Re: [PATCH] Don't block HOT update by BRIN index

On 7/12/21 11:02 PM, Alvaro Herrera wrote:
> On 2021-Jul-12, Josef Šimánek wrote:
>
>>> 2) Do we actually need to calculate and store hotblockingattrs
>>> separately in RelationGetIndexAttrBitmap? It seems to me it's either
>>> NULL (with amhotblocking=false) or equal to indexattrs. So why not to
>>> just get rid of hotblockingattr and rd_hotblockingattr, and do something
>>> like
>>>
>>> case INDEX_ATTR_BITMAP_HOT_BLOCKING:
>>> return (amhotblocking) ? bms_copy(rel->rd_hotblockingattr) : NULL;
>>>
>>> I haven't tried, so maybe I'm missing something?
>>
>> relation->rd_indexattr is currently not used (at least I have not
>> found anything) for anything, except looking if other values are
>> already loaded.
>
> Oh, that's interesting. What this means is that INDEX_ATTR_BITMAP_ALL
> is no longer used; its uses must have all been replaced by something
> else. It seems the only one that currently exists is for HOT in
> heap_update, which this patch replaces with the new one. In a quick
> search, no external code depends on it, so I'd be inclined to just
> remove it ...
>
> I think a boolean is much simpler. Consider a table with 1600 columns :-)
>

I'm not sure how to verify no external code depends on that flag. I have
no idea if there's a plausible use case for it, though.

Even with 1600 columns the amount of wasted memory is only about 200B,
which is not that bad I think. Not great, not terrible.

OTOH most tables won't have any BRIN indexes, in which case indexattr
and hotblockingattr are guaranteed to be exactly the same. So maybe
that's something we could leverage - we need to calculate the "hot"
bitmap, and in most cases we can use it for indexattr too.

Maybe let's leave that for a separate patch, though?

regards

--
Tomas Vondra
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