On Thu, 01 Apr 2004 12:22:58 +0200, I wrote:
>BTW, ANALYSE is basically a constant time operation.
On closer inspection, this is not the whole truth. ANALY[SZ]E is a two
stage process: First it collects a sample of rows, then these rows are
examined to produce various statistics.
The cost of the latter depends on the sample size, which itself depends
on the default or column-specific statistics target, and the number (and
types) of columns, so it *should* take more or less constant time.
The first step, however, (acquire_sample_rows() in analyze.c) has to
read more rows than finally end up in the sample. It visits less than
O(nblocks) pages but certainly more than O(1).
A vague feeling tries to tell me that the number of page reads is
somehow related to the harmonic numbers 1 + 1/2 + 1/3 + ... + 1/n, which
grow like O(ln(n)).
I have an idea how this could be done with O(1) page reads. If I'm able
to translate it into C, I'll send a patch ...
Servus
Manfred
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