postgresql的9.4版本出来有一段时间了，也更新了很多内容，其中之一是比较感兴趣的物化视图的更新，对比原先的物化视图语法，新增了一个CONCURRENTLY参数。

一、新语法：

--创建语法,未有更新
CREATE MATERIALIZED VIEW table_name[ (column_name [, ...] ) ][ WITH ( storage_parameter [= value] [, ... ] ) ][ TABLESPACE tablespace_name ]AS query[ WITH [ NO ] DATA ]--刷新语法
REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] name[ WITH [ NO ] DATA ]

二、数据准备：

[postgres@ ~]$ psql
psql (9.4.1)
Type "help" for help.postgres=# create table tbl_kenyon(id int,remark text);
CREATE TABLE
postgres=# insert into tbl_kenyon select generate_series(1,1000000),md5(random()::text);
INSERT 0 1000000
postgres=# select * from tbl_kenyon limit 10;id |              remark
----+----------------------------------1 | d4fc1c7440a4d1672028586c2bb765142 | 5c1590519fa47f02db2895146a5f62a43 | 1710ac4199746e9bfa188f1655d1f8574 | 6cae64191c2bc309a4884301e77b26ad5 | 813987a5c3af2d75bd0de6e288083b106 | c52baa42cda22c89719bfb59dde1f78b7 | 491003337ea4e887c5ac24d174c691c68 | 455cdf32b170fcf2b450c0b974fbf3109 | 43adb30aeb0a21ab35fdf97064ad1d2110 | 97dc1adc5484244a077e87ef36ecfe09
(10 rows)--创建简单的物化视图
postgres=# create materialized view mv_tbl_kenyon as select * from tbl_kenyon ;
SELECT 1000000
postgres=# \d+List of relationsSchema |     Name      |       Type        |  Owner   | Size  | Description
--------+---------------+-------------------+----------+-------+-------------public | mv_tbl_kenyon | materialized view | postgres | 65 MB | public | tbl_kenyon    | table             | postgres | 65 MB |
(2 rows)

三、测试用例:

--测试不带concurrently
postgres=# insert into tbl_kenyon values(1000001,md5(random()::text));
INSERT 0 1
postgres=# select max(id) from mv_tbl_kenyon ;max
---------1000000
(1 row)postgres=# \timing
Timing is on.
postgres=# refresh materialized view mv_tbl_kenyon ;
REFRESH MATERIALIZED VIEW
Time: 2056.460 ms--测试带concurrently，需要建一个唯一索引
postgres=# insert into tbl_kenyon values(1000002,md5(random()::text));
INSERT 0 1
Time: 9.434 mspostgres=# refresh materialized view concurrently mv_tbl_kenyon;
ERROR:  cannot refresh materialized view "public.mv_tbl_kenyon" concurrently
HINT:  Create a unique index with no WHERE clause on one or more columns of the materialized view.
Time: 22109.877 ms
postgres=# create unique index idx_ken on mv_tbl_kenyon(id);
CREATE INDEX
Time: 707.721 ms
postgres=# select max(id) from mv_tbl_kenyon ;max
---------1000001
(1 row)Time: 1.110 ms
postgres=# begin;
BEGIN
postgres=# refresh materialized view concurrently mv_tbl_kenyon;
REFRESH MATERIALIZED VIEW
Time: 24674.739 ms--如果在refresh的时候，前面加个begin;
--还能发现在开启的另外的session里面，是不会阻塞查询的,反之不加concurrently会阻塞
postgres=# select * from mv_tbl_kenyon limit 10;id |              remark
----+----------------------------------1 | d4fc1c7440a4d1672028586c2bb765142 | 5c1590519fa47f02db2895146a5f62a43 | 1710ac4199746e9bfa188f1655d1f8574 | 6cae64191c2bc309a4884301e77b26ad5 | 813987a5c3af2d75bd0de6e288083b106 | c52baa42cda22c89719bfb59dde1f78b7 | 491003337ea4e887c5ac24d174c691c68 | 455cdf32b170fcf2b450c0b974fbf3109 | 43adb30aeb0a21ab35fdf97064ad1d2110 | 97dc1adc5484244a077e87ef36ecfe09
(10 rows)

四、源码
 相关唯一索引的源码，在matview.c里面可以查看：

--先初始化唯一索引是false
foundUniqueIndex = false;--如果找到唯一索引赋值为true
if (foundUniqueIndex)appendStringInfoString(&querybuf, " AND ");colname = quote_identifier(NameStr((tupdesc->attrs[attnum - 1])->attname));appendStringInfo(&querybuf, "newdata.%s ", colname);type = attnumTypeId(matviewRel, attnum);op = lookup_type_cache(type, TYPECACHE_EQ_OPR)->eq_opr;mv_GenerateOper(&querybuf, op);appendStringInfo(&querybuf, " mv.%s", colname);foundUniqueIndex = true;--如果找不到唯一索引报error
if (!foundUniqueIndex)ereport(ERROR,(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),errmsg("cannot refresh materialized view \"%s\" concurrently",matviewname),errhint("Create a unique index with no WHERE clause on one or more columns of the materialized view.")));appendStringInfoString(&querybuf, " AND newdata OPERATOR(pg_catalog.*=) mv) ""WHERE newdata IS NULL OR mv IS NULL " "ORDER BY tid");

五、总结： 
1.新版的物化视图新增了concurrently参数，可以使在刷新视图时不会锁住该物化视图的查询工作 ,会对9.4之后，9.3之前的物化视图的查询，其会阻塞有关物化视图的查询
2.该参数的原理和优缺点与索引的concurrently类似，以时间来换取查询锁，刷新的速度会变得很慢 
3.增量刷新的参数还没有，比较遗憾