Ludwig Lim kirjutas T, 10.12.2002 kell 21:55:
> Hi:
>
> Is the performance overhead of creating a
> multi-column index greater than creating an individual
> index for each column? (i.e. Is the INSERT slower for
> a table with a three column index than a similar table
> with three single column indices?).
>
> I was wondering this because according to
> PostgreSQL Manual in the section on multi-columned
> indexes "Multicolumn indexes should be used sparingly.
> Most of the time, an index on a single column is
> sufficient and saves space and time".
You should create only as much indexes as you really need.
A multi-column index can not usually replace multiple single column
indexes and vice versa.
For example, while an index on a,b,c can be used for search on both a
and b it will not be used for search on b and c and will be used like
index on a for search on a and c.
While a multi-column index is slower than a single-column one, it is
definitely faster than multiple single column indexes - one 3-column
index should always be faster than 3 single-column indexes.
Also, currently even with multiple single-column indexes on a,b and c
the search on a and c will use only one index, either on a or c.
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--
Hannu Krosing <hannu@tm.ee>
