问题描述
我的 postgresql 版本是 10.6。我创建了一个索引，但它并未用于所有 where 子句条件检查。以下是更多详细信息：
Create index concurrently ticket_created_at_portal_id_created_by_id_assigned_group_id_idx on ticket(created_at,portal_id,created_by_id,assigned_group);
EXPLAIN (analyze true,verbose true,costs true,buffers true,timing true ) select * from ticket where status is not null
and (assigned_group in ('447') or created_by_id in ('39731566'))
and portal_id=8
and created_at>='2020-12-07T03:00:10.973'
and created_at<='2021-02-05T03:00:10.973'
order by updated_at DESC limit 10;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=18975.23..18975.25 rows=10 width=638) (actual time=278.340..278.345 rows=10 loops=1)
Output: id,action,assigned_agent,assigned_at,assigned_group,attachments,closed_at,created_at,created_by_email,description,first_response_time,parent_id,priority,reopened_at,resolution_id,resolved_at,resolved_by_id,resource_id,resource_type,source,status,subject,tags,ticket_category,ticket_id,ticket_sub_category,ticket_sub_sub_category,type,updated_at,custom_fields,updated_by_id,first_assigned_at,mode,sla_breached,agent_assist_tags,comm_vendor,from_email
Buffers: shared hit=2280 read=3105
-> Sort (cost=18975.23..18975.45 rows=87 width=638) (actual time=278.338..278.339 rows=10 loops=1)
Output: id,from_email
Sort Key: ticket.updated_at DESC
Sort Method: top-N heapsort Memory: 33kB
Buffers: shared hit=2280 read=3105
-> Bitmap Heap Scan on public.ticket (cost=17855.76..18973.35 rows=87 width=638) (actual time=111.871..275.835 rows=1256 loops=1)
Output: id,from_email
Recheck Cond: (((ticket.assigned_group = '447'::bigint) AND (ticket.portal_id = 8)) OR ((ticket.created_at >= '2020-12-07 03:00:10.973'::timestamp without time zone) AND (ticket.created_at <= '2021-02-05 03:00:10.973'::timestamp without time zone) AND (ticket.portal_id = 8) AND (ticket.created_by_id = '39731566'::bigint)))
Filter: ((ticket.status IS NOT NULL) AND (ticket.created_at >= '2020-12-07 03:00:10.973'::timestamp without time zone) AND (ticket.created_at <= '2021-02-05 03:00:10.973'::timestamp without time zone))
Rows Removed by Filter: 1517
Heap Blocks: exact=2638
Buffers: shared hit=2277 read=3105
-> BitmapOr (cost=17855.76..17855.76 rows=291 width=0) (actual time=106.215..106.216 rows=0 loops=1)
Buffers: shared hit=336 read=2408
-> Bitmap Index Scan on ticket_assigned_group_portal_id_assigned_agent_idx (cost=0.00..11.25 rows=282 width=0) (actual time=10.661..10.661 rows=2776 loops=1)
Index Cond: ((ticket.assigned_group = '447'::bigint) AND (ticket.portal_id = 8))
Buffers: shared hit=4 read=15
-> Bitmap Index Scan on ticket_created_at_portal_id_created_by_id_assigned_group_id_idx (cost=0.00..17844.47 rows=9 width=0) (actual time=95.551..95.551 rows=2 loops=1)
Index Cond: ((ticket.created_at >= '2020-12-07 03:00:10.973'::timestamp without time zone) AND (ticket.created_at <= '2021-02-05 03:00:10.973'::timestamp without time zone) AND (ticket.portal_id = 8) AND (ticket.created_by_id = '39731566'::bigint))
Buffers: shared hit=332 read=2393
Planning time: 43.083 ms
Execution time: 278.556 ms
(25 rows)
ticket_created_at_portal_id_created_by_id_assigned_group_id_idx 具有 where 子句的所有列，除了 status 不为空，但查询仍然使用单独的索引条件索引：((ticket.assigned_group = '447'::bigint) AND (ticket.portal_id = 8) ) 已经存在于第二个索引 ticket_created_at_portal_id_created_by_id_assigned_group_id_idx 中。
为什么会这样？即使我在索引中也包含状态列，查询仍然使用 2 个索引，并且对索引堆扫描进行了重过滤。
- 我们如何优化它？
我也对表的索引进行了实验，但仍然无法删除索引。对于差异查询，似乎在多个索引中重复了相同的列，如果我们可以减少这些索引数量，请提供帮助。 表的所有索引是：
"ticket_pkey" PRIMARY KEY,btree (id)
"ticket_ticket_id_idx" UNIQUE,btree (ticket_id)
"uk2uors84i0m8sjxc6oaocuy6oj" UNIQUE CONSTRAINT,btree (ticket_id)
"idx_resource_id" btree (resource_id)
"idx_ticket_created_at" btree (created_at)
"ticket_assigned_agent_idx" btree (assigned_agent)
"ticket_assigned_group_idx" btree (assigned_group)
"ticket_assigned_group_portal_id_assigned_agent_idx" btree (assigned_group,assigned_agent)
"ticket_created_at_portal_id_created_by_id_assigned_group_id_idx" btree (created_at,assigned_group)
"ticket_created_at_portal_id_status_idx" btree (created_at,status)
"ticket_id_resolved_at_assigned_group_status_idx" btree (id,status)
解决方法
使用电话簿（按姓氏和名字排序）找到名字为“Francis”且姓氏以 K 和 T 之间的字母开头的每个人有多容易？不是很容易，因为它不是按名字排序的。您将不得不浏览电话簿的整个中间部分，阅读每个人的名字。
这里也一样。当索引中的第一列用于范围/不等式查询而不是相等时，它会使之后的所有列效率低得多。您想先将用于相等的列放在 OR 中，而不是放在 OR 中。不幸的是，那只是portal_id。接下来的最佳选择取决于其他条件的选择性，我们无法从提供的信息中猜测这些条件。
在决定这一点时，status IS NULL 与相等是一回事，但 status IS NOT NULL 不是，因为它可能有任意数量的值同时仍然不为空，所以它实际上是相同的作为不平等。如果此条件具有高度选择性，则合并它的最佳方法是在部分索引的 WHERE 中。
由于 OR，您可能仍然最好使用 2 个可以组合在位图或中的索引。
...(portal_id,assigned_group,created_at) WHERE status IS NOT NULL;
...(portal_id,created_by_id,created_at) WHERE status IS NOT NULL;
另一种方法是避免获取和排序所有匹配的行，方法是通过 updated_at 使用索引按顺序遍历行并在找到 10 个后停止。只要在 ORDER BY 列之前仅发生经过相等性测试（并且没有 OR）的内容，就可以使用索引来按顺序遍历列，因此：
...(portal_id,updated_at) WHERE status IS NOT NULL;