Hi,
Adding DESC to both columns in the SORT BY did not make the query use
the multikey index. So both
SELECT DISTINCT ON (assetid) assetid, ts
FROM asset_positions
ORDER BY assetid, ts DESC;
and
SELECT DISTINCT ON (assetid) assetid, ts
FROM asset_positions
ORDER BY assetid DESC, ts DESC;
use the same query plans and both do sequential scans without using either the (assetid, ts) or (ts) indexes. Any other ideas on how to make this query use an index? Thanks,
--
Graham Davis
Refractions Research Inc.
gdavis@xxxxxxxxxxxxxxx
On Wed, Sep 27, 2006 at 20:56:32 +0000,
Graham Davis <gdavis@xxxxxxxxxxxxxxx> wrote:
SELECT assetid, max(ts) AS ts
FROM asset_positions
GROUP BY assetid;
I have an index on (ts), another index on (assetid) and a multikey index on
(assetid, ts). I know the assetid index is pointless since the multikey one
takes its place, but I put it there while testing just to make sure. The
ANALYZE EXPLAIN for this query is:
QUERY PLAN
----------------------------------------------------------------------------
-------------------------------------------------------------
HashAggregate (cost=125423.96..125424.21 rows=20 width=12) (actual
time=39693.995..39694.036 rows=20 loops=1)
-> Seq Scan on asset_positions (cost=0.00..116654.64 rows=1753864
width=12) (actual time=20002.362..34724.896 rows=1738693 loops=1)
Total runtime: 39694.245 ms
(3 rows)
You can see it is doing a sequential scan on the table when it should be
using the (assetid, ts) index, or at the very least the (ts) index. This
query takes about 40 seconds to complete with a table of 1.7 million rows.
I tested running the query without the group by as follows:
SELECT DISTINCT ON (assetid) assetid, ts
FROM asset_positions
ORDER BY assetid, ts DESC;
This is almost what you want to do to get an alternative plan. But you
need to ORDER BY assetid DESC, ts DESC to make use of the multicolumn
index. If you really need the other output order, reverse it in your
application or use the above as a subselect in another query that orders
by assetid ASC.
