postgres 数据库 一些操作

1. 开启/关闭seq scan

set session enable_seqscan=true;

set session enable_hashjoin=false;

参考：http://www.cnblogs.com/gaojian/archive/2012/11/07/2758750.html

2.hash join

http://www.cnblogs.com/gaojian/archive/2012/11/08/2759874.html

3.merge join

http://momjian.us/main/writings/pgsql/optimizer.pdf

4.PostgreSQL Hardware Performance Tuning

http://momjian.us/main/writings/pgsql/hw_performance/index.html

5.书籍

http://momjian.us/main/writings/pgsql/aw_pgsql_book/

6. analyze 作用

postgres=# create table t1(id int);
CREATE TABLE
postgres=# insert into t1 select generate_series(1,1000);
INSERT 0 1000
postgres=# SELECT relname, relkind, reltuples, relpages FROM pg_class
WHERE relname='t1';
relname | relkind | reltuples | relpages
---------+---------+-----------+----------
t1      | r       |         0 |        0
(1 row)

postgres=# analyze;
ANALYZE
postgres=# SELECT relname, relkind, reltuples, relpages FROM pg_class
WHERE relname='t1';
relname | relkind | reltuples | relpages
---------+---------+-----------+----------
t1      | r       |      1000 |        4
(1 row)

不管甚么时候，如果你在增加或者更新了大量数据之后， 运行 ANALYZE 或者 VACUUM ANALYZE 都是个好习惯．这样就可以保证规划器有最新的表的数据的统计．
如果没有统计数据或者统计数据太陈旧，那么规划器可能选择很差劲的 查询计划，导致检索你的表的查询性能的恶化。

7. pg_dump 导出数据均分多份

使用 split．. 你可以用下面的方法把输出分解成操作系统可以接受的大小． 比如，让每个块大小为
1 兆字节：

pg_dump dbname | split -b 1m - filename

用下面命令恢复：

createdb dbname
cat filename* | psql dbname

参考：http://www.php100.com/manual/pgsql/backup.html

8.单用户模式

postgres --single dbname

退出 ctrl+D

[wln@localhost ~]$ postgres --single postgres

PostgreSQL stand-alone backend 9.3beta2

backend> [wln@localhost ~]$ 

9. insert 返回打印信息

一个是last oid, 一个是有多少行被插入

postgres=# create table t2(id int) with oids;

CREATE TABLE

postgres=# insert into t1 values(1),(2);

INSERT 0 2

postgres=# insert into t2 values(1),(2);

INSERT 0 2

postgres=# insert into t1 values(1);

INSERT 0 1

postgres=# insert into t2 values(1);

INSERT 16389 1

参考：http://blog.163.com/digoal@126/blog/static/163877040201492452049196/

10.bytea 类型显示

postgres=# \d t4

      Table "public.t4"

 Column |  Type   | Modifiers

--------+---------+-----------

 id     | integer |

 id2    | bytea   |

postgres=# select * from t4;

 id |      id2      

----+----------------

  1 | \x31

  2 | \x3232

  2 | \x313233343536

(3 rows)

postgres=# set bytea_output = 'escape';
SET

postgres=# select * from t4;          

 id |  id2  

----+--------

  1 | 1

  2 | 22

  2 | 123456

(3 rows)

 

11. 对vacuum, vacuum full的认识

简单说vacuum 只是将标记为删除的标记为可用，以待后来利用；vacuum full 会将标记删除的还给操作系统。下面的文章写的很好

http://www.cnblogs.com/stephen-liu74/archive/2012/05/23/2304155.html

12. 如何避免create index 时表锁定

create index concurrently....  假如不加 concurrently 创建索引时只会扫描表1次，加了后会扫描2次。

13. create database

CREATE DATABASE db01

  WITH OWNER = postgres

       ENCODING = 'UTF8'

       TABLESPACE = pg_default

       LC_COLLATE = 'Chinese_People''s Republic of China.936'

       LC_CTYPE = 'Chinese_People''s Republic of China.936'

       CONNECTION LIMIT = -1;

14. begin transaction isolation level repeatable read

参考：http://blog.163.com/digoal@126/blog/static/163877040201326829943/

15.txid 

select txid_snapshot_xmax(txid_current_snapshot())::text::xid;

Current transaction details:

 txid_current(): int8

 txid_current_snapshot(): txid_snapshot

 Snapshot components:

 txid_snapshot_xmin(snap): int8

 txid_snapshot_xmax(snap): int8

 txid_snapshot_xip(snap): SETOF int8

 Visibility check:

 txid_visible_in_snapshot(txid, snap): bool

http://www.pgcon.org/2008/schedule/attachments/55_pgq.pdf