什麼是BRIN索引以及如何使用 - PolarDB

BRIN索引是塊級索引，有別於B-TREE等索引，BRIN記錄並不是以行號為單位記錄索引明細，而是記錄每個資料區塊或者每段連續的資料區塊的統計資訊。因此BRIN索引空間佔用特別的小，對資料寫入、更新、刪除的影響也很小。

操作符

應用情境

流式日誌資料，按時間順序不斷的順序插入。索引佔用空間小，效能要求高。

準備工作

建立表及插入資料

CREATE TABLE t_brin (id int, info text, crt_time timestamp);
INSERT INTO t_brin SELECT generate_series(1,1000000), md5(random()::text), clock_timestamp();

查詢資料

SELECT ctid,* FROM t_brin limit 3;

 ctid  | id | info                             | crt_time
 (0,1) |  1 | 81c3f4f603c0c17e45778b2dd2d72f4d | 2024-11-06 09:26:57.549121
 (0,2) |  2 | b4b77e95a1580480107b038776b3cc9c | 2024-11-06 09:26:57.551548
 (0,3) |  3 | 6ebf5ebdd3df3428c279de2d5c7aab9f | 2024-11-06 09:26:57.551558

建立索引

CREATE INDEX idx_t_brin_1 ON t_brin USING brin (id) WITH (pages_per_range=1);
CREATE INDEX idx_t_brin_2 ON t_brin USING brin (crt_time) WITH (pages_per_range=1);
CREATE INDEX idx_t_brin_3 ON t_brin(id);
CREATE INDEX idx_t_brin_4 ON t_brin(crt_time);

查詢索引

\di+

Schema   | Name         | Type  | Owner    | Table  | Size   | Description
wangjian | idx_t_brin_1 | index | wangjian | t_brin | 272 kB |
wangjian | idx_t_brin_2 | index | wangjian | t_brin | 352 kB |
wangjian | idx_t_brin_3 | index | wangjian | t_brin | 21 MB  |
wangjian | idx_t_brin_4 | index | wangjian | t_brin | 21 MB  |

樣本一

使用索引執行計畫。

EXPLAIN (analyze, verbose,timing,costs, buffers) SELECT * FROM t_brin WHERE id BETWEEN 100 AND 200;

Index Scan using idx_t_brin_3 on public.t_brin  (cost=0.42..5.79 rows=107 width=45) (actual time=0.006..0.020 rows=101 loops=1)
  Output: id, info, crt_time
  Index Cond: ((t_brin.id >= 100) AND (t_brin.id <= 200))
  Buffers: shared hit=5 (main=5 vm=0 fsm=0)
Query Identifier: -5761088690410512151
Planning:
  Buffers: shared hit=40 (main=38 vm=2 fsm=0)
Planning Time: 0.232 ms
Execution Time: 0.045 ms

刪除索引後，執行計畫。

DROP INDEX idx_t_brin_3;

EXPLAIN (analyze,verbose,timing,costs,buffers) SELECT * FROM t_brin WHERE id BETWEEN 100 AND 200;

Bitmap Heap Scan on public.t_brin  (cost=37.83..241.69 rows=107 width=45) (actual time=1.333..1.351 rows=101 loops=1)
  Output: id, info, crt_time
  Recheck Cond: ((t_brin.id >= 100) AND (t_brin.id <= 200))
  Rows Removed by Index Recheck: 93
  Heap Blocks: lossy=2
  Buffers: shared hit=51 (main=51 vm=0 fsm=0)
  ->  Bitmap Index Scan on idx_t_brin_1  (cost=0.00..37.80 rows=186 width=0) (actual time=1.327..1.327 rows=20 loops=1)
        Index Cond: ((t_brin.id >= 100) AND (t_brin.id <= 200))
        Buffers: shared hit=49 (main=49 vm=0 fsm=0)
Query Identifier: -5761088690410512151
Planning:
  Buffers: shared hit=5 (main=5 vm=0 fsm=0) dirtied=1 (main=0 vm=0 fsm=0)
Planning Time: 0.054 ms
Execution Time: 1.381 ms

樣本二

使用索引執行計畫。

EXPLAIN (analyze, verbose, timing, costs, buffers) SELECT * FROM t_brin WHERE crt_time BETWEEN '2017-06-27 22:50:19.172224' AND '2017-06-27 22:50:19.182224';

Index Scan using idx_t_brin_4 on public.t_brin  (cost=0.42..2.64 rows=1 width=45) (actual time=0.003..0.003 rows=0 loops=1)
  Output: id, info, crt_time
  Index Cond: ((t_brin.crt_time >= '2017-06-27 22:50:19.172224'::timestamp without time zone) AND (t_brin.crt_time <= '2017-06-27 22:50:19.182224'::timestamp without time zone))
  Buffers: shared hit=3 (main=3 vm=0 fsm=0)
Query Identifier: 2646955540723493075
Planning:
  Buffers: shared hit=9 (main=7 vm=2 fsm=0)
Planning Time: 0.061 ms
Execution Time: 0.019 ms

刪除索引後，執行計畫。

DROP INDEX idx_t_brin_4;

EXPLAIN (analyze, verbose, timing, costs, buffers) SELECT * FROM t_brin WHERE crt_time BETWEEN '2017-06-27 22:50:19.172224' AND '2017-06-27 22:50:19.182224';

Bitmap Heap Scan on public.t_brin  (cost=49.90..152.73 rows=1 width=45) (actual time=1.449..1.449 rows=0 loops=1)
  Output: id, info, crt_time
  Recheck Cond: ((t_brin.crt_time >= '2017-06-27 22:50:19.172224'::timestamp without time zone) AND (t_brin.crt_time <= '2017-06-27 22:50:19.182224'::timestamp without time zone))
  Buffers: shared hit=65 (main=65 vm=0 fsm=0)
  ->  Bitmap Index Scan on idx_t_brin_2  (cost=0.00..49.90 rows=93 width=0) (actual time=1.447..1.447 rows=0 loops=1)
     Index Cond: ((t_brin.crt_time >= '2017-06-27 22:50:19.172224'::timestamp without time zone) AND (t_brin.crt_time <= '2017-06-27 22:50:19.182224'::timestamp without time zone))
     Buffers: shared hit=65 (main=65 vm=0 fsm=0)
Query Identifier: 2646955540723493075
Planning:
  Buffers: shared hit=5 (main=5 vm=0 fsm=0) dirtied=1 (main=0 vm=0 fsm=0)
Planning Time: 0.058 ms
Execution Time: 1.477 ms