Composite index and min()

Hello,

I have the following table:

\d a
Table "phxconfig.a"
Column | Type | Modifiers
---------------+---------+-----------
phx_run_id | integer |
cell_id | integer |
Indexes:
"a_phx_run_id_cell_id_idx" btree (phx_run_id, cell_id)

When I use a min() query I get the following plans:

test=# explain select min(phx_run_id) from a;
QUERY PLAN
-------------------------------------------------------------------------------------------------------
Result (cost=0.22..0.23 rows=1 width=0)
InitPlan 1 (returns $0)
-> Limit (cost=0.14..0.22 rows=1 width=4)
-> Index Only Scan using a_phx_run_id_cell_id_idx on a
(cost=0.14..7.89 rows=100 width=4)
Index Cond: (phx_run_id IS NOT NULL)

test=# explain select min(cell_id) from a;
QUERY PLAN
---------------------------------------------------------
Aggregate (cost=2.25..2.26 rows=1 width=4)
-> Seq Scan on a (cost=0.00..2.00 rows=100 width=4)

Can anyone comment on why this happens?

The index kicks in when I do an explicit cell_id comparison. These are
large tables, and they are in a partition layout so it really hurts when I
do the min call on the parent table.

Cheers,

James Sewell,
PostgreSQL Team Lead / Solutions Architect
______________________________________

Level 2, 50 Queen St, Melbourne VIC 3000

*P *(+61) 3 8370 8000 *W* www.lisasoft.com *F *(+61) 3 8370 8099

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