PostgreSQL 为什么不要滥用unlogged table & hash index

unlogged table和hash index同样都不会写XLOG，所以如果你用流复制来搞HA，一定概要搞清楚一个问题，切换到备库的话unlogged table数据会被清掉，而hash index也没有，走hash index会失败。

unlogged table 的风险以及修复手段可以见 ：
http://blog.163.com/digoal@126/blog/static/163877040201582621345351/

hash index则风险略小，但是也必须重建，但是个人还是建议大家不要使用hash index，改用btree，因为性能确实相差无几。

批量将数据库集群的hash index修改为btree index的方法：
例子：
先并行创建一个btree索引，然后并行删除对应的hash 索引。
$ vi test.sh
#!/bin/bash

for db in `psql -n -q -t -h 127.0.0.1 -p 1921 -U postgres postgres -c "copy (select datname from pg_database where datname <>'template0') to stdout;"`
do 

psql -n -q -t -h 127.0.0.1 -p 1921 -U postgres $db -c "with t1(sql,nsp,idx) as (select regexp_replace(indexdef,'USING hash','USING btree'),schemaname,indexname from pg_indexes where indexdef ~ 'USING hash'), t2(sql_create,sql_drop) as (select regexp_replace(sql,'CREATE INDEX','CREATE INDEX CONCURRENTLY'), 'DROP INDEX CONCURRENTLY '||quote_ident(nsp)||'.'||quote_ident(idx) from t1) select regexp_replace(sql_create,'CONCURRENTLY (.*) ON','CONCURRENTLY \1_0926 ON')  ||'; '|| sql_drop ||'; ' from t2;"|psql -a -e -h 127.0.0.1 -p 1921 -U postgres $db -f -

done

$ . ./test.sh

这个with查询的结果如下举例：
postgres=# with t1(sql,nsp,idx) as (select regexp_replace(indexdef,'USING hash','USING btree'),schemaname,indexname from pg_indexes where indexdef ~ 'USING hash'),
t2(sql_create,sql_drop) as (select regexp_replace(sql,'CREATE INDEX','CREATE INDEX CONCURRENTLY'), 'DROP INDEX CONCURRENTLY '||quote_ident(nsp)||'.'||quote_ident(idx) from t1)
select regexp_replace(sql_create,'CONCURRENTLY (.*) ON','CONCURRENTLY \1_0926 ON')  ||'; '|| sql_drop ||'; ' from t2;
                                            ?column?
-------------------------------------------------------------------------------------------------
 CREATE INDEX CONCURRENTLY hi1_0926 ON t USING btree (id); DROP INDEX CONCURRENTLY public.hi1;
 CREATE INDEX CONCURRENTLY hi2_0926 ON s1.tbl USING btree (id); DROP INDEX CONCURRENTLY s1.hi2;
(2 rows)

或者在每个数据库调用这个inline code：
do language plpgsql $$
declare
  v_sql text;
  v_schema name;
  v_idx name;
  sql1 text;
begin
for v_sql,v_schema,v_idx in select regexp_replace(indexdef,'USING hash','USING btree'),schemaname,indexname from pg_indexes where indexdef ~ 'USING hash'
loop
  sql1='DROP INDEX '||quote_ident(v_schema)||'.'||quote_ident(v_idx);
  execute sql1;
  execute v_sql;
end loop;
end;
$$; 

postgres=# \d t
       Table "public.t"
 Column |  Type   | Modifiers
--------+---------+-----------
 id     | integer | not null
Indexes:
    "t_pkey" PRIMARY KEY, btree (id)
    "i1" btree (id)
    "i2" btree (id)

postgres=# \d s1.tbl
        Table "s1.tbl"
 Column |  Type   | Modifiers
--------+---------+-----------
 id     | integer |
Indexes:
    "i1" btree (id)

关于hash和btree性能：
查询性能：
postgres=# create table tbl(id int, info text);
CREATE TABLE
postgres=# insert into tbl select generate_series(1,1000000);
INSERT 0 1000000
postgres=# create index idx_tbl1 on tbl using hash (id);

$ vi test.sql
\setrandom id 1 1000000
select * from tbl where id=:id;

postgres@digoal-> pgbench -M prepared -n -r -P 1 -f ./test.sql -c 8 -j 8 -T 30
progress: 1.0 s, 24219.3 tps, lat 0.258 ms stddev 0.436
progress: 2.0 s, 29387.1 tps, lat 0.270 ms stddev 0.401
progress: 3.0 s, 29281.0 tps, lat 0.271 ms stddev 0.442
progress: 4.0 s, 29231.7 tps, lat 0.272 ms stddev 0.844
......
transaction type: Custom query
scaling factor: 1
query mode: prepared
number of clients: 8
number of threads: 8
duration: 30 s
number of transactions actually processed: 876806
latency average: 0.270 ms
latency stddev: 0.592 ms
tps = 29202.503991 (including connections establishing)
tps = 29379.956438 (excluding connections establishing)
statement latencies in milliseconds:
        0.003062        \setrandom id 1 1000000
        0.266481        select * from tbl where id=:id;

postgres=# drop index idx_tbl1 ;
DROP INDEX
postgres=# create index idx_tbl1 on tbl using btree (id);
CREATE INDEX

postgres@digoal-> pgbench -M prepared -n -r -P 1 -f ./test.sql -c 8 -j 8 -T 30
progress: 1.0 s, 28414.2 tps, lat 0.240 ms stddev 0.306
progress: 2.0 s, 31192.2 tps, lat 0.255 ms stddev 0.605
progress: 3.0 s, 31022.8 tps, lat 0.256 ms stddev 0.451
progress: 4.0 s, 29587.1 tps, lat 0.268 ms stddev 0.671
......
transaction type: Custom query
scaling factor: 1
query mode: prepared
number of clients: 8
number of threads: 8
duration: 30 s
number of transactions actually processed: 903467
latency average: 0.263 ms
latency stddev: 0.678 ms
tps = 30088.054150 (including connections establishing)
tps = 30229.295069 (excluding connections establishing)
statement latencies in milliseconds:
        0.002900        \setrandom id 1 1000000
        0.259402        select * from tbl where id=:id;

更新性能
$ vi test.sql
\setrandom id 1 1000000
update tbl set id=1+:id where id=:id;

postgres@digoal-> pgbench -M prepared -n -r -P 1 -f ./test.sql -c 8 -j 8 -T 30
progress: 1.0 s, 12500.2 tps, lat 0.570 ms stddev 0.864
progress: 2.0 s, 17456.9 tps, lat 0.456 ms stddev 0.641
progress: 3.0 s, 18242.3 tps, lat 0.435 ms stddev 0.234
progress: 4.0 s, 17693.0 tps, lat 0.450 ms stddev 0.909
progress: 5.0 s, 17753.3 tps, lat 0.448 ms stddev 0.758
......
transaction type: Custom query
scaling factor: 1
query mode: prepared
number of clients: 8
number of threads: 8
duration: 30 s
number of transactions actually processed: 521331
latency average: 0.456 ms
latency stddev: 0.702 ms
tps = 17372.386945 (including connections establishing)
tps = 17430.542432 (excluding connections establishing)
statement latencies in milliseconds:
        0.003072        \setrandom id 1 1000000
        0.452489        update tbl set id=1+:id where id=:id;

postgres=# drop index idx_tbl1 ;
DROP INDEX
postgres=# create index idx_tbl1 on tbl using hash (id);
CREATE INDEX

postgres@digoal-> pgbench -M prepared -n -r -P 1 -f ./test.sql -c 8 -j 8 -T 30
progress: 1.0 s, 16321.8 tps, lat 0.411 ms stddev 0.521
progress: 2.0 s, 17372.0 tps, lat 0.458 ms stddev 0.409
progress: 3.0 s, 16731.5 tps, lat 0.475 ms stddev 1.094
progress: 4.0 s, 16972.9 tps, lat 0.469 ms stddev 0.880
progress: 5.0 s, 17392.7 tps, lat 0.457 ms stddev 0.607
......
transaction type: Custom query
scaling factor: 1
query mode: prepared
number of clients: 8
number of threads: 8
duration: 30 s
number of transactions actually processed: 527778
latency average: 0.450 ms
latency stddev: 0.678 ms
tps = 17587.300360 (including connections establishing)
tps = 17671.181609 (excluding connections establishing)
statement latencies in milliseconds:
        0.002955        \setrandom id 1 1000000
        0.446435        update tbl set id=1+:id where id=:id;

SIZE
postgres=# create index idx_tbl1 on tbl using hash (id);
CREATE INDEX
postgres=# create index idx_tbl2 on tbl using btree (id);
CREATE INDEX
postgres=# \di+ idx_tbl*
                         List of relations
 Schema |   Name   | Type  |  Owner   | Table | Size  | Description
--------+----------+-------+----------+-------+-------+-------------
 public | idx_tbl1 | index | postgres | tbl   | 32 MB |
 public | idx_tbl2 | index | postgres | tbl   | 21 MB |
(2 rows)

小结:
查询和更新性能相差无几。
大小，btree是hash的2/3。