Hi HP
Thanks for your reply.
Quoting "Peter J. Holzer" <hjp-pgsql@xxxxxx>:
On 2018-12-25 11:54:11 +0000, Thiemo Kellner wrote:
[three different but functionally equivalent queries]
Explain analyze verbose showed for:
A (cost=264.72..626.97 rows=31 width=90) (actual time=1.117..1.117 rows=0
loops=1)
C (cost=0.42..611.19 rows=31 width=52) (actual time=2.217..2.217 rows=0
loops=1)
626.97 doesn't seem "much higher" to me than 611.19. I would call that
"about the same".
So would I but the cost is given as a range. Taking the the average
somewhat 400 compare to somewhat 300. I do not know whether averaging
is appropriate here.
This is weird. C takes almost exactly twice as long as A, and while
humans aren't very good at estimating times, One second should feel
faster than two, not slower, and certainly not slower by far. Is it
possible that your subjective impression wasn't based on the executions
you posted but on others? Caching and load spikes can cause quite large
variations in run time, so running the same query again may not take the
same time (usually the second time is faster - sometimes much faster).
I am pretty sure not to have confused the association of my felt time
measure to the query. I did run the script several times but as the
script create everything anew this might not have any impact caching
wise. However, if caching had an impact it just would add to the
discrepancy between feeling that the first statement was much faster
than the supposedly optimal statement. Being as may, there is still
fact that cost for A was estimated about the same as C or much higher
but A was executed in half of the time of C.
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