#25230: Change to Query.get_count() causes big performance hit
-------------------------------------+-------------------------------------
Reporter: dexity | Owner: nobody
Type: | Status: new
Cleanup/optimization |
Component: Database layer | Version: 1.8
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Comment (by charettes):
I do get a non negligible slowdown on PostgreSQL with a fully analyzed
table containing ~30M rows.
{{{
database=# EXPLAIN ANALYZE SELECT COUNT('*') FROM (SELECT DISTINCT * FROM
schema.table) subquery;
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=6213779.77..6213779.78 rows=1 width=0) (actual
time=88610.804..88610.804 rows=1 loops=1)
-> Unique (cost=5290067.41..5858505.79 rows=28421919 width=117)
(actual time=53265.590..86513.151 rows=28176350 loops=1)
-> Sort (cost=5290067.41..5361122.20 rows=28421919 width=117)
(actual time=53265.588..76718.379 rows=28176350 loops=1)
Sort Key: table.col0, table.col1, table.col2, table.col2,
table.col3, table.col4, table.col5
Sort Method: external merge Disk: 1296744kB
-> Seq Scan on table (cost=0.00..522350.19 rows=28421919
width=117) (actual time=0.840..15830.723 rows=28176350 loops=1)
Total runtime: 88695.934 ms
(7 rows)
database=# EXPLAIN ANALYZE SELECT COUNT(*) FROM schema.table;
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=593404.99..593405.00 rows=1 width=0) (actual
time=19139.072..19139.073 rows=1 loops=1)
-> Seq Scan on table (cost=0.00..522350.19 rows=28421919 width=0)
(actual time=5.683..17387.798 rows=28176350 loops=1)
Total runtime: 19139.108 ms
(3 rows)
}}}
Should we escalate this to release blocker?
--
Ticket URL: <https://code.djangoproject.com/ticket/25230#comment:6>
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