Josh Harrison <joshques@gmail.com> schrieb:
> My questions
> 1. I pg_dumped dummy and Shuffled_dummy (from database1) to another database
> (database2)
> When I issued the query in both database (database1 and database2)
>
> select * from dummy limit 1000 ( the planner chooses seq scan for this query)
> select * from shuffled_dummy limit 1000 (planner chooses seq scan)
>
>
> 2. Also when does the planner switch from choosing index scan to bitmap index
> scan? Is it dependent on the number of rows to be retrieved or the position of
> the relevant data in the blocks or something else?
For a select * ... without a WHERE the db can't use an index, this query
forced a seq-scan.
A index is used when:
- a index are created
- a propper WHERE or ORDER BY in the query
- this index is useful
(a index isn't useful, for instance, for a small table or when almost
all rows are in the result set)
A bitmap index scan performed when:
- 2 or more propper indexes available
- see above
Andreas
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