Suddenly all queries moved to seq scan

On 11/20/24 12:50, Sreejith P wrote:

Hi,

We are using PostgresQL 10 in our production database. We have around
890 req /s request on peak time.

We have 1 primary and 4 slave databases as well in the same postgres
cluster.

2 days back we applied some patches in the primary server and
restarted. We didn't do anything on the secondary server.

Next day, After 18 hours all our queries from secondary servers
started taking too much time.  queries were working in 2 sec started
taking 80 seconds. Almost all queries behaved the same way.

After half an hour of outage we restarted all db servers and system
back to normal.

Still we are not able to understand the root case. We couldn't find
any error log or fatal errors.  During the incident, in  one of the
read server disks was full. We couldn't see any replication lag or
query cancellation due to replication.

please help

Still you say seq scan, how do you know that? You run explain (buffers,
analyze, settings) ? If this is indeed the case, then you need to check
your indexes. Check when they were last used/scanned and if they are
usable. Did the disk error had any of your PostgreSQL tablespaces /
tables/indexes  ?

On 20 Nov 2024, at 11:50, Sreejith P <sreejith@lifetrenz.com> wrote:

We are using PostgresQL 10 in our production database. We have around 890 req /s request on peak time.

PostgreSQL 10 is well out of support and does not receive bugfixes or security
fixes, you should plan a migration to a supported version sooner rather than
later.

2 days back we applied some patches in the primary server and restarted. We didn't do anything on the secondary server.

Patches to the operating system, postgres, another application?

Next day, After 18 hours all our queries from secondary servers started taking too much time. queries were working in 2 sec started taking 80 seconds. Almost all queries behaved the same way.

After half an hour of outage we restarted all db servers and system back to normal.

Still we are not able to understand the root case. We couldn't find any error log or fatal errors. During the incident, in one of the read server disks was full. We couldn't see any replication lag or query cancellation due to replication.

You say that all queries started doing sequential scans, is that an assumption
from queries being slow or did you capture plans for the queries which be
compared against "normal" production plans?

--
Daniel Gustafsson

Make sure to run analyze on the entire database, possibly using vacuumdb would be faster.
Also, check for invalid indexes.
Efrain J. Berdecia

On Wednesday, November 20, 2024 at 08:02:36 AM EST, Daniel Gustafsson <daniel@yesql.se> wrote:

On 20 Nov 2024, at 11:50, Sreejith P <sreejith@lifetrenz.com> wrote:

We are using PostgresQL 10 in our production database.  We have around 890 req /s request on peak time.

PostgreSQL 10 is well out of support and does not receive bugfixes or security
fixes, you should plan a migration to a supported version sooner rather than
later.

2 days back we applied some patches in the primary server and restarted. We didn't do anything on the secondary server.

Patches to the operating system, postgres, another application?

Next day, After 18 hours all our queries from secondary servers started taking too much time.  queries were working in 2 sec started taking 80 seconds. Almost all queries behaved the same way.

After half an hour of outage we restarted all db servers and system back to normal.

Still we are not able to understand the root case. We couldn't find any error log or fatal errors.  During the incident, in  one of the read server disks was full. We couldn't see any replication lag or query cancellation due to replication.

You say that all queries started doing sequential scans, is that an assumption
from queries being slow or did you capture plans for the queries which be
compared against "normal" production plans?

--
Daniel Gustafsson

On 20 Nov 2024, at 6:32 PM, Daniel Gustafsson <daniel@yesql.se> wrote:

On 20 Nov 2024, at 11:50, Sreejith P <sreejith@lifetrenz.com> wrote:

We are using PostgresQL 10 in our production database. We have around 890 req /s request on peak time.

PostgreSQL 10 is well out of support and does not receive bugfixes or security
fixes, you should plan a migration to a supported version sooner rather than
later.

2 days back we applied some patches in the primary server and restarted. We didn't do anything on the secondary server.

Patches to the operating system, postgres, another application?

PostgreSQL Common 10.23-6

Next day, After 18 hours all our queries from secondary servers started taking too much time. queries were working in 2 sec started taking 80 seconds. Almost all queries behaved the same way.

After half an hour of outage we restarted all db servers and system back to normal.

Still we are not able to understand the root case. We couldn't find any error log or fatal errors. During the incident, in one of the read server disks was full. We couldn't see any replication lag or query cancellation due to replication.

You say that all queries started doing sequential scans, is that an assumption
from queries being slow or did you capture plans for the queries which be
compared against "normal" production plans?.

Queries were taking 20 ms started taking 60 seconds. So have done SQL analyse to understand about query plan. There we found that query planner taking seq scan instead in index scan.

I would like to add one ore point. A delete query were running in DB from 2 days for deleting around 80 million records.

--
Daniel Gustafsson

--

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On Wed, Nov 20, 2024 at 9:09 AM Sreejith P <sreejith@lifetrenz.com> wrote:

Queries were taking 20 ms started taking 60 seconds. So have done SQL
analyse to understand about query plan. There we found that query planner
taking seq scan instead in index scan.

I would like to add one ore point. A delete query were running in DB from
2 days for deleting around 80 million records.

This can cause this specific problem where the number of dead tuples and
lack of autovacuum running can cause the statistics to favor a sequential
scan over an index scan. Taking into account the length of time the delete
took it would hold a number of datapages and tuples in a lock state, which
can lead to blocking queries and prevent autovacuum/analyze.

It is best to do bulk deletes in batches and have a rest period between
batches to allow autovacuum and analyze to keep up.. Doing deletes in
batches reduces the number of resources being consumed.