Re: large number dead tup - Postgres 9.5

Hi:

On Mon, Sep 12, 2016 at 1:17 AM, Patrick B <patrickbakerbr@gmail.com> wrote:
>> schemaname relname       n_live_tup n_dead_tup
>> ---------- ------------- ---------- ----------
>> public     parts 191623953  182477402
...
> Because of that the table is very slow...
> When I do a select on that table it doesn't use an index, for example:
> \d parts;
>>     "index_parts_id" btree (company_id)
>>     "index_parts_id_and_country" btree (company_id, country)
> explain select * from parts WHERE company_id = 12;
>> Seq Scan on parts  (cost=0.00..6685241.40 rows=190478997 width=223)
>>   Filter: (company_id = 12)

You've already been directed to check table is really getting vacuumed
/ analyzed, but I'd like to point that if the count estimates are
nearly correct that plan is good ( it's estimating getting more than
99% of the table, a seq scan tends to beat index scan easily when
selecting that big part of the table, even accounting for dead tuples
it's more about 50% of the table, and a seq scan is much faster PER
TUPLE then an index scan ( and and index scan would likely touch every
data page for that big fraction, so reading all of them sequentially
and oing a quick filter is easier )).

Francisco Olarte.


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