Re: WIP: BRIN multi-range indexes

On Mon, Nov 9, 2020 at 1:39 PM Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com>
wrote:
>
>
> While investigating the failures, I've tried increasing the values a
> lot, without observing any measurable increase in runtime. IIRC I've
> even used (10 * target_partlen) or something like that. That tells me
> it's not very sensitive part of the code, so I'd suggest to simply use
> something that we know is large enough to be safe.

Okay, then it's not worth being clever.

> For example, the largest bloom filter we can have is 32kB, i.e. 262kb,
> at which point the largest gap is less than 95 (per the gap table). And
> we may use up to BLOOM_MAX_NUM_PARTITIONS, so let's just use
> BLOOM_MAX_NUM_PARTITIONS * 100

Sure.

> FWIW I wonder if we should do something about bloom filters that we know
> can get larger than page size. In the example I used, we know that
> nbits=575104 is larger than page, so as the filter gets more full (and
> thus more random and less compressible) it won't possibly fit. Maybe we
> should reject that right away, instead of "delaying it" until later, on
> the basis that it's easier to fix at CREATE INDEX time (compared to when
> inserts/updates start failing at a random time).

Yeah, I'd be inclined to reject that right away.

> The problem with this is of course that if the index is multi-column,
> this may not be strict enough (i.e. each filter would fit independently,
> but the whole index row is too large). But it's probably better to do at
> least something, and maybe improve that later with some whole-row check.

A whole-row check would be nice, but I don't know how hard that would be.

As a Devil's advocate proposal, how awful would it be to not allow
multicolumn brin-bloom indexes?

--
John Naylor
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