PostgreSQL 11 新特性之雜湊分割槽
阿新 • • 發佈:2019-01-09
文章目錄
PostgreSQL 10 引入了宣告式分割槽(declarative partitioning)特性,但是實現的功能有限。PostgreSQL 11 為此帶來了許多關於分割槽的增強功能。首先就是增加了 HASH 分割槽。雜湊分割槽基於分割槽鍵的雜湊值計算資料所在的分割槽。
CREATE TABLE htable (c1 bigint, c2 VARCHAR(10)) PARTITION BY HASH(c1);
為雜湊分割槽表建立分割槽時,使用 FOR VALUES WITH 子句指定分割槽的計算方法,其中的 MODULUS 子句用於指定除數,REMAINDER 子句用於指定雜湊值被除後的餘數。
CREATE TABLE htable_p0 PARTITION OF htable FOR VALUES WITH (MODULUS 4, REMAINDER 0);
CREATE TABLE htable_p1 PARTITION OF htable FOR VALUES WITH (MODULUS 4, REMAINDER 1);
CREATE TABLE htable_p2 PARTITION OF htable FOR VALUES WITH (MODULUS 4, REMAINDER 2);
CREATE TABLE htable_p3 PARTITION OF htable FOR VALUES WITH (MODULUS 4, REMAINDER 3);
REMAINDER 子句需要指定一個小於 MODULUS 子句的值。如果指定的分割槽數量少於 MODULES 子句的值,將會導致無法插入某些資料,因為沒有用於儲存這些值的分割槽。
\d+ htable
Table "public.htable"
Column | Type | Collation | Nullable | Default | Storage | Stats target | Description
--------+-----------------------+-----------+----------+---------+----------+--------------+-------------
c1 | bigint | | | | plain | |
c2 | character varying(10) | | | | extended | |
Partition key: HASH (c1)
Partitions: htable_p0 FOR VALUES WITH (modulus 4, remainder 0),
htable_p1 FOR VALUES WITH (modulus 4, remainder 1),
htable_p2 FOR VALUES WITH (modulus 4, remainder 2),
htable_p3 FOR VALUES WITH (modulus 4, remainder 3)
為表 htable 生成一些資料,檢視資料在各個分割槽的分佈是否均勻:
INSERT INTO htable SELECT val,'val:'||val FROM generate_series(1,100000) val;
INSERT 0 100000
SELECT COUNT(*) FROM htable_p0;
count
-------
25126
(1 row)
SELECT COUNT(*) FROM htable_p1;
count
-------
24978
(1 row)
SELECT COUNT(*) FROM htable_p2;
count
-------
24971
(1 row)
SELECT COUNT(*) FROM htable_p3;
count
-------
24925
(1 row)
每個分割槽大概包含四分之一(25000)的資料。
對於雜湊分割槽,同樣支援分割槽裁剪(Partition Pruning):
show enable_partition_pruning;
enable_partition_pruning
--------------------------
on
EXPLAIN SELECT * FROM htable where c1 = 200;
QUERY PLAN
------------------------------------------------------------------
Append (cost=0.00..470.57 rows=1 width=17)
-> Seq Scan on htable_p3 (cost=0.00..470.56 rows=1 width=17)
Filter: (c1 = 200)
(3 rows)
官方文件:Table Partitioning