On 10/03/15 10:54, Amit Kapila wrote:
On Tue, Mar 10, 2015 at 3:03 PM, Petr Jelinek <p...@2ndquadrant.com
<mailto:p...@2ndquadrant.com>> wrote:
 > Ok now I think I finally understand what you are suggesting - you are
saying let's go over whole page while tsmnexttuple returns something,
and do the visibility check and other stuff in that code block under the
buffer lock and cache the resulting valid tuples in some array and then
return those tuples one by one from that cache?

Yes, this is what I am suggesting.

 >>  > And if the caller will try to do it in one step and cache the
 >> visibility info then we'll end up with pretty much same structure as
 >> rs_vistuples - there isn't saner way to cache this info other than
 >> ordered vector of tuple offsets, unless we assume that most pages have
 >> close to MaxOffsetNumber of tuples which they don't, so why not just use
 >> the heapgetpage directly and do the binary search over rs_vistuples.
 >>  >
 >> The downside of doing it via heapgetpage is that it will do
 >> visibility test for tuples which we might not even need (I think
 >> we should do visibility test for tuples retrurned by tsmnexttuple).
 > Well, heapgetpage can either read visibility data for whole page or
not, depending on if we want pagemode reading or not. So we can use the
pagemode for sampling methods where it's feasible (like system) and not
use pagemode where it's not (like bernoulli) and then either use the
rs_vistuples or call HeapTupleSatisfiesVisibility individually again
depending if the method is using pagemode or not.

Yeah, but as mentioned above, this has some downside, but go
for it only if you feel that above suggestion is making code complex,
which I think should not be the case as we are doing something similar
in acquire_sample_rows().

I think your suggestion is actually simpler code wise, I am just somewhat worried by the fact that no other scan node uses that kind of caching and there is probably reason for that.

 Petr Jelinek                  http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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