为什么jsonb列上的gin索引减慢了查询速度，我该怎么办？

10

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->类型和限制进行过滤：

-- 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->类型过滤，按数据->索引和限制排序

-- 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-与第二个查询（上一个查询）相同，但在数据->索引上具有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进行过滤

-- 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"
*/

第五查询-与第四查询相同，但对数据具有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（也是最后一个）查询-与第三查询相同（按数据->类型查询，按数据->索引排序，限制）：

-- 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（与第三）查询似乎慢得多。可能是因为data-> type字段没有很多不同的值（只有“ type1”或“ type2”）？我该怎么办？我需要杜松子酒索引来进行其他查询，以受益于它...

postgresql  postgresql-9.4 
— 用户名
source

Answers:

5

好像您遇到了一个问题，即jsonb列的统计率只有1％，如此处报道的那样。解决jsonb缺少统计信息的问题？。查看您的查询计划，估计与实际执行之间的差异是巨大的。据估计，大概有200行，而实际收益为100158行，这使计划者偏向于某些策略。

由于第六个查询中的选择似乎归结为倾向于使用位图索引扫描而不是索引扫描，因此您可以轻推计划程序，并SET enable_bitmapscan=off尝试使其恢复为第三个示例中的行为。

这对我来说是这样的：

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:=#

如果您打算采用这种方式，请确保仅对显示此类行为的查询禁用该扫描，否则，在其他查询计划上也会出现不良行为。做这样的事情应该很好：

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=）

— 卡桑德里
source
我不确定我是否理解正确（我不熟悉PG内部）-这种现象是由jsonb列中“类型”字段的基数低引起的（内部是由统一统计率引起的），对吗？这也意味着，如果要优化查询，必须知道要查询的jsonb字段的大致基数，以确定是否应该启用enable_bitmapscan，对吗？
— 2015年
1
是的，您似乎在两个方面都了解这一点。基本的1％选择性有利于查看WHEREgin索引中子句中的字段，因为它认为它将返回较少的行，这是不正确的。由于您可以更好地估计行数，因此您可以看到，由于正在执行ORDER BY data->'index' LIMIT 25，因此扫描另一个索引的前几个条目（50个左右，已丢弃的行）将减少行数，因此对于计划程序，它实际上不应该尝试使用位图扫描结果来使用更快的查询计划。希望这能说明问题。=）
— 卡桑德里2015年
1
这里还有其他澄清信息：databasesoup.com/2015/01/tag-all-things-part-3.html，在此演示文稿中，还提供了build.com/presentations/json2015-pgconfus.pdf来提供帮助。
— 卡萨德里（Kassandry）
1
我所知道的唯一工作是Oleg Bartunov，Tedor Sigaev和Alexander Kotorov对JsQuery扩展及其选择性的改进。幸运的是，它在9.6或更高版本中进入了PostgreSQL核心。
— 卡萨德里（Kassandry）
1
我在PostgreSQL核心团队成员Josh Berkus的回答中引用了电子邮件中1％的数字。抱歉，这是从何而来的呢？需要比我现在拥有的要多得多的内部知识。=（您可以尝试回复pgsql-performance@postgresql.orgfreenode IRC或在该节点上检查#postgresql该数字的确切来源
— 。– Kassandry
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