On Wed, 19 May 2021 at 22:55, Jan Kort <jan.kort@genetics.nl> wrote:
> Ok so the problem is that the MCV list is created, because there are groups of common values, but then this has no
effect,because there are not enough single values to create a histogram?
The problem is mainly that the join costing code for the merge join
looks at the statistics on the column to get the upper bound of the
merge. Ideally this value would be 1000000 in my "three" table.
However, the MCV list sees the value of 1 as a most common value due
to it appearing twice. When we consider building a histogram with the
remaining values, we see we don't have enough to build one. We need at
least 2 values. This means when the merge join costing code tries to
get the upper bound of the merge (ideally 1000000), it only has the
MCV list to go on, which just contains the value 1. The 1000000 value
is not tracked at all. This makes it appear that the merge join will
start and end with the same value, which is very cheap, and it costs
that path accordingly.
> That is not such a rare case as I would have hoped, for example this would then also cause the same behavior:
>
> values(1,1),(2,1),(3,1000000),(4,2),(5,2),(6,3),(7,3);
Yeah, technically you can add enough duplicate values to fill the MCV
list then just not enough to build the histogram. However, to
maintain the problem case, the MCVs that you do add will need to be
very early in the range. Your last MCV here is 3, which would make
the estimated merge range of 1 to 3, which is not as cheap as 1 to 1
> And it seems to do.
>
> Ranges can then be longer too:
>
> values(1,1),(2,1),(4,1),(3,1000000);
>
> for some reason I can't reverse the ranges:
>
> values(1,1000000),(2,1000000),(3,1);
>
> I don’t understand why that doesn't create a bad plan, there seems to be order involved too.
That's because the 1000000 value will make it into the MCV list and
extend the estimated range of the merge join.
> Using text as primary key seems to perform good too, even if it keeps the same "merge" plan:
That will be because of the change in sort order between text and int.
e.g 1000000 sorts before 2, for example. I don't think you should
work around the problem this way.
> What the high value is and how close it is to the low value seems to matter:
Yeah, that's because merge join can end by reading fewer values from
the "million" table. When the highest value is 1000000 it needs to
read the entire "million" table.
> values(1,1),(2,1),(3,100000);
> values(1,1),(2,1),(3,10000);
> values(1,1),(2,1),(3,1000);
>
> Gets progressively less bad, again the plan is not changed, but the execution of it becomes more efficient.
>
> It seems like it goes through the records from 1 till say 10000 one by one.
This is how Merge Join works. See
https://en.wikipedia.org/wiki/Sort-merge_join . There's a c# example
there that shows how you have to consume a value from the side with
the lowest value until you find an equal value, where you join, or a
higher value, where you must consume the other side until you find
another equal value. The join ends when you run out of values on one
side.
David