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Date: Fri, 29 Dec 2023 21:14:34 +0100
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Subject: Re: Statistics Import and Export
To: Corey Huinker <corey.huinker@gmail.com>
Cc: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>,
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From: Tomas Vondra <tomas.vondra@enterprisedb.com>
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On 12/29/23 17:27, Corey Huinker wrote:
>     But maybe it's enough to just do what you did - if we get an MCELEM
>     slot, can it ever contain anything else than array of elements of the
>     attribute array type? I'd bet that'd cause all sorts of issues, no?
> 
> 
> Thanks for the explanation of why it wasn't working for me. Knowing that
> the case of MCELEM + is-array-type is the only case where we'd need to
> do that puts me at ease.
> 

But I didn't claim MCELEM is the only slot where this might be an issue.
I merely asked if a MCELEM slot can ever contain an array with element
type different from the "original" attribute.

After thinking about this a bit more, and doing a couple experiments
with a trivial custom data type, I think this is true:

1) MCELEM slots for "real" array types are OK

I don't think we allow "real" arrays created by users directly, all
arrays are created implicitly by the system. Those types always have
array_typanalyze, which guarantees MCELEM has the correct element type.

I haven't found a way to either inject my custom array type or alter the
typanalyze to some custom function. So I think this is OK.


2) I'm not sure we can extend this regular data types / other slots

For example, I think I can implement a data type with custom typanalyze
function (and custom compute_stats function) that fills slots with some
other / strange stuff. For example I might build MCV with hashes of the
original data, a CountMin sketch, or something like that.

Yes, I don't think people do that often, but as long as the type also
implements custom selectivity functions for the operators, I think this
would work.


regards

-- 
Tomas Vondra
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