jet*_*157 5 postgresql execution-plan postgresql-performance
我这里有一个非常奇怪的问题。我们的搜索查询接收多个参数并通过不同表中的多个联接进行搜索。查询在我们运行的前几次运行良好，然后有时会严重减慢，例如：从 200 毫秒到 15000/20000 毫秒。在同一个查询上。
最初我认为这是后端 ORM 问题，但在彻底检查并打开慢速查询日志、打开 auto_explain 功能后，我意识到查询计划在几次调用后发生了变化。我们在后端放置一个循环，在同一线程上多次执行查询（用于测试），我发现几乎总是在执行 8 次后数据库开始记录慢速查询。最后结果是查询计划发生了变化。最重要的变化是（我认为）TimeSlots 表上的变化，从 seq 扫描到位图索引扫描，它估计的行数比表中的行数少得多。估计：566 实际：100 000 左右。
我之前尝试过的事情：
这是选择语句：
SELECT DISTINCT u."UserId" AS UserId,
cast(cuj.company_id as bigint) AS CompanyId,
cast(ts.SlotsCount as bigint) as SlotsCount,
case when cuj.vip is null then false else cuj.vip end vip,
u.is_vip as userVip,
case
when ts2.EarliestDate isnull then '7777-12-01 21:00:00.000000'
else ts2.EarliestDate end EarliestDate
from "Users" u
left JOIN "UserSpecialty" usp ON usp."UserId" = u."UserId"
left join "company_users_join" cuj ON cuj.user_id = u."UserId" and cuj.company_user_role =2 and cuj.status in(1,3)
LEFT JOIN "company_users_join_providers" cujp
ON cuj.id = cujp.company_users_join_id
left join ( SELECT T."DoctorId", dse.company_user_join_id, COALESCE(COUNT(*),0) as SlotsCount
FROM "TimeSlots" T
inner JOIN
"DoctorSchedule" dse
ON T."DoctorScheduleId" = dse."DoctorScheduleId"
WHERE (T."SlotDateTime" > :dateStart
AND T."SlotDateTime" < :dateEnd)
and (:specialtyId = 0 or t."SpecialityId" = :specialtyId)
and (:specialtyId = 0 or dse."SpecialityId" = :specialtyId)
AND (dse."EndDate" < T."SlotDateTime"
OR dse."EndDate" IS NULL)
AND T."DeleteDate" IS NULL
AND T."IsActive" = TRUE
and dse."DeleteDate" is null
GROUP BY T."DoctorId",dse.company_user_join_id) as ts
on ts."DoctorId" = u."UserId" and ((ts.company_user_join_id is null and cuj.company_id is null) or ts.company_user_join_id = cuj.id)
left join (SELECT t1."DoctorId", dse.company_user_join_id, min(t1."SlotDateTime") as EarliestDate
FROM "TimeSlots" t1
inner JOIN
"DoctorSchedule" dse
ON t1."DoctorScheduleId" = dse."DoctorScheduleId"
WHERE (
t1."SlotDateTime" > :dateStart
AND t1."SlotDateTime" < :dateEnd
)
and (:specialtyId =0 or t1."SpecialityId" = :specialtyId)
AND (
dse."EndDate" < t1."SlotDateTime"
OR dse."EndDate" IS NULL
)
and(:serviceTypeId = 0 or t1."ServiceTypeId" = :serviceTypeId )
AND t1."DeleteDate" IS NULL
AND dse."DeleteDate" is null
AND t1."IsActive" = TRUE
GROUP BY t1."DoctorId", dse.company_user_join_id) ts2
on ts2."DoctorId" = u."UserId" and ((ts2.company_user_join_id is null and cuj.company_id is null) or ts2.company_user_join_id = cuj.id)
and(:companyId =0 or cuj.company_id = :companyId)
and(:providerId =0 or cujp.provider_id = :providerId)
and(:specialtyId =0 or usp."SpecialtyId" = :specialtyId)
and (:onlyForeignDoctors = false or u.is_foreign_doctor = true)
and (:onlyForeignDoctors = true or LOWER(u."FullName") = :fullName or (u.is_foreign_doctor is null or u.is_foreign_doctor = false))
and LOWER(u."FullName") LIKE concat('%', :fullName, '%')
and ((length(:fullName) =0 and ts.SlotsCount >0) or (length(:fullName) >0 ))
and ts.SlotsCount >=0
ORDER BY userVip desc, vip desc, EarliestDate asc, SlotsCount desc offset 0 limit 20
这是一开始的查询计划（快速）：
Limit (cost=9689.40..9689.42 rows=1 width=30) (actual time=127.650..127.682 rows=20 loops=1)
Buffers: shared hit=2229 read=2013
-> Unique (cost=9689.40..9689.42 rows=1 width=30) (actual time=127.639..127.668 rows=20 loops=1)
Buffers: shared hit=2229 read=2013
-> Sort (cost=9689.40..9689.40 rows=1 width=30) (actual time=127.637..127.653 rows=51 loops=1)
Sort Key: u.is_vip DESC, (CASE WHEN (cuj.vip IS NULL) THEN false ELSE cuj.vip END) DESC, (CASE WHEN ((min(t1."SlotDateTime")) IS NULL) THEN '7777-12-01 21:00:00'::timestamp without time zone ELSE (min(t1."SlotDateTime")) END), (COALESCE(count(*), '0'::bigint)) DESC, u."UserId", cuj.company_id
Sort Method: quicksort Memory: 57kB
Buffers: shared hit=2229 read=2013
-> Nested Loop Left Join (cost=9297.95..9689.39 rows=1 width=30) (actual time=122.171..127.367 rows=419 loops=1)
Join Filter: (cuj.id = cujp.company_users_join_id)
Rows Removed by Join Filter: 26015
Buffers: shared hit=2215 read=2013
-> Hash Right Join (cost=9297.95..9686.68 rows=1 width=38) (actual time=122.107..122.421 rows=391 loops=1)
Hash Cond: (t1."DoctorId" = u."UserId")
Join Filter: (((dse.company_user_join_id IS NULL) AND (cuj.company_id IS NULL)) OR (dse.company_user_join_id = cuj.id))
Rows Removed by Join Filter: 26
Buffers: shared hit=1825 read=2012
-> HashAggregate (cost=4100.24..4263.91 rows=16367 width=20) (actual time=37.779..37.953 rows=133 loops=1)
Group Key: t1."DoctorId", dse.company_user_join_id
Batches: 1 Memory Usage: 801kB
Buffers: shared hit=1268 read=53
-> Hash Join (cost=853.30..3977.49 rows=16367 width=20) (actual time=3.710..28.964 rows=45298 loops=1)
Hash Cond: (t1."DoctorScheduleId" = dse."DoctorScheduleId")
Join Filter: ((dse."EndDate" < t1."SlotDateTime") OR (dse."EndDate" IS NULL))
Buffers: shared hit=1268 read=53
-> Bitmap Heap Scan on "TimeSlots" t1 (cost=624.79..3638.82 rows=41926 width=16) (actual time=2.127..16.859 rows=45300 loops=1)
Recheck Cond: ("ServiceTypeId" = 1)
Filter: (("DeleteDate" IS NULL) AND "IsActive" AND ("SlotDateTime" > '2021-03-29 23:22:36.152'::timestamp without time zone) AND ("SlotDateTime" < '2021-04-28 23:22:36.152'::timestamp without time zone))
Rows Removed by Filter: 8302
Heap Blocks: exact=1157
Buffers: shared hit=1157 read=53
-> Bitmap Index Scan on timeslots4 (cost=0.00..614.31 rows=53602 width=0) (actual time=1.960..1.961 rows=53602 loops=1)
Index Cond: ("ServiceTypeId" = 1)
Buffers: shared read=53
-> Hash (cost=197.21..197.21 rows=2504 width=20) (actual time=1.537..1.538 rows=2504 loops=1)
Buckets: 4096 Batches: 1 Memory Usage: 132kB
Buffers: shared hit=111
-> Seq Scan on "DoctorSchedule" dse (cost=0.00..197.21 rows=2504 width=20) (actual time=0.009..1.110 rows=2504 loops=1)
Filter: ("DeleteDate" IS NULL)
Rows Removed by Filter: 3897
Buffers: shared hit=111
-> Hash (cost=5197.70..5197.70 rows=1 width=30) (actual time=84.315..84.322 rows=391 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 35kB
Buffers: shared hit=557 read=1959
-> Nested Loop Left Join (cost=4728.99..5197.70 rows=1 width=30) (actual time=83.475..84.219 rows=391 loops=1)
Buffers: shared hit=557 read=1959
-> Nested Loop Left Join (cost=4728.72..5197.30 rows=1 width=30) (actual time=83.461..83.934 rows=138 loops=1)
Filter: (((dse_1.company_user_join_id IS NULL) AND (cuj.company_id IS NULL)) OR (dse_1.company_user_join_id = cuj.id))
Rows Removed by Filter: 8
Buffers: shared hit=280 read=1959
-> Hash Join (cost=4728.57..5186.85 rows=39 width=21) (actual time=83.411..83.619 rows=139 loops=1)
Hash Cond: (t."DoctorId" = u."UserId")
Buffers: shared hit=2 read=1956
-> HashAggregate (cost=4668.48..5087.57 rows=3104 width=20) (actual time=81.842..82.001 rows=147 loops=1)
Group Key: t."DoctorId", dse_1.company_user_join_id
Filter: ((COALESCE(count(*), '0'::bigint) > 0) AND (COALESCE(count(*), '0'::bigint) >= 0))
Batches: 1 Memory Usage: 801kB
Buffers: shared hit=2 read=1839
-> Hash Join (cost=228.51..4236.18 rows=34584 width=12) (actual time=2.341..63.449 rows=95524 loops=1)
Hash Cond: (t."DoctorScheduleId" = dse_1."DoctorScheduleId")
Join Filter: ((dse_1."EndDate" < t."SlotDateTime") OR (dse_1."EndDate" IS NULL))
Buffers: shared hit=2 read=1839
-> Seq Scan on "TimeSlots" t (cost=0.00..3774.93 rows=88591 width=16) (actual time=0.018..38.614 rows=95531 loops=1)
Filter: (("DeleteDate" IS NULL) AND "IsActive" AND ("SlotDateTime" > '2021-03-29 23:22:36.152'::timestamp without time zone) AND ("SlotDateTime" < '2021-04-28 23:22:36.152'::timestamp without time zone))
Rows Removed by Filter: 17731
Buffers: shared hit=2 read=1728
-> Hash (cost=197.21..197.21 rows=2504 width=20) (actual time=2.293..2.294 rows=2504 loops=1)
Buckets: 4096 Batches: 1 Memory Usage: 132kB
Buffers: shared read=111
-> Seq Scan on "DoctorSchedule" dse_1 (cost=0.00..197.21 rows=2504 width=20) (actual time=0.012..1.782 rows=2504 loops=1)
Filter: ("DeleteDate" IS NULL)
Rows Removed by Filter: 3897
Buffers: shared read=111
-> Hash (cost=57.25..57.25 rows=227 width=5) (actual time=1.560..1.561 rows=218 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 16kB
Buffers: shared read=117
-> Index Scan using users__index_is_published on "Users" u (cost=0.29..57.25 rows=227 width=5) (actual time=0.180..1.507 rows=218 loops=1)
Index Cond: ("IsPublished" = true)
Filter: (((lower(("FullName")::text) = ''::text) OR (is_foreign_doctor IS NULL) OR (NOT is_foreign_doctor)) AND (lower(("FullName")::text) ~~ concat('%', ''::character varying, '%')))
Rows Removed by Filter: 9
Buffers: shared read=117
-> Index Scan using company_users_join__index_sch1 on company_users_join cuj (cost=0.14..0.26 rows=1 width=21) (actual time=0.002..0.002 rows=1 loops=139)
Index Cond: ((user_id = u."UserId") AND (company_user_role = 2))
Filter: (status = ANY ('{1,3}'::integer[]))
Buffers: shared hit=278 read=3
-> Index Only Scan using us1 on "UserSpecialty" usp (cost=0.28..0.38 rows=2 width=4) (actual time=0.001..0.001 rows=3 loops=138)
Index Cond: ("UserId" = u."UserId")
Heap Fetches: 0
Buffers: shared hit=277
-> Seq Scan on company_users_join_providers cujp (cost=0.00..1.87 rows=67 width=8) (actual time=0.001..0.006 rows=67 loops=391)
Buffers: shared hit=390 read=1
这是慢的一个
Limit (cost=2664.61..2664.63 rows=1 width=30) (actual time=377.322..377.348 rows=20 loops=1)
Buffers: shared hit=383194
-> Unique (cost=2664.61..2664.63 rows=1 width=30) (actual time=377.320..377.343 rows=20 loops=1)
Buffers: shared hit=383194
-> Sort (cost=2664.61..2664.62 rows=1 width=30) (actual time=377.319..377.330 rows=51 loops=1)
Sort Key: u.is_vip DESC, (CASE WHEN (cuj.vip IS NULL) THEN false ELSE cuj.vip END) DESC, (CASE WHEN ((min(t1."SlotDateTime")) IS NULL) THEN '7777-12-01 21:00:00'::timestamp without time zone ELSE (min(t1."SlotDateTime")) END), (COALESCE(count(*), '0'::bigint)) DESC, u."UserId", cuj.company_id
Sort Method: quicksort Memory: 57kB
Buffers: shared hit=383194
-> Merge Left Join (cost=2629.35..2664.60 rows=1 width=30) (actual time=318.926..377.019 rows=419 loops=1)
Merge Cond: (u."UserId" = t1."DoctorId")
Join Filter: (((dse_1.company_user_join_id IS NULL) AND (cuj.company_id IS NULL)) OR (dse_1.company_user_join_id = cuj.id))
Rows Removed by Join Filter: 26
Buffers: shared hit=383194
-> Nested Loop Left Join (cost=1363.46..1398.63 rows=1 width=30) (actual time=206.475..254.659 rows=419 loops=1)
Join Filter: (cuj.id = cujp.company_users_join_id)
Rows Removed by Join Filter: 26015
Filter: (($15 = 0) OR (cujp.provider_id = $16))
Buffers: shared hit=290878
-> Nested Loop Left Join (cost=1363.46..1395.58 rows=1 width=30) (actual time=206.460..249.574 rows=391 loops=1)
Join Filter: (cuj.user_id = u."UserId")
Rows Removed by Join Filter: 89798
Filter: ((((dse.company_user_join_id IS NULL) AND (cuj.company_id IS NULL)) OR (dse.company_user_join_id = cuj.id)) AND (($13 = 0) OR (cuj.company_id = $14)))
Rows Removed by Filter: 37
Buffers: shared hit=290487
-> Nested Loop Left Join (cost=1363.46..1384.83 rows=1 width=21) (actual time=206.445..226.437 rows=394 loops=1)
Filter: (($17 = 0) OR (usp."SpecialtyId" = $18))
Buffers: shared hit=289305
-> Nested Loop (cost=1359.17..1375.28 rows=1 width=21) (actual time=206.418..225.252 rows=139 loops=1)
Buffers: shared hit=288749
-> GroupAggregate (cost=1358.88..1358.93 rows=1 width=20) (actual time=206.366..224.117 rows=147 loops=1)
Group Key: t."DoctorId", dse.company_user_join_id
Filter: ((COALESCE(count(*), '0'::bigint) >= 0) AND (((length(($23)::text) = 0) AND (COALESCE(count(*), '0'::bigint) > 0)) OR (length(($24)::text) > 0)))
Buffers: shared hit=288308
-> Sort (cost=1358.88..1358.89 rows=1 width=12) (actual time=205.791..212.762 rows=95524 loops=1)
Sort Key: t."DoctorId", dse.company_user_join_id
Sort Method: quicksort Memory: 7550kB
Buffers: shared hit=288308
-> Nested Loop (cost=10.24..1358.87 rows=1 width=12) (actual time=3.708..181.475 rows=95524 loops=1)
Buffers: shared hit=288308
-> Bitmap Heap Scan on "TimeSlots" t (cost=9.96..1245.79 rows=16 width=16) (actual time=3.687..31.267 rows=95531 loops=1)
Recheck Cond: (("SlotDateTime" > $1) AND ("SlotDateTime" < $2))
Filter: (("DeleteDate" IS NULL) AND "IsActive" AND (($3 = 0) OR ("SpecialityId" = $4)))
Rows Removed by Filter: 389
问题是，在您的情况下，PostgreSQL 在前 5 次执行后切换到通用计划，并且通用计划的性能更差。特别是，对索引扫描的估计slotdatetime在通用计划中严重偏离。
PostgreSQL 有一个内置的启发式表单语句，其计划可以缓存：前 5 次它使用实际参数值来计划它们，然后如果该计划保证性能不会更差，则切换到通用计划。不幸的是，在这种情况下，通用计划并没有兑现承诺。
由于没有好的方法来修复通用计划，所以最好的办法就是避免它。您应该升级到 PostgreSQL v12 或更高版本，并在运行此查询时将参数设置plan_cache_mode为。force_custom_plan如果查询是在 PL/pgSQL 函数中执行的，您可以简单地
ALTER FUNCTION myfunc(argtype1,argtype2) SET plan_cache_mode = force_custom_plan;
另一种方法是使用动态 SQL EXECUTE，它将始终使用自定义计划。
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