t_p为父表，t_p_1,t_p_2是子表

hank=> \d+ t_p
                                                 Table "hank.t_p"
 Column |            Type             |             Modifiers             | Storage  | Stats target | Description 
--------+-----------------------------+-----------------------------------+----------+--------------+-------------
 c1     | integer                     |                                   | plain    |              | 
 c2     | character varying(20)       |                                   | extended |              | 
 c3     | timestamp without time zone |                                   | plain    |              | 
 c4     | character varying(20)       |                                   | extended |              | 
 c5     | character varying(20)       | default 'test'::character varying | extended |              | 
 c6     | character varying(20)       | default 'hank'::character varying | extended |              | 
Child tables: t_p_1,
              t_p_2

hank=> \d t_p_2
                            Table "hank.t_p_2"
 Column |            Type             |             Modifiers             
--------+-----------------------------+-----------------------------------
 c1     | integer                     | 
 c2     | character varying(20)       | 
 c3     | timestamp without time zone | 
 c4     | character varying(20)       | 
 c5     | character varying(20)       | default 'test'::character varying
 c6     | character varying(20)       | default 'hank'::character varying
Check constraints:
    "t_p_201802" CHECK (c3 >= '2018-02-01'::date AND c3 < '2018-03-01'::date)
Inherits: t_p

hank=> \d t_p_1
                            Table "hank.t_p_1"
 Column |            Type             |             Modifiers             
--------+-----------------------------+-----------------------------------
 c1     | integer                     | 
 c2     | character varying(20)       | 
 c3     | timestamp without time zone | 
 c4     | character varying(20)       | 
 c5     | character varying(20)       | default 'test'::character varying
 c6     | character varying(20)       | default 'hank'::character varying
Check constraints:
    "t_p_201801" CHECK (c3 >= '2018-01-01'::date AND c3 < '2018-02-01'::date)
Inherits: t_p

查看分区筛选参数：
hank=> show constraint_exclusion ;
 constraint_exclusion 
----------------------
 partition
(1 row)
如果是to_date无法裁剪,扫描了父表和所有子表
hank=> explain  select * from t_p where c3>to_date('2018-02-02','yyyy-mm-dd');
                                   QUERY PLAN                                   
--------------------------------------------------------------------------------
 Gather  (cost=1000.00..76112.02 rows=3 width=208)
   Workers Planned: 3
   ->  Append  (cost=0.00..75111.72 rows=3 width=208)
         ->  Parallel Seq Scan on t_p  (cost=0.00..0.00 rows=1 width=244)
               Filter: (c3 > to_date('2018-02-02'::text, 'yyyy-mm-dd'::text))
         ->  Parallel Seq Scan on t_p_1  (cost=0.00..75098.46 rows=1 width=190)
               Filter: (c3 > to_date('2018-02-02'::text, 'yyyy-mm-dd'::text))
         ->  Parallel Seq Scan on t_p_2  (cost=0.00..13.26 rows=1 width=190)
               Filter: (c3 > to_date('2018-02-02'::text, 'yyyy-mm-dd'::text))
(9 rows)

date可以裁剪，过滤掉无关分区，只扫描父表和t_p_2,达到优化效果
hank=> explain  select * from t_p where c3>date'2018-02-02'; 
                          QUERY PLAN                          
--------------------------------------------------------------
 Append  (cost=0.00..14.62 rows=2 width=217)
   ->  Seq Scan on t_p  (cost=0.00..0.00 rows=1 width=244)
         Filter: (c3 > '2018-02-02'::date)
   ->  Seq Scan on t_p_2  (cost=0.00..14.62 rows=1 width=190)
         Filter: (c3 > '2018-02-02'::date)
(5 rows)

这里to_date为什么不能过滤掉无效分区呢，而date可以，因为过滤分区的首要条件是要immutable函数，to_date是stable函数，所以无效
查看to_date的稳定性
这里s是stable，i是immutable
hank=> select proname,provolatile,proargtypes from pg_proc where proname='to_date';
 proname | provolatile | proargtypes 
---------+-------------+-------------
 to_date | s           | 25 25
(1 row)
hank=> select oid,typname from pg_type where oid in (25);                                  
 oid | typname 
-----+---------
  25 | text
(1 row)

查看date稳定性
hank=> select proname,provolatile,proargtypes from pg_proc where proname='date';   
 proname | provolatile | proargtypes 
---------+-------------+-------------
 date    | s           | 1184
 date    | s           | 702
 date    | i           | 1114
(3 rows)

hank=> select oid,typname from pg_type where oid in (1184,702,1114);
 oid  |   typname   
------+-------------
  702 | abstime
 1114 | timestamp
 1184 | timestamptz