Re: Slow query on a one-tuple table - Mailing list pgsql-performance

Em 19/09/2019 17:41, Luís Roberto Weck escreveu:
> Em 19/09/2019 17:24, Luís Roberto Weck escreveu:
>> Em 19/09/2019 17:11, Igor Neyman escreveu:
>>> With LIMIT 1, I get 3 shared buffers hit, pretty much always.
>>>
>>> ____________________________________________________________________________________ 
>>>
>>>
>>> Check if assessoria_pkey index is bloated.
>>>
>>> Regards,
>>> Igor Neyman
>>>
>>>
>>
>> With this query[1] it shows:
>>
>> current_database|schemaname|tblname   |idxname 
>> |real_size|extra_size|extra_ratio|fillfactor|bloat_size|bloat_ratio|is_na|
>>
----------------|----------|----------|---------------|---------|----------|-----------|----------|----------|-----------|-----|

>>
>> database_name   |public    |assessoria|assessoria_pkey| 
>> 16384|         0|        0.0|        90|       0.0| 0.0|false|
>>
>> [1]https://github.com/ioguix/pgsql-bloat-estimation/blob/master/btree/btree_bloat-superuser.sql 
>>
>>
>>
>
> Using the quer provided here[1] I see this comment:
>
>   /*
>    * distinct_real_item_keys is how many distinct "data" fields on page
>    * (excludes highkey).
>    *
>    * If this is less than distinct_block_pointers on an internal page, 
> that
>    * means that there are so many duplicates in its children that 
> there are
>    * duplicate high keys in children, so the index is probably pretty 
> bloated.
>    *
>    * Even unique indexes can have duplicates.  It's sometimes 
> interesting to
>    * watch out for how many distinct real items there are within leaf 
> pages,
>    * compared to the number of live items, or total number of items.  
> Ideally,
>    * these will all be exactly the same for unique indexes.
>    */
>
> In my case, I'm seeing:
>
> distinct_real_item_keys|distinct_block_pointers|
> -----------------------|-----------------------|
>                       1|                     63|
>
> This is about half an hour after running VACUUM FULL ANALYZE on the 
> table.
>
> What can I do to reduce this?
>
>
> [1] 
> https://wiki.postgresql.org/wiki/Index_Maintenance#Summarize_keyspace_of_a_B-Tree_inde
Like Igor suggested, the index bloat seems to be at fault here. After 
dropping the PK, I'm getting these plans:

First run (SELECT asscod, asscambol FROM ASSESSORIA WHERE asscod = 1 
ORDER BY asscod):

  Seq Scan on public.assessoria  (cost=0.00..88.01 rows=1 width=62) 
(actual time=0.242..0.810 rows=1 loops=1)
    Output: asscod, asscambol
    Filter: (assessoria.asscod = 1)
    Buffers: shared hit=88
  Planning Time: 0.312 ms
  Execution Time: 0.876 ms
(6 rows)

Subsequent runs get increasingly faster, up to 0.080ms execution times.

Using LIMIT 1, I get on the first run:

  Limit  (cost=0.00..88.01 rows=1 width=62) (actual time=0.252..0.254 
rows=1 loops=1)
    Output: asscod, asscambol
    Buffers: shared hit=17
    ->  Seq Scan on public.assessoria  (cost=0.00..88.01 rows=1 
width=62) (actual time=0.250..0.250 rows=1 loops=1)
          Output: asscod, asscambol
          Filter: (assessoria.asscod = 1)
          Buffers: shared hit=17
  Planning Time: 0.334 ms
  Execution Time: 0.296 ms


Subsequent runs look more like this:

  Limit  (cost=0.00..88.01 rows=1 width=62) (actual time=0.057..0.057 
rows=1 loops=1)
    Output: asscod, asscambol
    Buffers: shared hit=17
    ->  Seq Scan on public.assessoria  (cost=0.00..88.01 rows=1 
width=62) (actual time=0.056..0.056 rows=1 loops=1)
          Output: asscod, asscambol
          Filter: (assessoria.asscod = 1)
          Buffers: shared hit=17
  Planning Time: 0.082 ms
  Execution Time: 0.068 ms

I have about 6 bigint fields in this table that are very frequently 
updated, but none of these are indexed. I thought that by not having an 
index on them, would make all updates HOT, therefore not bloating the 
primary key index. Seems I was wrong?