On 9/6/07, Tom Lane <tgl@sss.pgh.pa.us> wrote: "Heikki Linnakangas" <heikki@enterprisedb.com> writes:
> When I suggested that we get rid of the LP_DELETE flag for heap tuples,
> the tuple-level fragmentation and all that, and just take the vacuum
> lock and call PageRepairFragmentation, I was thinking that we'd do it in
> heap_update and only when we run out of space on the page. But as Greg
> said, it doesn't work because you're already holding a reference to at
> least one tuple on the page, the one you're updating, by the time you
> get to heap_update. That's why I put the pruning code to heap_fetch
> instead. Yes, though the amortized cost is the same, it does push the
> pruning work to the foreground query path.
The amortized cost is only "the same" if every heap_fetch is associated
with a heap update. I feel pretty urgently unhappy about this choice.
Have you tested the impact of the patch on read-mostly workloads?
For read-mostly workloads, only the first SELECT after an UPDATE
would trigger pruning/defragmentation. heap_page_prune_defrag()
would be a no-op for subsequent SELECTs (PageIsPrunable() would
be false until the next UPDATE)
I think Heikki's recent test confirms this.
Thanks,
Pavan
--
Pavan Deolasee
EnterpriseDB
http://www.enterprisedb.com
