Re: Strange nested loop for an INSERT

On 9/23/16 12:59 PM, phb07 wrote:
>
> Le 21/09/2016 à 23:42, Jim Nasby a écrit :
>> On 9/12/16 1:05 PM, phb07 wrote:
>>> The drawback is the overhead of this added ANALYZE statement. With a
>>> heavy processing like in this test case, it is worth to be done. But for
>>> common cases, it's a little bit expensive.
>>
>> You could always look at the number of rows affected by a command and
>> make a decision on whether to ANALYZE based on that, possibly by
>> looking at pg_stat_all_tables.n_mod_since_analyze.
> I have solved the issue by adding an ANALYZE between both statements. To
> avoid the associated overhead for cases when it is not worth to be done,
> the ANALYZE is only performed when more than 1000 rows have just been
> deleted by the first statement (as the logic is embeded into a plpgsql
> function, the GET DIAGNOSTICS statement provides the information). This
> threshold is approximately the point where the potential loss due to bad
> estimates equals the ANALYZE cost.
> But the idea of using the n_mod_since_analyze data to also take into
> account other recent updates not yet reflected into the statistics is
> very interesting.

Another interesting possibility would be to look at
pg_catalog.pg_stat_xact_all_tables; if you add n_tup_ins, _upd, and _del
that will tell you how much n_mod_since_analyze will be increased when
your transaction commits, so you could guage exactly how much the
current transaction has changed things.
--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
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