Re: seq vs index scan in join query

Andres Freund wrote:
> 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.

Thanks for explaining that.

Yours,
Laurenz Albe