I have created a dummy panda table with 1.000.000 rows of random data. Table has id primary key column of uuid type.
Using prisma I do a cursor based pagination forward and backward for the same id cursor. Forward generated query takes 900ms (~1s), backward takes 0.06ms (~0.006s)
Why first is slower and is there a way to improve it? Strange thing is that if I replace "order_cmp"."Panda_id_0" with actual value (second example) performance will be good again 0.06ms (~0.006s).
Forward:
explain analyze
select * from
"public"."panda",
(
select
"public"."panda"."id" as "Panda_id_0"
from
"public"."panda"
where
("public"."panda"."id") = ('ffff581b-ef0c-4cb8-9ece-25504e420348')) as "order_cmp"
where
"public"."panda"."id" >= "order_cmp"."Panda_id_0"
order by
"public"."panda"."id" asc
limit 4 offset 1;
Limit (cost=0.98..1.52 rows=4 width=86) (actual time=919.298..919.303 rows=4 loops=1)
-> Nested Loop (cost=0.85..44539.57 rows=333333 width=86) (actual time=919.295..919.301 rows=5 loops=1)
Join Filter: (panda.id >= panda_1.id)
Rows Removed by Join Filter: 999990
-> Index Scan using panda_pkey on panda (cost=0.42..29536.92 rows=1000000 width=70) (actual time=0.017..607.298 rows=999995 loops=1)
-> Materialize (cost=0.42..2.65 rows=1 width=16) (actual time=0.000..0.000 rows=1 loops=999995)
-> Index Only Scan using panda_pkey on panda panda_1 (cost=0.42..2.64 rows=1 width=16) (actual time=0.011..0.012 rows=1 loops=1)
Index Cond: (id = 'ffff581b-ef0c-4cb8-9ece-25504e420348'::uuid)
Heap Fetches: 0
Planning Time: 0.166 ms
Execution Time: 919.332 ms
Previous forward query with replaced "order_cmp"."Panda_id_0" to actual value
explain analyze
select * from
"public"."panda",
(
select
"public"."panda"."id" as "Panda_id_0"
from
"public"."panda"
where
("public"."panda"."id") = ('ffff581b-ef0c-4cb8-9ece-25504e420348')) as "order_cmp"
where
"public"."panda"."id" >= 'ffff581b-ef0c-4cb8-9ece-25504e420348'
order by
"public"."panda"."id" asc
limit 4 offset 1;
Limit (cost=1.73..5.26 rows=4 width=86) (actual time=0.032..0.040 rows=4 loops=1)
-> Nested Loop (cost=0.85..4412.29 rows=5001 width=86) (actual time=0.013..0.038 rows=5 loops=1)
-> Index Scan using panda_pkey on panda (cost=0.42..4347.13 rows=5001 width=70) (actual time=0.007..0.028 rows=5 loops=1)
Index Cond: (id >= 'ffff581b-ef0c-4cb8-9ece-25504e420348'::uuid)
-> Materialize (cost=0.42..2.65 rows=1 width=16) (actual time=0.001..0.001 rows=1 loops=5)
-> Index Only Scan using panda_pkey on panda panda_1 (cost=0.42..2.64 rows=1 width=16) (actual time=0.003..0.004 rows=1 loops=1)
Index Cond: (id = 'ffff581b-ef0c-4cb8-9ece-25504e420348'::uuid)
Heap Fetches: 0
Planning Time: 0.224 ms
Execution Time: 0.073 ms
Unrelated, just to compare:
Backward:
explain analyze
select * from
"public"."panda",
(
select
"public"."panda"."id" as "Panda_id_0"
from
"public"."panda"
where
("public"."panda"."id") = ('ffff581b-ef0c-4cb8-9ece-25504e420348')) as "order_cmp"
where
"public"."panda"."id" <= "order_cmp"."Panda_id_0"
order by
"public"."panda"."id" desc
limit 4 offset 1;
Limit (cost=0.97..1.51 rows=4 width=85) (actual time=0.025..0.030 rows=4 loops=1)
-> Nested Loop (cost=0.83..4503.05 rows=33280 width=85) (actual time=0.023..0.029 rows=5 loops=1)
Join Filter: (panda.id <= panda_1.id)
Rows Removed by Join Filter: 2
-> Index Scan Backward using panda_pkey on panda (cost=0.42..3002.81 rows=99840 width=69) (actual time=0.011..0.017 rows=7 loops=1)
-> Materialize (cost=0.42..2.64 rows=1 width=16) (actual time=0.001..0.001 rows=1 loops=7)
-> Index Only Scan using panda_pkey on panda panda_1 (cost=0.42..2.64 rows=1 width=16) (actual time=0.005..0.005 rows=1 loops=1)
Index Cond: (id = 'ffff581b-ef0c-4cb8-9ece-25504e420348'::uuid)
Heap Fetches: 0
Planning Time: 0.099 ms
Execution Time: 0.046 ms
Offset based:
explain analyze
select * from
"public"."panda"
order by
"public"."panda"."id" asc
limit 4 offset 500000;
Limit (cost=14768.67..14768.79 rows=4 width=70) (actual time=316.159..316.161 rows=4 loops=1)
-> Index Scan using panda_pkey on panda (cost=0.42..29536.92 rows=1000000 width=70) (actual time=0.012..298.960 rows=500004 loops=1)
Planning Time: 0.051 ms
Execution Time: 316.179 ms
Related
Indexes on table:
create index shifts_start_at_idx
on shifts (start_at);
Query 1 with at time zone:
SELECT shifts.id
FROM shifts
JOIN stores ON shifts.store_id = stores.id AND stores.deleted_at IS NULL
JOIN cities ON stores.city_id = cities.id
WHERE TRUE
AND (shifts.start_at >= '2022-05-06 03:00:00'::timestamp AT TIME ZONE
(EXTRACT(timezone FROM cities.time_zone) * INTERVAL '1 second'))
ORDER BY shifts.start_at DESC, shifts.end_at DESC, shifts.id DESC
LIMIT 100;
Explain query 1:
Limit (cost=0.86..298.93 rows=100 width=24) (actual time=0.143..25.257 rows=100 loops=1)
-> Nested Loop (cost=0.86..1485256.59 rows=498300 width=24) (actual time=0.131..23.317 rows=100 loops=1)
" Join Filter: (shifts.start_at >= timezone((date_part('timezone'::text, cities.time_zone) * '00:00:01'::interval), '2022-05-06 03:00:00'::timestamp without time zone))"
-> Nested Loop (cost=0.72..1209695.67 rows=1494900 width=32) (actual time=0.096..17.621 rows=100 loops=1)
-> Index Scan Backward using shifts_admin_order_by_idx on shifts (cost=0.43..291132.79 rows=3000000 width=32) (actual time=0.036..6.780 rows=205 loops=1)
-> Index Scan using stores_id_deleted_at_null_idx on stores (cost=0.29..0.31 rows=1 width=16) (actual time=0.025..0.025 rows=0 loops=205)
Index Cond: (id = shifts.store_id)
-> Index Scan using cities_pkey on cities (cost=0.14..0.16 rows=1 width=20) (actual time=0.017..0.017 rows=1 loops=100)
Index Cond: (id = stores.city_id)
Planning Time: 0.632 ms
Execution Time: 26.436 ms
Postgres doesn't use index
Query 2 without at time zone:
SELECT shifts.id
FROM shifts
JOIN stores ON shifts.store_id = stores.id AND stores.deleted_at IS NULL
JOIN cities ON stores.city_id = cities.id
WHERE TRUE
AND (shifts.start_at >= '2022-05-06 03:00:00')
ORDER BY shifts.start_at DESC, shifts.end_at DESC, shifts.id DESC
LIMIT 100;
Explain query 2:
Limit (cost=0.86..108.84 rows=100 width=24) (actual time=0.125..8.866 rows=100 loops=1)
-> Nested Loop (cost=0.86..898691.17 rows=832261 width=24) (actual time=0.115..7.886 rows=100 loops=1)
-> Nested Loop (cost=0.72..761958.37 rows=832261 width=32) (actual time=0.066..5.570 rows=100 loops=1)
-> Index Scan Backward using shifts_admin_order_by_idx on shifts (cost=0.43..248984.02 rows=1670200 width=32) (actual time=0.014..1.380 rows=205 loops=1)
Index Cond: (start_at >= '2022-05-06 03:00:00+00'::timestamp with time zone)
-> Index Scan using stores_id_deleted_at_null_idx on stores (cost=0.29..0.31 rows=1 width=16) (actual time=0.008..0.008 rows=0 loops=205)
Index Cond: (id = shifts.store_id)
-> Index Only Scan using cities_pkey on cities (cost=0.14..0.16 rows=1 width=8) (actual time=0.008..0.008 rows=1 loops=100)
Index Cond: (id = stores.city_id)
Heap Fetches: 100
Planning Time: 0.327 ms
Execution Time: 9.394 ms
It is not entirely clear why it does not want to use the index when converting the time to a time format with a timezone
I am trying to pass some ids into an in-clause on a sorted index with the same order by condition but the query planner is explicitly sorting the data after performing index search. below are my queries.
Generate a temporary table.
SELECT a.n/20 as n, md5(a.n::TEXT) as b INTO temp_table
From generate_series(1, 100000) as a(n);
create an index
CREATE INDEX idx_temp_table ON temp_table(n ASC, b ASC);
In below query, planner uses index ordering and doesn't explicitly sorts the data.(expected)
EXPLAIN ANALYSE
SELECT * from
temp_table WHERE n = 10
ORDER BY n, b
limit 5;
Query Plan
QUERY PLAN Limit (cost=0.42..16.07 rows=5 width=36) (actual time=0.098..0.101 rows=5 loops=1)
-> Index Only Scan using idx_temp_table on temp_table (cost=0.42..1565.17 rows=500 width=36) (actual time=0.095..0.098 rows=5 loops=1)
Index Cond: (n = 10)
Heap Fetches: 5 Planning time: 0.551 ms Execution time: 0.128 ms
but when i use one or more ids from a cte and pass them in clause then planner only uses index to fetch the values but explicitly sorts them afterwards (not expected).
EXPLAIN ANALYSE
WITH cte(x) AS (VALUES (10))
SELECT * from temp_table
WHERE n IN ( SELECT x from cte)
ORDER BY n, b
limit 5;
then planner uses below query plan
QUERY PLAN
QUERY PLAN
Limit (cost=85.18..85.20 rows=5 width=37) (actual time=0.073..0.075 rows=5 loops=1)
CTE cte
-> Values Scan on "*VALUES*" (cost=0.00..0.03 rows=2 width=4) (actual time=0.001..0.002 rows=2 loops=1)
-> Sort (cost=85.16..85.26 rows=40 width=37) (actual time=0.072..0.073 rows=5 loops=1)
Sort Key: temp_table.n, temp_table.b
Sort Method: top-N heapsort Memory: 25kB
-> Nested Loop (cost=0.47..84.50 rows=40 width=37) (actual time=0.037..0.056 rows=40 loops=1)
-> Unique (cost=0.05..0.06 rows=2 width=4) (actual time=0.009..0.010 rows=2 loops=1)
-> Sort (cost=0.05..0.06 rows=2 width=4) (actual time=0.009..0.010 rows=2 loops=1)
Sort Key: cte.x
Sort Method: quicksort Memory: 25kB
-> CTE Scan on cte (cost=0.00..0.04 rows=2 width=4) (actual time=0.004..0.005 rows=2 loops=1)
-> Index Only Scan using idx_temp_table on temp_table (cost=0.42..42.02 rows=20 width=37) (actual time=0.012..0.018 rows=20 loops=2)
Index Cond: (n = cte.x)
Heap Fetches: 40
Planning time: 0.166 ms
Execution time: 0.101 ms
I tried putting an explicit sorting while passing the ids in where clause so that sorted order in ids is maintained but still planner sorted explicitly
EXPLAIN ANALYSE
WITH cte(x) AS (VALUES (10))
SELECT * from temp_table
WHERE n IN ( SELECT x from cte)
ORDER BY n, b
limit 5;
Query plan
QUERY PLAN
Limit (cost=42.62..42.63 rows=5 width=37) (actual time=0.042..0.044 rows=5 loops=1)
CTE cte
-> Result (cost=0.00..0.01 rows=1 width=4) (actual time=0.000..0.000 rows=1 loops=1)
-> Sort (cost=42.61..42.66 rows=20 width=37) (actual time=0.042..0.042 rows=5 loops=1)
Sort Key: temp_table.n, temp_table.b
Sort Method: top-N heapsort Memory: 25kB
-> Nested Loop (cost=0.46..42.28 rows=20 width=37) (actual time=0.025..0.033 rows=20 loops=1)
-> HashAggregate (cost=0.05..0.06 rows=1 width=4) (actual time=0.009..0.009 rows=1 loops=1)
Group Key: cte.x
-> Sort (cost=0.03..0.04 rows=1 width=4) (actual time=0.006..0.006 rows=1 loops=1)
Sort Key: cte.x
Sort Method: quicksort Memory: 25kB
-> CTE Scan on cte (cost=0.00..0.02 rows=1 width=4) (actual time=0.003..0.003 rows=1 loops=1)
-> Index Only Scan using idx_temp_table on temp_table (cost=0.42..42.02 rows=20 width=37) (actual time=0.014..0.020 rows=20 loops=1)
Index Cond: (n = cte.x)
Heap Fetches: 20
Planning time: 0.167 ms
Execution time: 0.074 ms
Can anyone explain why planner is using an explicit sort on the data? Is there a way to by pass this and make planner use the index sorting order so additional sorting on the records can be saved. In production, we have similar case but size of our selection is too big but only a handful of records needs to fetched with pagination. Thanks in anticipation!
It is actually a decision made by the planner, with a larger set of values(), Postgres will switch to a smarter plan, with the sort done before the merge.
select version();
\echo +++++ Original
EXPLAIN ANALYSE
WITH cte(x) AS (VALUES (10))
SELECT * from temp_table
WHERE n IN ( SELECT x from cte)
ORDER BY n, b
limit 5;
\echo +++++ TEN Values
EXPLAIN ANALYSE
WITH cte(x) AS (VALUES (10),(11),(12),(13),(14),(15),(16),(17),(18),(19)
)
SELECT * from temp_table
WHERE n IN ( SELECT x from cte)
ORDER BY n, b
limit 5;
\echo ++++++++ one row from table
EXPLAIN ANALYSE
WITH cte(x) AS (SELECT n FROM temp_table WHERE n = 10)
SELECT * from temp_table
WHERE n IN ( SELECT x from cte)
ORDER BY n, b
limit 5;
\echo ++++++++ one row from table TWO ctes
EXPLAIN ANALYSE
WITH val(x) AS (VALUES (10))
, cte(x) AS (
SELECT n FROM temp_table WHERE n IN (select x from val)
)
SELECT * from temp_table
WHERE n IN ( SELECT x from cte)
ORDER BY n, b
limit 5;
Resulting plans:
version
-------------------------------------------------------------------------------------------------------
PostgreSQL 11.3 on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 4.8.4-2ubuntu1~14.04.4) 4.8.4, 64-bit
(1 row)
+++++ Original
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=13.72..13.73 rows=5 width=37) (actual time=0.197..0.200 rows=5 loops=1)
CTE cte
-> Result (cost=0.00..0.01 rows=1 width=4) (actual time=0.001..0.001 rows=1 loops=1)
-> Sort (cost=13.71..13.76 rows=20 width=37) (actual time=0.194..0.194 rows=5 loops=1)
Sort Key: temp_table.n, temp_table.b
Sort Method: top-N heapsort Memory: 25kB
-> Nested Loop (cost=0.44..13.37 rows=20 width=37) (actual time=0.083..0.097 rows=20 loops=1)
-> HashAggregate (cost=0.02..0.03 rows=1 width=4) (actual time=0.018..0.018 rows=1 loops=1)
Group Key: cte.x
-> CTE Scan on cte (cost=0.00..0.02 rows=1 width=4) (actual time=0.007..0.008 rows=1 loops=1)
-> Index Only Scan using idx_temp_table on temp_table (cost=0.42..13.14 rows=20 width=37) (actual time=0.058..0.068 rows=20 loops=1)
Index Cond: (n = cte.x)
Heap Fetches: 20
Planning Time: 1.328 ms
Execution Time: 0.360 ms
(15 rows)
+++++ TEN Values
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=0.91..89.11 rows=5 width=37) (actual time=0.179..0.183 rows=5 loops=1)
CTE cte
-> Values Scan on "*VALUES*" (cost=0.00..0.12 rows=10 width=4) (actual time=0.001..0.007 rows=10 loops=1)
-> Merge Semi Join (cost=0.78..3528.72 rows=200 width=37) (actual time=0.178..0.181 rows=5 loops=1)
Merge Cond: (temp_table.n = cte.x)
-> Index Only Scan using idx_temp_table on temp_table (cost=0.42..3276.30 rows=100000 width=37) (actual time=0.030..0.123 rows=204 loops=1)
Heap Fetches: 204
-> Sort (cost=0.37..0.39 rows=10 width=4) (actual time=0.023..0.023 rows=1 loops=1)
Sort Key: cte.x
Sort Method: quicksort Memory: 25kB
-> CTE Scan on cte (cost=0.00..0.20 rows=10 width=4) (actual time=0.003..0.013 rows=10 loops=1)
Planning Time: 0.197 ms
Execution Time: 0.226 ms
(13 rows)
++++++++ one row from table
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=14.39..58.52 rows=5 width=37) (actual time=0.168..0.173 rows=5 loops=1)
CTE cte
-> Index Only Scan using idx_temp_table on temp_table temp_table_1 (cost=0.42..13.14 rows=20 width=4) (actual time=0.010..0.020 rows=20 loops=1)
Index Cond: (n = 10)
Heap Fetches: 20
-> Merge Semi Join (cost=1.25..3531.24 rows=400 width=37) (actual time=0.167..0.170 rows=5 loops=1)
Merge Cond: (temp_table.n = cte.x)
-> Index Only Scan using idx_temp_table on temp_table (cost=0.42..3276.30 rows=100000 width=37) (actual time=0.025..0.101 rows=204 loops=1)
Heap Fetches: 204
-> Sort (cost=0.83..0.88 rows=20 width=4) (actual time=0.039..0.039 rows=1 loops=1)
Sort Key: cte.x
Sort Method: quicksort Memory: 25kB
-> CTE Scan on cte (cost=0.00..0.40 rows=20 width=4) (actual time=0.012..0.031 rows=20 loops=1)
Planning Time: 0.243 ms
Execution Time: 0.211 ms
(15 rows)
++++++++ one row from table TWO ctes
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=14.63..58.76 rows=5 width=37) (actual time=0.224..0.229 rows=5 loops=1)
CTE val
-> Result (cost=0.00..0.01 rows=1 width=4) (actual time=0.001..0.001 rows=1 loops=1)
CTE cte
-> Nested Loop (cost=0.44..13.37 rows=20 width=4) (actual time=0.038..0.052 rows=20 loops=1)
-> HashAggregate (cost=0.02..0.03 rows=1 width=4) (actual time=0.007..0.007 rows=1 loops=1)
Group Key: val.x
-> CTE Scan on val (cost=0.00..0.02 rows=1 width=4) (actual time=0.003..0.003 rows=1 loops=1)
-> Index Only Scan using idx_temp_table on temp_table temp_table_1 (cost=0.42..13.14 rows=20 width=4) (actual time=0.029..0.038 rows=20 loops=1)
Index Cond: (n = val.x)
Heap Fetches: 20
-> Merge Semi Join (cost=1.25..3531.24 rows=400 width=37) (actual time=0.223..0.226 rows=5 loops=1)
Merge Cond: (temp_table.n = cte.x)
-> Index Only Scan using idx_temp_table on temp_table (cost=0.42..3276.30 rows=100000 width=37) (actual time=0.038..0.114 rows=204 loops=1)
Heap Fetches: 204
-> Sort (cost=0.83..0.88 rows=20 width=4) (actual time=0.082..0.082 rows=1 loops=1)
Sort Key: cte.x
Sort Method: quicksort Memory: 25kB
-> CTE Scan on cte (cost=0.00..0.40 rows=20 width=4) (actual time=0.040..0.062 rows=20 loops=1)
Planning Time: 0.362 ms
Execution Time: 0.313 ms
(21 rows)
Beware of CTEs!.
For the planner, CTEs are more or less black boxes, and very little is known about expected number of rows, statistics distribution, or ordering inside.
In cases where CTEs result in a bad plan (the original question is not such a case), a CTE can often be replaced by a (temp) view, which is seen by the planner in its full naked glory.
Update
Starting with version 11, CTEs are handled differently by the planner: if they do not have side effects, they are candidates for being merged with the main query. (but is still a good idea to check your query plans)
The optimizet isn't aware that the CTE is sorted. If you scan an index for multiple values and have an ORDER BY, PostgreSQL will always sort.
The only thing that comes to my mind is to create a temporary table with the values from the IN list and put an index on that temporary table. Then when you join with that table, PostgreSQL will be aware of the ordering and might for example choose a merge join that can use the indexes.
Of course that means a lot of overhead, and it could easily be that the original sort wins out.
I am having these results for analyze for a simple query that does not return more than 150 records from tables less than 200 records most of them, as I have a table that stores latest value and the other fields are FK of the data.
Update: see the new results from same query some our later. The site is not public and/or there should be not users right now as it is in development.
explain analyze
SELECT lv.station_id,
s.name AS station_name,
s.latitude,
s.longitude,
s.elevation,
lv.element_id,
e.symbol AS element_symbol,
u.symbol,
e.name AS element_name,
lv.last_datetime AS datetime,
lv.last_value AS valor,
s.basin_id,
s.municipality_id
FROM (((element_station lv /*350 records*/
JOIN stations s ON ((lv.station_id = s.id))) /*40 records*/
JOIN elements e ON ((lv.element_id = e.id))) /*103 records*/
JOIN units u ON ((e.unit_id = u.id))) /* 32 records */
WHERE s.id = lv.station_id AND e.id = lv.element_id AND lv.interval_id = 6 and
lv.last_datetime >= ((now() - '06:00:00'::interval) - '01:00:00'::interval)
I have already tried VACUUM and after that some is saved, but again after some times it goes up. I have implemented an index on the fields.
Nested Loop (cost=0.29..2654.66 rows=1 width=92) (actual time=1219.390..35296.253 rows=157 loops=1)
Join Filter: (e.unit_id = u.id)
Rows Removed by Join Filter: 4867
-> Nested Loop (cost=0.29..2652.93 rows=1 width=92) (actual time=1219.383..35294.083 rows=157 loops=1)
Join Filter: (lv.element_id = e.id)
Rows Removed by Join Filter: 16014
-> Nested Loop (cost=0.29..2648.62 rows=1 width=61) (actual time=1219.301..35132.373 rows=157 loops=1)
-> Seq Scan on element_station lv (cost=0.00..2640.30 rows=1 width=20) (actual time=1219.248..1385.517 rows=157 loops=1)
Filter: ((interval_id = 6) AND (last_datetime >= ((now() - '06:00:00'::interval) - '01:00:00'::interval)))
Rows Removed by Filter: 168
-> Index Scan using stations_pkey on stations s (cost=0.29..8.31 rows=1 width=45) (actual time=3.471..214.941 rows=1 loops=157)
Index Cond: (id = lv.station_id)
-> Seq Scan on elements e (cost=0.00..3.03 rows=103 width=35) (actual time=0.003..0.999 rows=103 loops=157)
-> Seq Scan on units u (cost=0.00..1.32 rows=32 width=8) (actual time=0.002..0.005 rows=32 loops=157)
Planning time: 8.312 ms
Execution time: 35296.427 ms
update, same query running it tonight; no changes:
Sort (cost=601.74..601.88 rows=55 width=92) (actual time=1.822..1.841 rows=172 loops=1)
Sort Key: lv.last_datetime DESC
Sort Method: quicksort Memory: 52kB
-> Nested Loop (cost=11.60..600.15 rows=55 width=92) (actual time=0.287..1.680 rows=172 loops=1)
-> Hash Join (cost=11.31..248.15 rows=55 width=51) (actual time=0.263..0.616 rows=172 loops=1)
Hash Cond: (e.unit_id = u.id)
-> Hash Join (cost=9.59..245.60 rows=75 width=51) (actual time=0.225..0.528 rows=172 loops=1)
Hash Cond: (lv.element_id = e.id)
-> Bitmap Heap Scan on element_station lv (cost=5.27..240.25 rows=75 width=20) (actual time=0.150..0.359 rows=172 loops=1)
Recheck Cond: ((last_datetime >= ((now() - '06:00:00'::interval) - '01:00:00'::interval)) AND (interval_id = 6))
Heap Blocks: exact=22
-> Bitmap Index Scan on element_station_latest (cost=0.00..5.25 rows=75 width=0) (actual time=0.136..0.136 rows=226 loops=1)
Index Cond: ((last_datetime >= ((now() - '06:00:00'::interval) - '01:00:00'::interval)) AND (interval_id = 6))
-> Hash (cost=3.03..3.03 rows=103 width=35) (actual time=0.062..0.062 rows=103 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 15kB
-> Seq Scan on elements e (cost=0.00..3.03 rows=103 width=35) (actual time=0.006..0.031 rows=103 loops=1)
-> Hash (cost=1.32..1.32 rows=32 width=8) (actual time=0.019..0.019 rows=32 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 10kB
-> Seq Scan on units u (cost=0.00..1.32 rows=32 width=8) (actual time=0.003..0.005 rows=32 loops=1)
-> Index Scan using stations_pkey on stations s (cost=0.29..6.39 rows=1 width=45) (actual time=0.005..0.006 rows=1 loops=172)
Index Cond: (id = lv.station_id)
Planning time: 2.390 ms
Execution time: 2.009 ms
The problem is the misestimate of the number of rows in the sequential scan on element_station. Either autoanalyze has kicked in and calculated new statistics for the table or the data changed.
The problem is probably that PostgreSQL doesn't know the result of
((now() - '06:00:00'::interval) - '01:00:00'::interval)
at query planning time.
If that is possible for you, do it in two steps: First, calculate the expression above (either in PostgreSQL or on the client side). Then run the query with the result as a constant. That will make it easier for PostgreSQL to estimate the result count.
I'm using postgres v9.6.5. I have a query which seems not that complicated and was wondering why is it so "slow" (it's not really that slow, but I don't have a lot of data actually - like a few thousand rows).
Here is the query:
SELECT o0.*
FROM "orders" AS o0
JOIN "balances" AS b1 ON b1."id" = o0."balance_id"
JOIN "users" AS u3 ON u3."id" = b1."user_id"
WHERE (u3."partner_id" = 3)
ORDER BY o0."id" DESC LIMIT 10;
And that's query plan:
Limit (cost=0.43..12.84 rows=10 width=148) (actual time=0.062..53.866 rows=4 loops=1)
-> Nested Loop (cost=0.43..4750.03 rows=3826 width=148) (actual time=0.061..53.864 rows=4 loops=1)
Join Filter: (b1.user_id = u3.id)
Rows Removed by Join Filter: 67404
-> Nested Loop (cost=0.43..3945.32 rows=17856 width=152) (actual time=0.025..38.457 rows=16852 loops=1)
-> Index Scan Backward using orders_pkey on orders o0 (cost=0.29..897.80 rows=17856 width=148) (actual time=0.016..11.558 rows=16852 loops=1)
-> Index Scan using balances_pkey on balances b1 (cost=0.14..0.16 rows=1 width=8) (actual time=0.001..0.001 rows=1 loops=16852)
Index Cond: (id = o0.balance_id)
-> Materialize (cost=0.00..1.19 rows=3 width=4) (actual time=0.000..0.000 rows=4 loops=16852)
-> Seq Scan on users u3 (cost=0.00..1.18 rows=3 width=4) (actual time=0.023..0.030 rows=4 loops=1)
Filter: (partner_id = 3)
Rows Removed by Filter: 12
Planning time: 0.780 ms
Execution time: 54.053 ms
I actually tried without LIMIT and I got quite different plan:
Sort (cost=874.23..883.80 rows=3826 width=148) (actual time=11.361..11.362 rows=4 loops=1)
Sort Key: o0.id DESC
Sort Method: quicksort Memory: 26kB
-> Hash Join (cost=3.77..646.55 rows=3826 width=148) (actual time=11.300..11.346 rows=4 loops=1)
Hash Cond: (o0.balance_id = b1.id)
-> Seq Scan on orders o0 (cost=0.00..537.56 rows=17856 width=148) (actual time=0.012..8.464 rows=16852 loops=1)
-> Hash (cost=3.55..3.55 rows=18 width=4) (actual time=0.125..0.125 rows=24 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 9kB
-> Hash Join (cost=1.21..3.55 rows=18 width=4) (actual time=0.046..0.089 rows=24 loops=1)
Hash Cond: (b1.user_id = u3.id)
-> Seq Scan on balances b1 (cost=0.00..1.84 rows=84 width=8) (actual time=0.011..0.029 rows=96 loops=1)
-> Hash (cost=1.18..1.18 rows=3 width=4) (actual time=0.028..0.028 rows=4 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 9kB
-> Seq Scan on users u3 (cost=0.00..1.18 rows=3 width=4) (actual time=0.014..0.021 rows=4 loops=1)
Filter: (partner_id = 3)
Rows Removed by Filter: 12
Planning time: 0.569 ms
Execution time: 11.420 ms
And also without WHERE (but with LIMIT):
Limit (cost=0.43..4.74 rows=10 width=148) (actual time=0.023..0.066 rows=10 loops=1)
-> Nested Loop (cost=0.43..7696.26 rows=17856 width=148) (actual time=0.022..0.065 rows=10 loops=1)
Join Filter: (b1.user_id = u3.id)
Rows Removed by Join Filter: 139
-> Nested Loop (cost=0.43..3945.32 rows=17856 width=152) (actual time=0.009..0.029 rows=10 loops=1)
-> Index Scan Backward using orders_pkey on orders o0 (cost=0.29..897.80 rows=17856 width=148) (actual time=0.007..0.015 rows=10 loops=1)
-> Index Scan using balances_pkey on balances b1 (cost=0.14..0.16 rows=1 width=8) (actual time=0.001..0.001 rows=1 loops=10)
Index Cond: (id = o0.balance_id)
-> Materialize (cost=0.00..1.21 rows=14 width=4) (actual time=0.001..0.001 rows=15 loops=10)
-> Seq Scan on users u3 (cost=0.00..1.14 rows=14 width=4) (actual time=0.005..0.007 rows=16 loops=1)
Planning time: 0.286 ms
Execution time: 0.097 ms
As you can see, without WHERE it's much faster. Can someone provide me with some information where can I look for explanations for those plans to better understand them? And also what can I do to make those queries faster (or I shouldn't worry cause with like 100 times more data they will still be fast enough? - 50ms is fine for me tbh)
PostgreSQL thinks that it will be fastest if it scans orders in the correct order until it finds a matching users entry that satisfies the WHERE condition.
However, it seems that the data distribution is such that it has to scan almost 17000 orders before it finds a match.
Since PostgreSQL doesn't know how values correlate across tables, there is nothing much you can do to change that.
You can force PostgreSQL to plan the query without the LIMIT clause like this:
SELECT *
FROM (<your query without ORDER BY and LIMIT> OFFSET 0) q
ORDER BY id DESC LIMIT 10;
With a top-N-sort this should perform better.
I use Postgresql 9.5. The settings are default.
I have split a table into ~7000 partitions. Then I've inserted one row.
When I query SELECT * FROM "Offer";, it is running 1,5 seconds.
When I query SELECT * FROM "Offer" WHERE bid=4793;, where bid -- partition's constraint (one table per bid), it is running 1 second.
Here is an EXPLAIN ANALYZE for second query:
Append (cost=0.00..12.14 rows=2 width=596) (actual time=0.014..0.014 rows=1 loops=1)
-> Seq Scan on "Offer" (cost=0.00..1.01 rows=1 width=344) (actual time=0.011..0.011 rows=0 loops=1)
Filter: (bid = 4793)
Rows Removed by Filter: 1
-> Seq Scan on "Offer-4793" (cost=0.00..11.12 rows=1 width=848) (actual time=0.002..0.002 rows=1 loops=1)
Filter: (bid = 4793)
Planning time: 996.243 ms
Execution time: 0.261 ms
Why so slow? What can I use to profile it?
I have only one guess -- postgresql does not keep partitioning constrains in RAM and reads them from HDD every time.
Expecting some help!
UPDATE:
I've tried to create cascading partitioning (as #jmelesky has written). Results are worse:
Append (cost=0.00..12.24 rows=5 width=848) (actual time=0.013..0.013 rows=1 loops=1)
-> Seq Scan on "Offer" (cost=0.00..1.11 rows=1 width=848) (actual time=0.006..0.006 rows=0 loops=1)
Filter: (bid = 4793)
-> Seq Scan on "Offer-ddd-3" (cost=0.00..0.00 rows=1 width=848) (actual time=0.001..0.001 rows=0 loops=1)
Filter: (bid = 4793)
-> Seq Scan on "Offer-dd-33" (cost=0.00..0.00 rows=1 width=848) (actual time=0.000..0.000 rows=0 loops=1)
Filter: (bid = 4793)
-> Seq Scan on "Offer-d-336" (cost=0.00..0.00 rows=1 width=848) (actual time=0.000..0.000 rows=0 loops=1)
Filter: (bid = 4793)
-> Seq Scan on "Offer-4793" (cost=0.00..11.12 rows=1 width=848) (actual time=0.006..0.006 rows=1 loops=1)
Filter: (bid = 4793)
Planning time: 1449.872 ms
Execution time: 0.354 ms