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?
