Hi Oscar,
Thank you for the suggestion.
Unfortunately, I didn't mention that on prod we performed the upgrade
from Postgres 12 to 15 using replication to another instance with
pglogical, so I assume that the index was filled from scratch by
Postgres 15.
We upgraded stage instance by changing Postgres version only, so
potentially could run into the index issue there. I've tried to execute
REINDEX CONCURRENTLY, but the performance issue hasn't gone. The problem
is probably somewhere else. However, I do not exclude that we'll perform
REINDEX on prod.
Kind regards,
Alexander
On 13.10.2023 11:44, Oscar van Baten wrote:
Hi Alexander,
I think this is caused by the de-duplication of B-tree index entries
which was added to postgres in version 13
https://www.postgresql.org/docs/release/13.0/
"
More efficiently store duplicates in B-tree indexes (Anastasia
Lubennikova, Peter Geoghegan)
This allows efficient B-tree indexing of low-cardinality columns by
storing duplicate keys only once. Users upgrading with pg_upgrade will
need to use REINDEX to make an existing index use this feature.
"
When we upgraded from 12->13 we had a similar issue. We had to rebuild
the indexes and it was fixed..
regards,
Oscar
Op do 12 okt 2023 om 18:41 schreef Alexander Okulovich
<aokulov...@stiltsoft.com>:
Hello everyone!
Recently, we upgraded the AWS RDS instance from Postgres 12.14 to
15.4 and noticed extremely high disk consumption on the following
query execution:
select (exists (select 1 as "one" from "public"."indexed_commit"
where "public"."indexed_commit"."repo_id" in
(964992,964994,964999, ...);
For some reason, the query planner starts using Seq Scan instead
of the index on the "repo_id" column when requesting under user
limited with RLS. On prod, it happens when there are more than 316
IDs in the IN part of the query, on stage - 3. If we execute the
request from Superuser, the planner always uses the "repo_id" index.
Luckily, we can easily reproduce this on our stage database (which
is smaller). If we add a multicolumn "repo_id, tenant_id" index,
the planner uses it (Index Only Scan) with any IN params count
under RLS.
Could you please clarify if this is a Postgres bug or not? Should
we include the "tenant_id" column in all our indexes to make them
work under RLS?
Postgres version / Operating system+version
PostgreSQL 15.4 on aarch64-unknown-linux-gnu, compiled by gcc
(GCC) 7.3.1 20180712 (Red Hat 7.3.1-6), 64-bit
Full Table and Index Schema
\d indexed_commit
Table "public.indexed_commit"
Column | Type | Collation |
Nullable | Default
---------------+-----------------------------+-----------+----------+---------
id | bigint | | not null |
commit_hash | character varying(40) | | not null |
parent_hash | text | | |
created_ts | timestamp without time zone | | not null |
repo_id | bigint | | not null |
lines_added | bigint | | |
lines_removed | bigint | | |
tenant_id | uuid | | not null |
author_id | uuid | | not null |
Indexes:
"indexed-commit-repo-idx" btree (repo_id)
"indexed_commit_commit_hash_repo_id_key" UNIQUE CONSTRAINT,
btree (commit_hash, repo_id) REPLICA IDENTITY
"indexed_commit_repo_id_without_loc_idx" btree (repo_id) WHERE
lines_added IS NULL OR lines_removed IS NULL
Policies:
POLICY "commit_isolation_policy"
USING ((tenant_id =
(current_setting('app.current_tenant_id'::text))::uuid))
Table Metadata
SELECT relname, relpages, reltuples, relallvisible, relkind,
relnatts, relhassubclass, reloptions, pg_table_size(oid) FROM
pg_class WHERE relname='indexed_commit';
relname | relpages | reltuples | relallvisible |
relkind | relnatts | relhassubclass | reloptions | pg_table_size
----------------+----------+--------------+---------------+---------+----------+----------------+---------------------------------------------------------------------------------------------------------------------------------------------+---------------
indexed_commit | 18170522 | 7.451964e+08 | 18104744 |
r | 9 | f |
{autovacuum_vacuum_scale_factor=0,autovacuum_analyze_scale_factor=0,autovacuum_vacuum_threshold=200000,autovacuum_analyze_threshold=100000}
| 148903337984
EXPLAIN (ANALYZE, BUFFERS), not just EXPLAIN
Production queries:
316 ids under RLS limited user
<https://explain.depesz.com/s/X7Iq>
392 ids under RLS limited user <https://explain.depesz.com/s/lbkX>
392 ids under Superuser <https://explain.depesz.com/s/uKSG>
History
It became slow after the upgrade to 15.4. We never had any issues
before.
Hardware
AWS DB class db.t4g.large + GP3 400GB disk
Maintenance Setup
Are you running autovacuum? Yes
If so, with what settings?
autovacuum_vacuum_scale_factor=0,autovacuum_analyze_scale_factor=0,autovacuum_vacuum_threshold=200000,autovacuum_analyze_threshold=100000
SELECT * FROM pg_stat_user_tables WHERE relname='indexed_commit';
relid | schemaname | relname | seq_scan | seq_tup_read |
idx_scan | idx_tup_fetch | n_tup_ins | n_tup_upd | n_tup_del |
n_tup_hot_upd | n_live_tup | n_dead_tup | n_mod_since_analyze |
n_ins_since_vacuum | last_vacuum | last_autovacuum |
last_analyze | last_autoanalyze | vacuum_count |
autovacuum_count | analyze_count | autoanalyze_count
-------+------------+----------------+----------+--------------+-----------+---------------+-----------+-----------+-----------+---------------+------------+------------+---------------------+--------------------+-------------+-------------------------------+--------------+-------------------------------+--------------+------------------+---------------+-------------------
24662 | public | indexed_commit | 2485 | 49215378424 |
374533865 | 4050928807 | 764089750 | 2191615 | 18500311
| 0 | 745241398 | 383 | 46018
| 45343 | | 2023-10-11 23:51:29.170378+00
| | 2023-10-11 23:50:18.922351+00 | 0
| 672 | 0 | 753
WAL Configuration
For data writing queries: have you moved the WAL to a different
disk? Changed the settings? No.
GUC Settings
What database configuration settings have you changed? We use
default settings.
What are their values?
SELECT * FROM pg_settings WHERE name IN ('effective_cache_size',
'shared_buffers', 'work_mem');
name | setting | unit | category |
short_desc | extra_desc | context | vartype |
source | min_val | max_val | enumvals | boot_val |
reset_val | sourcefile | sourceline | pending_restart
----------------------+---------+------+---------------------------------------+------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------+---------+--------------------+---------+------------+----------+----------+-----------+------------+------------+-----------------
effective_cache_size | 494234 | 8kB | Query Tuning / Planner
Cost Constants | Sets the planner's assumption about the total
size of the data caches. | That is, the total size of the caches
(kernel cache and shared buffers) used for PostgreSQL data files.
This is measured in disk pages, which are normally 8 kB each. |
user | integer | configuration file | 1 | 2147483647
| | 524288 | 494234 | | | f
shared_buffers | 247117 | 8kB | Resource Usage /
Memory | Sets the number of shared memory buffers
used by the server. | | postmaster | integer | configuration file
| 16 | 1073741823 | | 16384 | 247117 |
| | f
work_mem | 4096 | kB | Resource Usage /
Memory | Sets the maximum memory to be used for
query workspaces. | This much memory can be used by
each internal sort operation and hash table before switching to
temporary disk
files. |
user | integer | default | 64 | 2147483647
| | 4096 | 4096 | | | f
Statistics: n_distinct, MCV, histogram
Useful to check statistics leading to bad join plan. SELECT
(SELECT sum(x) FROM unnest(most_common_freqs) x) frac_MCV,
tablename, attname, inherited, null_frac, n_distinct,
array_length(most_common_vals,1) n_mcv,
array_length(histogram_bounds,1) n_hist, correlation FROM pg_stats
WHERE attname='...' AND tablename='...' ORDER BY 1 DESC;
Returns 0 rows.
Kind regards,
Alexander
