The following bug has been logged online:

Bug reference: 2341
Logged by: Slavcho Ivanov
Email address: slavcho@gmail.com
PostgreSQL version: 8.1.2
Operating system: slackware
Description: Wrong results when using index
Details:

It happend so that a query started returning wrong results.

The query was as simple as possible
"select * from data where "SensorID" = 18",
and the result had a couple of rows where SensorID was NOT 18, but 3.

There was a index at the table using only ("SensorID").

After we dropped the index. It was OK. The query started again to show good
results.

We created the index again. And still it was OK.

Unfortunatelly (for you, good for us), there are no more problems of that
kind.

There are about 85 000 rows in that table, and we don't use "delete" or
"update" there - only insert new rows. Hope it helps.

Sincerely:
Slavcho Ivanov

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"Slavcho Ivanov" <slavcho@gmail.com> writes:
> The query was as simple as possible
> "select * from data where "SensorID" = 18",
> and the result had a couple of rows where SensorID was NOT 18, but 3.

That's not too surprising if the index had gotten corrupted somehow,
but since you haven't provided any details that would let someone else
reproduce the problem, there's not much we can do with this report :-(

regards, tom lane

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TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
match