Re: Query hitting empty tables taking 48 minutes - Mailing list pgsql-general

Robert Creager wrote:
> I have a system running FreeBSD 11.1-STABLE, PostgreSQL 9.6.8,Java OpenJDK 1.8.0_131,
> jdbc 9.3-1104-jdbc41 which is exhibiting very bizarre behavior.
> A query is executing against a couple of tables that have 1 and 0 records in them.
> ds3.job_entry has 0 rows, ds3.blob has 1 row.
> If I execute the query manually via command line, it executes fine.  There are no other active queries, no locks.
> The system is idle, in our Dev Test group, so I can leave it this way for a bit of time.
> The general software setup is in production and I’ve seen nothing like this before.
> Even a system with 300M ds3.blob entries executes this query fine.
> 
> Jun  7 17:24:21 blackpearl postgres[10670]: [7737-1] db=tapesystem,user=Administrator,app=[unknown],client=127.0.0.1
LOG: duration: 2903612.206 ms  execute fetch from S_2037436/C_2037437: SELECT * FROM ds3.blob WHERE EXISTS (SELECT *
FROMds3.job_entry WHERE blob_id = ds3.blob.id AND (job_id = $1))
 
> 
> tapesystem=# explain analyze SELECT * FROM ds3.blob WHERE EXISTS (SELECT * FROM ds3.job_entry WHERE blob_id =
ds3.blob.idAND (job_id = 'b51357cd-e07a-4c87-a50b-999c347a5c71'));                       
 
>                                                               QUERY PLAN
>
---------------------------------------------------------------------------------------------------------------------------------------
                                                              
 
>  Nested Loop  (cost=0.84..6.89 rows=1 width=77) (actual time=0.044..0.044 rows=0 loops=1)
>    ->  Index Scan using job_entry_job_id_idx on job_entry  (cost=0.42..2.44 rows=1 width=16) (actual
time=0.042..0.042rows=0 loops=1)
 
>          Index Cond: (job_id = 'b51357cd-e07a-4c87-a50b-999c347a5c71'::uuid)
>    ->  Index Scan using blob_pkey on blob  (cost=0.42..4.44 rows=1 width=77) (never executed)
>          Index Cond: (id = job_entry.blob_id)                                  
>  Planning time: 0.388 ms
>  Execution time: 0.118 ms
                                                                  
 
> 
> 
> pid    client_port    runtime    query_start    datname    state    query    usename
> 10670    11211    0 years 0 mons 0 days 0 hours 43 mins 28.852273 secs    2018-06-07 17:24:22.026384    tapesystem
active   SELECT * FROM ds3.blob WHERE EXISTS (SELECT * FROM ds3.job_entry WHERE blob_id = ds3.blob.id AND (job_id =
$1))   Administrator
 
> 
> 
> From the system with 300M ds3.blob entries, which works fine (along with every other system in house):
> 
> QUERY PLAN
> Nested Loop  (cost=0.57..738052.90 rows=164379 width=75) (actual time=1.001..1947.029 rows=50000 loops=1)
>   ->  Seq Scan on job_entry  (cost=0.00..10039.50 rows=164379 width=16) (actual time=0.871..56.442 rows=50000
loops=1)
>         Filter: (job_id = 'ef27d2fa-2727-424e-8f44-da9e33a5ca05'::uuid)
>         Rows Removed by Filter: 60001
>   ->  Index Scan using blob_pkey on blob  (cost=0.57..4.42 rows=1 width=75) (actual time=0.037..0.037 rows=1
loops=50000)
>         Index Cond: (id = job_entry.blob_id)
> Planning time: 6.330 ms
> Execution time: 1951.564 ms

The JDBC query is using a cursor since you are using setFetchSize(10000).

I can see two reasons why the plan might be different:

1. It is a prepared statement that has been executed often enough
   for the generic plan to kick in, and the generic plan is bad.

2. PostgreSQL chooses a fast startup plan because a cursor is used,
   and that plan performs much worse.

To see if 1. causes the problem, you could set
  m_statement.setPrepareThreshold(0);
and see if that makes a difference.

To test if 2. has an influence, try setting cursor_tuple_fraction to 1.0
and see if it makes a difference.

You can capture the execution plan of the bad statement by using auto_explain,
that would certainly shed more light on the problem.

Yours,
Laurenz Albe
-- 
Cybertec | https://www.cybertec-postgresql.com