Re: Creating multiple indexes
- From: Nick Pierpoint <mail@xxxxxxxxxxxxx>
- To: Laimutis.Nedzinskas@xxxxxx
- Date: Thu, 18 Nov 2010 09:12:53 +0000
On Wed, 2010-11-17 at 08:18 +0200, Laimutis.Nedzinskas@xxxxxx wrote:
> >I'm trying to find the reason why there isn't a
> mechanism to create multiple indexes at the same time - a "create
> indexes" statement or similar.
>
> > Each create index needs to scan through every row in the table to form the
> > index.
> > i.e. 10 indexes = 10 full scans.
>
> b-tree initial construction requires sorting
> (http://en.wikipedia.org/wiki/B-tree#Initial_construction)
> Let's use a merge-sort algorithm (http://en.wikipedia.org/wiki/Merge_sort),
> ok?
> Merge algorithm does scan all rows but it also does more than that.
>
>
Are you saying that index creation requires more work than the initial
table scan so any benefits derived from creating multiple indexes at the
same time with a single scan would be countered by the additional
overhead?
I don't see this. I'd have thought that the first "data gathering" stage
of index creation would benefit from caching each column as part of a
single table scan.
--
Nick
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