From: | Andres Freund <andres(at)anarazel(dot)de> |
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To: | Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at> |
Cc: | Emanuel Alvarez <ema(at)abductedcow(dot)com(dot)ar>, pgsql-general(at)postgresql(dot)org |
Subject: | Re: seq vs index scan in join query |
Date: | 2017-11-29 17:46:40 |
Message-ID: | 20171129174640.o7q2ge4f3lk25cmo@alap3.anarazel.de |
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Lists: | pgsql-general |
On 2017-11-29 18:17:18 +0100, Laurenz Albe wrote:
> That is because the execution with the sequential scan touched
> 26492 + 80492 = 106984 blocks, while the second execution touched
> 311301 + 48510 = 359811 blocks, more than three times as many.
That's not necessarily said. What those count are buffer *accesses*,
*not* the number of distinct blocks accessed. You'll very commonly have
more buffer accesses in indexscans but still fewer total reads because a
lot of those accesses will be reads previously done in the same
scan. Just imagine a scan of an index with a leaf page pointing to 100
tuples of the same value - that'd result in at least a 100 buffer
accesses, but it'd be highly likely that they'll be in cache.
Greetings,
Andres Freund
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