为什么 jsonb 列上的 gin 索引会减慢我的查询速度，我该怎么办？

Question

初始化测试数据：

CREATE EXTENSION IF NOT EXISTS pgcrypto;
CREATE TABLE docs (data JSONB NOT NULL DEFAULT '{}');
-- generate 200k documents, ~half with type: "type1" and another half with type: "type2", unique incremented index and random uuid per each row
INSERT INTO docs (data)
SELECT json_build_object('id', gen_random_uuid(), 'type', (CASE WHEN random() > 0.5 THEN 'type1' ELSE 'type2' END) ,'index', n)::JSONB
FROM generate_series(1, 200000) n;
-- inset one more row with explicit uuid to query by it later
INSERT INTO docs (data) VALUES (json_build_object('id', '30e84646-c5c5-492d-b7f7-c884d77d1e0a', 'type', 'type1' ,'index', 200001)::JSONB);

第一个查询 - 按数据过滤->类型和限制：

-- FAST ~19ms
EXPLAIN ANALYZE
SELECT * FROM docs
WHERE data @> '{"type": "type1"}'::JSONB
LIMIT 25;
/* "Limit  (cost=0.00..697.12 rows=25 width=90) (actual time=0.029..0.070 rows=25 loops=1)"
   "  ->  Seq Scan on docs  (cost=0.00..5577.00 rows=200 width=90) (actual time=0.028..0.061 rows=25 loops=1)"
   "        Filter: (data @> '{"type": "type1"}'::jsonb)"
   "        Rows Removed by Filter: 17"
   "Planning time: 0.069 ms"
   "Execution time: 0.098 ms" 
*/

第二个查询 - 按数据过滤->类型，按数据排序->索引和限制

-- SLOW ~250ms
EXPLAIN ANALYZE
SELECT * FROM docs
WHERE data @> '{"type": "type1"}'::JSONB
ORDER BY data->'index' -- added ORDER BY
LIMIT 25;

/* "Limit  (cost=5583.14..5583.21 rows=25 width=90) (actual time=236.750..236.754 rows=25 loops=1)"
   "  ->  Sort  (cost=5583.14..5583.64 rows=200 width=90) (actual time=236.750..236.750 rows=25 loops=1)"
   "        Sort Key: ((data -> 'index'::text))"
   "        Sort Method: top-N heapsort  Memory: 28kB"
   "        ->  Seq Scan on docs  (cost=0.00..5577.50 rows=200 width=90) (actual time=0.020..170.797 rows=100158 loops=1)"
   "              Filter: (data @> '{"type": "type1"}'::jsonb)"
   "              Rows Removed by Filter: 99842"
   "Planning time: 0.075 ms"
   "Execution time: 236.785 ms"
*/

第三个查询 - 与第二个（上一个）相同，但在 data->index 上有 btree 索引：

CREATE INDEX docs_data_index_idx ON docs ((data->'index'));

-- FAST ~19ms
EXPLAIN ANALYZE
SELECT * FROM docs
WHERE data @> '{"type": "type1"}'::JSONB
ORDER BY data->'index' -- added BTREE index on this field
LIMIT 25;
/* "Limit  (cost=0.42..2473.98 rows=25 width=90) (actual time=0.040..0.125 rows=25 loops=1)"
   "  ->  Index Scan using docs_data_index_idx on docs  (cost=0.42..19788.92 rows=200 width=90) (actual time=0.038..0.119 rows=25 loops=1)"
   "        Filter: (data @> '{"type": "type1"}'::jsonb)"
   "        Rows Removed by Filter: 17"
   "Planning time: 0.127 ms"
   "Execution time: 0.159 ms"
*/

第四个查询 - 现在按 data->id 和 limit = 1 过滤：

-- SLOW ~116ms
EXPLAIN ANALYZE
SELECT * FROM docs
WHERE data @> ('{"id": "30e84646-c5c5-492d-b7f7-c884d77d1e0a"}')::JSONB -- querying by "id" field now
LIMIT 1;
/* "Limit  (cost=0.00..27.89 rows=1 width=90) (actual time=97.990..97.990 rows=1 loops=1)"
   "  ->  Seq Scan on docs  (cost=0.00..5577.00 rows=200 width=90) (actual time=97.989..97.989 rows=1 loops=1)"
   "        Filter: (data @> '{"id": "30e84646-c5c5-492d-b7f7-c884d77d1e0a"}'::jsonb)"
   "        Rows Removed by Filter: 189999"
   "Planning time: 0.064 ms"
   "Execution time: 98.012 ms"
*/ 

第五个查询 - 与第四个相同，但在数据上有 gin (json_path_ops) 索引：

CREATE INDEX docs_data_idx ON docs USING GIN (data jsonb_path_ops);

-- FAST ~17ms
EXPLAIN ANALYZE
SELECT * FROM docs
WHERE data @> '{"id": "30e84646-c5c5-492d-b7f7-c884d77d1e0a"}'::JSONB -- added gin index with json_path_ops
LIMIT 1;
/* "Limit  (cost=17.55..20.71 rows=1 width=90) (actual time=0.027..0.027 rows=1 loops=1)"
   "  ->  Bitmap Heap Scan on docs  (cost=17.55..649.91 rows=200 width=90) (actual time=0.026..0.026 rows=1 loops=1)"
   "        Recheck Cond: (data @> '{"id": "30e84646-c5c5-492d-b7f7-c884d77d1e0a"}'::jsonb)"
   "        Heap Blocks: exact=1"
   "        ->  Bitmap Index Scan on docs_data_idx  (cost=0.00..17.50 rows=200 width=0) (actual time=0.016..0.016 rows=1 loops=1)"
   "              Index Cond: (data @> '{"id": "30e84646-c5c5-492d-b7f7-c884d77d1e0a"}'::jsonb)"
   "Planning time: 0.095 ms"
   "Execution time: 0.055 ms"
*/

第六个（也是最后一个）查询 - 与第三个查询相同（按数据->类型查询，按数据->索引，限制排序）：

-- SLOW AGAIN! ~224ms
EXPLAIN ANALYZE
SELECT * FROM docs
WHERE data @> '{"type": "type1"}'::JSONB
ORDER BY data->'index'
LIMIT 25;
/* "Limit  (cost=656.06..656.12 rows=25 width=90) (actual time=215.927..215.932 rows=25 loops=1)"
   "  ->  Sort  (cost=656.06..656.56 rows=200 width=90) (actual time=215.925..215.925 rows=25 loops=1)"
   "        Sort Key: ((data -> 'index'::text))"
   "        Sort Method: top-N heapsort  Memory: 28kB"
   "        ->  Bitmap Heap Scan on docs  (cost=17.55..650.41 rows=200 width=90) (actual time=33.134..152.618 rows=100158 loops=1)"
   "              Recheck Cond: (data @> '{"type": "type1"}'::jsonb)"
   "              Heap Blocks: exact=3077"
   "              ->  Bitmap Index Scan on docs_data_idx  (cost=0.00..17.50 rows=200 width=0) (actual time=32.468..32.468 rows=100158 loops=1)"
   "                    Index Cond: (data @> '{"type": "type1"}'::jsonb)"
   "Planning time: 0.157 ms"
   "Execution time: 215.992 ms"
*/

因此，当数据列上有 gin 索引时，第六次（与第三次相同）查询似乎要慢得多。可能是因为 data->type 字段没有很多不同的值（只有“type1”或“type2”）？我能做些什么？我需要 gin 索引来进行其他查询以从中受益...

Answer 1

看起来您遇到了jsonb列具有 1% 的统一统计率的问题，如此处报告的那样解决 jsonb 缺乏统计数据？. 查看您的查询计划，估计和实际执行之间的差异是巨大的。估计说可能有 200 行，实际返回 100158 行，这导致计划者偏爱某些策略而不是其他策略。

由于第六个查询中的选择似乎归结为支持位图索引扫描而不是索引扫描，因此您可以轻推规划器并SET enable_bitmapscan=off尝试让它恢复到您在第三个示例中的行为。

这是它对我的工作方式：

postgres@[local]:5432:postgres:=# EXPLAIN (ANALYZE, BUFFERS)
SELECT * FROM docs
WHERE data @> '{"type": "type1"}'::JSONB
ORDER BY data->'index'
LIMIT 25;
                                                                QUERY PLAN                                                                 
-------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=656.06..656.12 rows=25 width=90) (actual time=117.338..117.343 rows=25 loops=1)
   Buffers: shared hit=3096
   ->  Sort  (cost=656.06..656.56 rows=200 width=90) (actual time=117.336..117.338 rows=25 loops=1)
         Sort Key: ((data -> 'index'::text))
         Sort Method: top-N heapsort  Memory: 28kB
         Buffers: shared hit=3096
         ->  Bitmap Heap Scan on docs  (cost=17.55..650.41 rows=200 width=90) (actual time=12.838..80.584 rows=99973 loops=1)
               Recheck Cond: (data @> '{"type": "type1"}'::jsonb)
               Heap Blocks: exact=3077
               Buffers: shared hit=3096
               ->  Bitmap Index Scan on docs_data_idx  (cost=0.00..17.50 rows=200 width=0) (actual time=12.469..12.469 rows=99973 loops=1)
                     Index Cond: (data @> '{"type": "type1"}'::jsonb)
                     Buffers: shared hit=19
 Planning time: 0.088 ms
 Execution time: 117.405 ms
(15 rows)

Time: 117.813 ms
postgres@[local]:5432:postgres:=# SET enable_bitmapscan = off;
SET
Time: 0.130 ms
postgres@[local]:5432:postgres:=# EXPLAIN (ANALYZE, BUFFERS)
SELECT * FROM docs
WHERE data @> '{"type": "type1"}'::JSONB
ORDER BY data->'index'
LIMIT 25;
                                                               QUERY PLAN                                                               
----------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.42..1320.48 rows=25 width=90) (actual time=0.017..0.050 rows=25 loops=1)
   Buffers: shared hit=4
   ->  Index Scan using docs_data_index_idx on docs  (cost=0.42..10560.94 rows=200 width=90) (actual time=0.015..0.045 rows=25 loops=1)
         Filter: (data @> '{"type": "type1"}'::jsonb)
         Rows Removed by Filter: 27
         Buffers: shared hit=4
 Planning time: 0.083 ms
 Execution time: 0.071 ms
(8 rows)

Time: 0.402 ms
postgres@[local]:5432:postgres:=#

如果您想走这条路线，请确保仅对显示此类行为的查询禁用该扫描，否则，您也会在其他查询计划中获得不良行为。做这样的事情应该可以正常工作：

BEGIN;
SET enable_bitmapscan=off;
SELECT * FROM docs
WHERE data @> '{"type": "type1"}'::JSONB
ORDER BY data->'index'
LIMIT 25;
SET enable_bitmapscan=on;
COMMIT;

希望有帮助=）