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 * FROM ds3.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.id AND (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..0.042 rows=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
