Re: Index not being used

[Ralph Smith <smithrn@xxxxxxxxxxxxxxxx>]

On Aug 13, 2007, at 4:39 PM, Scott Marlowe wrote:

On 8/13/07, Ralph Smith <smithrn@xxxxxxxxxxxxxxxx> wrote:

I'm confused.  Shouldn't this index be used?

(It's running on v7.4.7)

airburst=> \d stats2

             Table "public.stats2"

  Column   |         Type          | Modifiers

-----------+-----------------------+-----------

 lab       | character varying(30) |

 name      | character varying(50) |

 status    | character varying(40) |

 eventtime | integer               |

 username  | character varying(30) |

 pkey      | character varying(60) |

Indexes:

    "stats2_etime_index" btree (eventtime)

airburst=> \d stats2_etime_index

Index "public.stats2_etime_index"

  Column   |  Type

-----------+---------

 eventtime | integer

btree, for table "public.stats2"

airburst=> explain select count(*) from stats2 where eventtime > 1167638400

;

                              QUERY PLAN

-----------------------------------------------------------------------

 Aggregate  (cost=185247.97..185247.97 rows=1 width=0)

   ->  Seq Scan on stats2  (cost=0.00..179622.45 rows=2250205 width=0)

         Filter: (eventtime > 1167638400)

(3 rows)

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That really depends.  how many rows are actually returned?  If it's

2250205 like the query planner thinks, and that's a fair chunk of the

table, then no, it shouldn't use an index, a seq scan will be faster.

What does explain analyze select ... say?

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Somewhere between 40,000 and 48,000 rows returned the index kicks in.

Out of a table of 7 million rows, that's a fairly common count I have to work with.

It's the amount of activity since August 2nd, this year; NOT that long ago.

Any suggestions on speeding up these queries, other than using more and more tables, thus ruling out the reasonable use of command-line queries?

Ultimately we'll move to some datawarehousing solution, but that's not a 'tomorrow' kind of thing...

Thanks again all,

Ralph

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