged表數據插入速度區別

uber_geocoder=# \dt+
                                      List of relations
          Schema          |          Name          | Type  |  Owner   |  Size  | Description 
--------------------------+------------------------+-------+----------+--------+-------------
 uber_geocoder_tw_15q3_v2 | compiling_info         | table | postgres | 16 kB  | 
 uber_geocoder_tw_15q3_v2 | data_process_reports   | table | postgres | 16 kB  | 
 uber_geocoder_tw_15q3_v2 | info                   | table | postgres | 16 kB  | 
 uber_geocoder_tw_15q3_v2 | twn_addr_compact       | table | postgres | 774 MB | 
(4 rows)

uber_geocoder=# \timing 
Timing is on.

uber_geocoder=# create table twn_addr_compact_loggod as select * from twn_addr_compact ;
SELECT 258902
Time: 977250.581 ms

可以看到在新建正常表並插770M的數據的情況下耗時近16分鍾

新建一張unlogged表並插入770M數據進行測試

uber_geocoder=# create unlogged table twn_addr_compact_unloggod as select * from twn_addr_compact ;
SELECT 258902
Time: 300683.321 ms

可以看到在同等條件下unlogged表的插入速度為5分鍾，性能提高了三倍。

 感興趣的可以將postgres進程kill掉，然后再啟動數據庫，就會發現我們剛才創建的unlogged表數據丟失。

如何查看當前數據庫中所有的unlogged表

uber_geocoder=# select n.nspname as schema_name,c.relname as table_name from pg_catalog.pg_namespace n, pg_catalog.pg_class c where c.relnamespace=n.oid and n.nspname != 'pg_toast' and c.relkind='r' and c.relpersistence = 'u';
       schema_name        |        table_name         
--------------------------+---------------------------
 edbstore                 | emp
 uber_geocoder_tw_15q3_v2 | twn_addr_compact_unloggod
(2 rows)

如果需要批量將unlogged表修改為正常的表，則執行如下

uber_geocoder=# select 'ALTER TABLE'||' '||concat(n.nspname,'.' ,c.relname)||' '||'SET LOGGED ;' AS convert_logged_sql from pg_catalog.pg_namespace n, pg_catalog.pg_class c where c.relnamespace=n.oid and n.nspname != 'pg_toast' and c.relkind='r' and c.relpersistence = 'u';
                             convert_logged_sql                              
-----------------------------------------------------------------------------
 ALTER TABLE edbstore.emp SET LOGGED ;
 ALTER TABLE uber_geocoder_tw_15q3_v2.twn_addr_compact_unloggod SET LOGGED ;
(2 rows)

 

select n.nspname ||'.'|| c.relname as table_name, pg_table_size(n.nspname ||'.'|| c.relname) as table_size from pg_catalog.pg_namespace n, pg_catalog.pg_class c where c.relnamespace=n.oid and n.nspname != 'pg_toast' and c.relkind='r' and c.relpersistence = 'u' order by table_size desc;