| From: | Peter Geoghegan <pg(at)bowt(dot)ie> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | "Jonathan S(dot) Katz" <jkatz(at)postgresql(dot)org>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: SP-GiST failing to complete SP-GiST index build |
| Date: | 2018-05-28 00:24:37 |
| Message-ID: | CAH2-Wzmp_QG+iS7v4jqa3KRwUSweN=jSEQJOzKi4SEnMiX2xfw@mail.gmail.com |
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On Sun, May 27, 2018 at 5:10 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Instrumenting the test case suggests that getQuadrant pretty much always
> returns 1, resulting in a worst-case unbalanced SPGiST tree. I think this
> is related to the fact that the test case inserts the values in increasing
> order, so that new values are always greater than existing values in the
> index.
I suspected the same. It reminded me of the weird behavior that the
Postgres qsort() sometimes exhibits.
> SPGiST is unable to rebalance its tree on the fly, so it's pretty
> well screwed in this situation. It does finish eventually, but in about
> 50x longer than GiST. I imagine the index's query performance would be
> equally awful.
Can you think of some way of side-stepping the issue? It's unfortunate
that SP-GiST is potentially so sensitive to input order.
--
Peter Geoghegan
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