> On Aug 11, 2021, at 5:08 AM, Dean Rasheed <[email protected]> wrote:
>
> This feels like rather an artificial example though. Is there any real
> use for this sort of clause?
The test generated random combinations of clauses and then checked if any had
consistently worse performance. These came up. I don't know that they
represent anything real.
What was not random in the tests was the data in the tables. I've gotten
curious if these types of clauses (with columns compared against themselves)
would still be bad for random rather than orderly data sets. I'll go check....
testing....
Wow. Randomizing the data makes the problems even more extreme. It seems my
original test set was actually playing to this patch's strengths, not its
weaknesses. I've changed the columns to double precision and filled the
columns with random() data, where column1 gets random()^1, column2 gets
random()^2, etc. So on average the larger numbered columns will be smaller,
and the mcv list will be irrelevant, since values should not tend to repeat.
Over all queries, 47791 have better estimates after the patch, but 34802 had
worse estimates after the patch (with the remaining 17407 queries having
roughly equal quality).
The worst estimates are still ones that have a column compared to itself:
better:0, worse:33: A <= B or A <= A or A <= A
better:0, worse:33: A <= B or A = A or not A <> A
better:0, worse:33: A <= B or A >= A or not A <> A
better:0, worse:33: A <> B or A <= A
better:0, worse:33: A <> B or A <= A or A <> A
better:0, worse:33: A <> B or A <= A or A >= A
better:0, worse:33: A <> B or A <= A or not A = A
better:0, worse:33: A <> B or A > A or not A < A
better:0, worse:33: A <> B or A >= A
better:0, worse:33: A <> B or A >= A and A <= A
better:0, worse:33: A = B or not A > A or not A > A
better:0, worse:33: A >= B or not A <> A or A = A
better:0, worse:39: B <= A or B <= B or B <= B
better:0, worse:39: B <= A or B = B or not B <> B
better:0, worse:39: B <= A or B >= B or not B <> B
better:0, worse:39: B <> A or B <= B
better:0, worse:39: B <> A or B <= B or B <> B
better:0, worse:39: B <> A or B <= B or B >= B
better:0, worse:39: B <> A or B <= B or not B = B
better:0, worse:39: B <> A or B > B or not B < B
better:0, worse:39: B <> A or B >= B
better:0, worse:39: B <> A or B >= B and B <= B
better:0, worse:39: B = A or not B > B or not B > B
better:0, worse:39: B >= A or not B <> B or B = B
But there are plenty that got worse without that, such as the following
examples:
better:25, worse:39: A < B and A < B or B > A
better:10, worse:48: A < B and A < C
better:10, worse:54: A < B and A < C or C > A
I'll go test random data designed to have mcv lists of significance....
—
Mark Dilger
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
