Andrew Dunstan <and...@dunslane.net> writes:
> On 08/07/2014 11:17 PM, Tom Lane wrote:
>> I looked into the issue reported in bug #11109. The problem appears to be
>> that jsonb's on-disk format is designed in such a way that the leading
>> portion of any JSON array or object will be fairly incompressible, because
>> it consists mostly of a strictly-increasing series of integer offsets.
> Ouch.
> Back when this structure was first presented at pgCon 2013, I wondered
> if we shouldn't extract the strings into a dictionary, because of key
> repetition, and convinced myself that this shouldn't be necessary
> because in significant cases TOAST would take care of it.
That's not really the issue here, I think. The problem is that a
relatively minor aspect of the representation, namely the choice to store
a series of offsets rather than a series of lengths, produces
nonrepetitive data even when the original input is repetitive.
> Maybe we should have pglz_compress() look at the *last* 1024 bytes if it
> can't find anything worth compressing in the first, for values larger
> than a certain size.
Not possible with anything like the current implementation, since it's
just an on-the-fly status check not a trial compression.
> It's worth noting that this is a fairly pathological case. AIUI the
> example you constructed has an array with 100k string elements. I don't
> think that's typical. So I suspect that unless I've misunderstood the
> statement of the problem we're going to find that almost all the jsonb
> we will be storing is still compressible.
Actually, the 100K-string example I constructed *did* compress. Larry's
example that's not compressing is only about 12kB. AFAICS, the threshold
for trouble is in the vicinity of 256 array or object entries (resulting
in a 1kB JEntry array). That doesn't seem especially high. There is a
probabilistic component as to whether the early-exit case will actually
fire, since any chance hash collision will probably result in some 3-byte
offset prefix getting compressed. But the fact that a beta tester tripped
over this doesn't leave me with a warm feeling about the odds that it
won't happen much in the field.
regards, tom lane
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