Thread: [GENERAL] Problems with Greatest

As a quick follow up I just did an explain on the query,

Aggregate  (cost=258007258.87..258007258.88 rows=1 width=8)
->  Nested Loop  (cost=0.00..184292254.83 rows=14743000807 width=16)
->  Seq Scan on t1  (cost=0.00..3796.41 rows=263141 width=8)
->  Materialize  (cost=0.00..1088.40 rows=56027 width=8)
->  Seq Scan on t2  (cost=0.00..808.27 rows=56027 width=8)

It seems it has to do a loop on 14 billion rows?  Can someone explain why this would happen?

Thanks,

Teddy

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From: Teddy Schmitz
Sent: Thursday, February 16, 2017 12:32:41 PM
To: pgsql-general@postgresql.org
Subject: Problems with Greatest

 

Hello,

I have a query using Greatest that hangs and never returns when called with two tables.

Postgres Version: 9.6

Tables

t1{ id bigint }

t2 { id bigint }

they are sharing a sequence

the query

select greatest(max(t1.id), max(t2.id)) from t1, t2;

The purpose was to call setval on the sequence after doing a bulk data load into the database.  But this query never returns.  I have tried it with various combinations,

select greatest(max(t1.id), 6) from t1; -> This returns

select greatest(max(t1.id), 6) from t1, t2; -> This never returns.

The query does work if there is only a few hundred items between the tables but I'm importing about ~300,000 rows between the two tables.  I looked at pg_stat_activity and it says the query is active

I worked around this problem using a union all query but I'm wondering if this is a bug or I am just using greatest wrong.