Re: Can the V7.3 EXPLAIN ANALYZE be trusted?

Hi, @all,

Greg Stark schrieb:
> "Leeuw van der, Tim" <tim.leeuwvander@nl.unisys.com> writes:
>
>>I don't think EXPLAIN ANALYZE puts that much overhead on a query.
>
> EXPLAIN ANALYZE does indeed impose a significant overhead.

Additional note:

In some rare cases, you can experience just the opposite effect, explain
analyze can be quicker then the actual query.

This is the case for rather expensive send/output functions, like the
PostGIS ones:

lwgeom=# \timing
Zeitmessung ist an.
lwgeom=# explain analyze select setsrid(geom,4326) from adminbndy1;
                                                QUERY PLAN

-----------------------------------------------------------------------------------------------------------
 Seq Scan on adminbndy1  (cost=0.00..4.04 rows=83 width=89) (actual
time=11.793..2170.184 rows=83 loops=1)
 Total runtime: 2170.834 ms
(2 Zeilen)

Zeit: 2171,688 ms
lwgeom=# \o /dev/null
lwgeom=# select setsrid(geom,4326) from adminbndy1;
Zeit: 9681,001 ms


BTW: I use the cheap setsrid(geom,4326) to force deTOASTing of the
geometry column. Not using it seems to ignore TOASTed columns in
sequential scan simulation.)

lwgeom=# explain analyze select geom from adminbndy1;
                                              QUERY PLAN

-------------------------------------------------------------------------------------------------------
 Seq Scan on adminbndy1  (cost=0.00..3.83 rows=83 width=89) (actual
time=0.089..0.499 rows=83 loops=1)
 Total runtime: 0.820 ms
(2 Zeilen)


Markus

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