On 2022-10-20 09:56:23 -0700, Christophe Pettus wrote: > On Oct 20, 2022, at 09:52, Vince McMahon <sippingonesandze...@gmail.com> > wrote: > > The number of rows are different. > > This isn't unexpected. EXPLAIN does not actually run the query and > determine how many rows are returned; it calculates an estimate based > on the current system statistics, which vary constantly depending on > activity in the database.
EXPLAIN ANALYZE (which is what he did) does run the query and return the
actual number of rows:
#v+
wdsah=> explain (analyze, buffers) select * from
facttable_eurostat_comext_cpa2_1 ;
╔══════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════╗
║ QUERY PLAN
║
╟──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╢
║ Seq Scan on facttable_eurostat_comext_cpa2_1 (cost=0.00..1005741.32
rows=39633432 width=85) (actual time=0.396..6541.701 rows=39633591 loops=1) ║
║ Buffers: shared read=609407
║
║ Planning Time: 1.650 ms
║
║ Execution Time: 7913.027 ms
║
╚══════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════╝
(4 rows)
#v-
The first tuple (cost=0.00..1005741.32 rows=39633432 width=85) is an
estimate used to plan the query. But the second one
(actual time=0.396..6541.701 rows=39633591 loops=1)
contains measurements from actually running the query.
I think it's possible that the rows estimate in the first tuple changes
without any actual data change (although the only reason I can think of
right now would be an ANALYZE (in another session or by autovacuum)).
But the actual rows definitely shouldn't change.
hp
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| | | h...@hjp.at | -- Charles Stross, "Creative writing
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