postgresql----temporary table和unlogged table

Posted 2020-07-28 alian

一.TEMPORARY|TEMP TABLE

会话级或事务级的临时表，临时表在会话结束或事物结束自动删除，任何在临时表上创建的索引也会被自动删除。除非用模式修饰的名字引用，否则现有的同名永久表在临时表存在期间，在本会话或事务中是不可见的。另外临时表对其他会话也是不可见的，但是会话级的临时表也可以使用临时表所在模式修饰的名字引用。

创建临时表的语法：

CREATE TEMP tbl_name()ON COMMIT{PRESERVE ROWS|DELETE ROWS|DROP};

PRESERVE ROWS：默认值，事务提交后保留临时表和数据

DELETE ROWS：事务提交后删除数据，保留临时表

DROP：事务提交后删除表

 

示例1

会话A:

创建临时表

test=# create temp table tbl_temp(a int);
CREATE TABLE

 

会话B：

1.在会话B查询临时表tbl_temp,提示表不存在

test=# select * from tbl_temp;
ERROR:  relation "tbl_temp" does not exist
LINE 1: select * from tbl_temp;

2.但是在会话B查询pg_class中可以查到tbl_temp的记录

test=# select relname,relnamespace from pg_class where relname = ‘tbl_temp‘;
 relname  | relnamespace 
----------+--------------
 tbl_temp |        16488
(1 row)

3.从上述查询结果中可以看到临时表tbl_temp属于16488的模式

test=# select nspname from pg_namespace where oid = 16488;
  nspname  
-----------
 pg_temp_3
(1 row)

4.直接使用模式修饰的表名访问成功

test=# select * from pg_temp_3.tbl_temp ;
 a 
---
(0 rows)

 

会话A:

退出会话A

 

会话B：

再次查询tbl_temp时提示不存在

test=# select * from pg_temp_3.tbl_temp ;
ERROR:  relation "pg_temp_3.tbl_temp" does not exist
LINE 1: select * from pg_temp_3.tbl_temp ;
                      ^

 

示例2.创建ON COMMIT DELETE ROWS的临时表

test=# begin ;
BEGIN
test=# create temp table tbl_temp(a int) on commit delete rows;
CREATE TABLE
test=# insert into tbl_temp values (1);
INSERT 0 1
test=# select * from tbl_temp ;
 a 
---
 1
(1 row)

test=# commit ;
COMMIT
test=# select * from tbl_temp ;
 a 
---
(0 rows)

 

示例3.创建ON COMMIT DROP临时表

test=# begin ;
BEGIN
test=# create temp table tbl_temp(a int) on commit drop;
CREATE TABLE
test=# commit ;
COMMIT
test=# select * from tbl_temp;
ERROR:  relation "tbl_temp" does not exist
LINE 1: select * from tbl_temp;
                      ^

 

二.UNLOGGED TABLE

unlogged table是为临时数据设计的，写入性能较高，但是当postgresql进程崩溃时会丢失数据。

创建一张普通表test和一张unlogged表test，测试性能情况

普通表：

 

test=# create table test(a int);
CREATE TABLE
test=# \timing
Timing is on.
test=# insert into test select generate_series(1,1000000);
INSERT 0 1000000
Time: 3603.715 ms

 

 

unlogged表

test=# create unlogged table testu(a int);
CREATE TABLE
Time: 12.920 ms
test=# insert into testu select generate_series(1,1000000);
INSERT 0 1000000
Time: 801.376 ms

比较以上两个结果，unlogged表的写性能是普通表的4.5倍。

杀死postgresql的主进程，重启DB服务

[[email protected] ~]# ps -elf | grep postgres
0 S postgres  2129     1  0  80   0 - 66830 poll_s 04:24 ?        00:00:00 /opt/pg9.6/bin/postgres -D /mnt/pgdata
1 S postgres  2130  2129  0  80   0 - 29645 ep_pol 04:24 ?        00:00:00 postgres: logger process              
1 S postgres  2132  2129  0  80   0 - 66898 poll_s 04:24 ?        00:00:00 postgres: checkpointer process        
1 S postgres  2133  2129  0  80   0 - 66830 ep_pol 04:24 ?        00:00:00 postgres: writer process              
1 S postgres  2134  2129  0  80   0 - 66871 ep_pol 04:24 ?        00:00:00 postgres: wal writer process          
1 S postgres  2135  2129  0  80   0 - 66954 ep_pol 04:24 ?        00:00:00 postgres: autovacuum launcher process   
1 S postgres  2136  2129  0  80   0 - 29677 ep_pol 04:24 ?        00:00:00 postgres: stats collector process     
0 S root      2262  2099  0  80   0 - 28768 n_tty_ 04:52 pts/1    00:00:00 /opt/pg9.6/bin/psql -d test -U postgres
1 S postgres  2264  2129  0  80   0 - 67351 ep_pol 04:52 ?        00:00:02 postgres: postgres test [local] idle  
0 S root      2334  2198  0  80   0 - 25813 pipe_w 05:15 pts/2    00:00:00 grep postgres
[[email protected]-R1CL-srv ~]# kill -9 2129
[[email protected]-R1CL-srv ~]# rm -rf /mnt/pgdata/postmaster.pid 
[[email protected]-R1CL-srv ~]# su -l postgres -c ‘/opt/pg9.6/bin/pg_ctl -D /mnt/pgdata start‘
server starting
[[email protected]-R1CL-srv ~]# 2016-06-22 05:16:04.399 CST 2372    LOG:  redirecting log output to logging collector process
2016-06-22 05:16:04.399 CST 2372    HINT:  Future log output will appear in directory "/var/log/pg_log".

再次查询unlogged表testu，发现数据已丢失

test=# select * from testu ;
 a 
---
(0 rows)