RE: Optimizer estimated cardinality very low
Date: Wed, 28 Oct 2020 13:16:03 +0000
Message-ID: <1492_1603890999_5F996F37_1492_14514_1_cf9eaac0f1ea4b86b419a1b229ff93df_at_vontobel.com>
Jonathan,
The usage of column group statistics obviously depends on many factors, such as the presence of histograms, matching columns on col group (or index) stats in both tables, sanity checks, etc..
The problem is that the optimizer switches to completely different formula for calculating selectivity if it decides to use col group stats:
With column groups: sel = 1/greater(NDV(t1),NDV(t2)
Without column groups sel = 1
The difference can be in orders of magnitude, like in Daniel’s case.
The problem isn’t only with semi-join. The join selectivity calculation is also broken for multi-joins with a filter.
Best regards,
Nenad
From: oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> On Behalf Of Jonathan Lewis
Sent: Mittwoch, 28. Oktober 2020 14:03
I've got to say that I think there's a logic error in the way this whole thing works.
A very small change in statistics shouldn't result in a massive change in estimated outcome. (This is different from the nlj/hj switch which can look as if it's an analogy, but the nlj/hj switch happens when the statistics say that both should cost the same).
For this existence to go (effectively) from 100% to 0% on the basis of a tiny difference really does suggest that someone hasn't thought through the implications (or, possibly, two independent pieces of code have been hijacked as the A/B options without any consideration of consistency).
Regards
On Wed, 28 Oct 2020 at 12:40, Jonathan Lewis <jlewisoracle_at_gmail.com<mailto:jlewisoracle_at_gmail.com>> wrote:
I've modelled this problem on a pair of non-partitioned tables. (Since the join is exactly across an exact matching pair of partitions I think this is probably valid).
I've got 100,000 rows in claim_header and 110,000 rows in claim_line.
The actual number of distinct trnsct_cntl_nbr in both tables is exactly 100,000, with 4 dates
The basic stats collection for tables reports 4 dates and 99,848 distinct values for trnsctl_ctl_nbr.
Any set of tests below starts with no indexes, no column groups and no histograms
From c''' and c''''' it appears that there's a break point for the sanity check at 99% of the rows in the table.
Trying to untangle this to get something useful is going to be a pain - and it's still not a complete set of tests.
Question - For the two tables, for the two columns in the join/existence, what indexes, column groups and histograms do you have.
Regards
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To: ORACLE-L (oracle-l_at_freelists.org) <oracle-l_at_freelists.org>
Subject: Re: Optimizer estimated cardinality very low
Jonathan Lewis
Jonathan Lewis
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Received on Wed Oct 28 2020 - 14:16:03 CET