From: | Joe Conway <mail(at)joeconway(dot)com> |
---|---|
To: | Ron Johnson <ron(dot)l(dot)johnson(at)cox(dot)net>, pgsql-general(at)lists(dot)postgresql(dot)org |
Subject: | Re: Reindex doesn't eliminate bloat |
Date: | 2018-03-13 23:10:18 |
Message-ID: | cee840be-ed58-f7a1-c481-55c4281a327b@joeconway.com |
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Lists: | pgsql-general |
On 03/12/2018 09:16 PM, Ron Johnson wrote:
> On 03/12/2018 10:48 PM, Nikolay Samokhvalov wrote:
>> Those queries from wiki for table and index bloat estimation are for
>> estimation only. In many cases they show very wrong results. Better
>> (yet not ideal) approach is using pgstattuple extension (though I'm
>> not sure it existed back in 2009).
>>
>> Can you provide table and index definition and, if you can, some
>> sample data?
>
> Sadly, no sample data. (It's all PCI controlled.)
>
> Index idx_item_mapping_rp7_y2016m03itemmapping_custom_userfield_801 has
> 40% bloat.
Assuming the data in the indexed column(s) is not highly correlated with
the physical table order (i.e. it is roughly random), about 50% density
is theoretically expected. In fact, in some empirical testing, I have
seen a long term steady state value of closer to 44% if I remember
correctly (but perhaps that was related to the way I was testing). For a
discussion on why this is the case, see for example:
So what is being reported at 40% bloat is probably actually not really
bloat.
HTH,
Joe
--
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PostgreSQL Support for Secure Enterprises
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