Re: Timestamp indicies not being used!

Pedro Doria Meunier <pdoria(at)netmadeira(dot)com> wrote:

> Hi All,
>
> I *really* need some help with this one...
>
> I have a table ~18M rows with a 'timestamp with time zone' column. It's
> indexed thus:
>
> CREATE INDEX my_table_timestamp_idx
> ON my_table
> USING btree
> (zulu_timestamp);
>
> whenever I issue a command like:
> SELECT speed, digital_input_1, digital_input_2, digital_input_3,
> digital_input_4, priority FROM my_table WHERE id='123456789012345'
> AND zulu_timestamp < '2009-07-10 15:24:45+01'
> ORDER BY zulu_timestamp DESC LIMIT 1
>
> it takes FOREVER in instances where there's only 1 row or 0 rows in the
> table
>
> the column id is also indexed.
>
> this is the query plan:
>
> "Limit (cost=0.00..83.88 rows=1 width=20) (actual
> time=810784.212..810784.212 rows=0 loops=1)"
> " -> Index Scan Backward using my_table_timestamp_idx on my_table
> (cost=0.00..3706639.95 rows=44192 width=20) (actual
> time=810784.210..810784.210 rows=0 loops=1)"
> " Index Cond: (zulu_timestamp < '2009-07-10
> 15:24:45+01'::timestamp with time zone)"
> " Filter: (id = '123456789012345'::bpchar)"
> "Total runtime: 810808.298 ms"

There is a BIG difference between estimated and real rows (rows=44192
vs. rows=0. Try to increase the statistics for this column (and of
course, run analyse for this table)

Which PG-Version? And show us the table-definition (for the id-column).

Andreas
--
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