Re: [HACKERS] <> join selectivity estimate question

On 6 September 2017 at 04:14, Ashutosh Bapat
<ashutosh.bapat@enterprisedb.com> wrote:
> On Fri, Jul 21, 2017 at 4:10 AM, Thomas Munro
> <thomas.munro@enterprisedb.com> wrote:
>>
>> Thanks.  Bearing all that in mind, I ran through a series of test
>> scenarios and discovered that my handling for JOIN_ANTI was wrong: I
>> thought that I had to deal with inverting the result, but I now see
>> that that's handled elsewhere (calc_joinrel_size_estimate() I think).
>> So neqjoinsel should just treat JOIN_SEMI and JOIN_ANTI exactly the
>> same way.
>
> I agree, esp. after looking at eqjoinsel_semi(), which is used for
> both semi and anti joins, it becomes more clear.
>
>>
>> That just leaves the question of whether we should try to handle the
>> empty RHS and single-value RHS cases using statistics.  My intuition
>> is that we shouldn't, but I'll be happy to change my intuition and
>> code that up if that is the feedback from planner gurus.
>
> Empty RHS can result from dummy relations also, which are produced by
> constraint exclusion, so may be that's an interesting case. Single
> value RHS may be interesting with partitioned table with all rows in a
> given partition end up with the same partition key value. But may be
> those are just different patches. I am not sure.
>
>>
>> Please find attached a new version, and a test script I used, which
>> shows a bunch of interesting cases.  I'll add this to the commitfest.
>
> I added some "stable" tests to your patch taking inspiration from the
> test SQL file. I think those will be stable across machines and runs.
> Please let me know if those look good to you.

Why isn't this an open item for PG10?

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