[英]Postgres seems to be using the wrong index?
我正在使用Postgres 9.4。 这是我的桌子:
Table "public.frontend_prescription"
Column | Type | Modifiers
-------------------+-------------------------+--------------------------------------------------------------------
id | integer | not null default nextval('frontend_prescription_id_seq'::regclass)
presentation_code | character varying(15) | not null
total_items | integer | not null
processing_date | date | not null
practice_id | character varying(6) | not null
Indexes:
"frontend_prescription_pkey" PRIMARY KEY, btree (id)
"frontend_prescription_6ea07fe3" btree (practice_id)
"frontend_prescription_by_practice" btree (presentation_code, practice_id)
"frontend_prescription_by_practice_and_code" btree (practice_id, presentation_code varchar_pattern_ops)
"frontend_prescription_idx_date_and_code" btree (processing_date, presentation_code)
这是我的查询:
EXPLAIN (analyse, verbose)
SELECT SUM(total_items) AS items, SUM(total_items) AS numerator
FROM frontend_prescription
WHERE ((presentation_code LIKE '0601012Z0%') OR (presentation_code LIKE '0601012X0%') OR (presentation_code LIKE '0601012V0%'))
AND (practice_id='A81001')
AND (processing_date='2016-01-01')
这是输出:
Aggregate (cost=12.26..12.27 rows=1 width=4) (actual time=16898.277..16898.277 rows=1 loops=1)
Output: sum(total_items), sum(total_items)
-> Index Scan using frontend_prescription_idx_date_and_code on public.frontend_prescription (cost=0.57..12.26 rows=1 width=4) (actual time=9220.091..16898.251 rows=6 loops=1)
Output: id, presentation_code, presentation_name, total_items, net_cost, actual_cost, quantity, processing_date, price_per_unit, chemical_id, pct_id, practice_id, sha_id
Index Cond: (frontend_prescription.processing_date = '2016-01-01'::date)
Filter: (((frontend_prescription.practice_id)::text = 'A81001'::text) AND (((frontend_prescription.presentation_code)::text ~~ '0601012Z0%'::text) OR ((frontend_prescription.presentation_code)::text ~~ '0601012X0%'::text) OR ((frontend_prescription.presentation_code)::text ~~ '0601012V0%'::text)))
Rows Removed by Filter: 10036400
Planning time: 6.054 ms
Execution time: 16898.366 ms
有人可以建议我如何加快这些查询的速度吗? 当frontend_prescription_by_practice_and_code
索引具有varchar选项时,Postgres为什么使用frontend_prescription_idx_date_and_code
索引,这会使事情变得更快,所以我真的不明白。
如果我创建三列索引,这可能会有所帮助?
对于此查询:
SELECT SUM(total_items) AS items, SUM(total_items) AS numerator
FROM frontend_prescription
WHERE ((presentation_code LIKE '0601012Z0%') OR (presentation_code LIKE '0601012X0%') OR (presentation_code LIKE '0601012V0%')
) AND
(practice_id = 'A81001') AND
(processing_date = '2016-01-01');
最好的索引是frontend_prescription(practice_id, processing_date, presentation_code, total_items)
上的复合索引。 最后一列不是严格必需的。 它使索引成为查询的覆盖索引; 换句话说,查询所需的所有信息都在索引中,因此数据页不是必需的。
前两列可以采用任何顺序,因为where
子句使用相等性。
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