Re: Huge shared hit for small table - Mailing list pgsql-performance

Thanks to Justin for the clarification around pgstatindex:

Staging:

version2
tree_level1
index_size425984
root_block_no3
internal_pages1
leaf_pages50
empty_pages0
deleted_pages0
avg_leaf_density70.86
leaf_fragmentation16

Production:

version2
tree_level1
index_size360448
root_block_no3
internal_pages1
leaf_pages41
empty_pages0
deleted_pages1
avg_leaf_density60.44
leaf_fragmentation39.02

On 11/4/19, 3:07 PM, "Peter Geoghegan" <pg@bowt.ie> wrote:

    On Mon, Nov 4, 2019 at 11:56 AM Justin Pryzby <pryzby@telsasoft.com> wrote:
    > I think it's because some heap pages are being visited many times, due to the
    > index tuples being badly "fragmented".  Note, I'm not talking about
    > fragmentation of index *pages*, which is what pgstattuple reports (which
    > wouldn't have nearly so detrimental effect).  I could probably say that the
    > index tuples are badly "correlated" with the heap.

    But this is a unique index, and Scott indicates that the problem seems
    to go away for a while following a REINDEX.

    > In PG v12 you can use REINDEX CONCURRENTLY (but beware there's a crash
    > affecting its progress reporting, fix to be included in v12.1).

    PG v12 will store B-Tree duplicates in heap TID order, so if that's
    the problem then upgrading to v12 (and REINDEXing if the upgrade was
    performed using pg_upgrade) will fix it for good.

    --
    Peter Geoghegan



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