From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
---|---|
To: | Andres Freund <andres(at)2ndquadrant(dot)com> |
Cc: | pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: [RFC] indirect toast tuple support |
Date: | 2013-02-19 14:12:02 |
Message-ID: | CA+Tgmoa0nL_F=8hZec6D=J051YovssiJy40uHxztNfW3pbBAqQ@mail.gmail.com |
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Lists: | pgsql-hackers |
On Tue, Feb 19, 2013 at 9:00 AM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
>> So the other way that we could do this is to use something that's the
>> same size as a TOAST pointer but has different content - the
>> seemingly-obvious choice being va_toastrelid == 0.
>
> Unfortunately that would mean you need to copy the varatt_external (or
> whatever it would be called) to aligned storage to check what it
> is. Thats why I went the other way.
How big a problem is that, though?
>> I'd be a little
>> reluctant to do it the way you propose because we might, at some
>> point, want to try to reduce the size of toast pointers. If you have
>> a tuple with many attributes, the size of the TOAST pointers
>> themselves starts to add up. It would be nice to be able to have 8
>> byte or even 4 byte toast pointers to handle those situations. If we
>> steal one or both of those lengths to mean "the data is cached in
>> memory somewhere" then we can't use those lengths in a smaller on-disk
>> representation, which would seem a shame.
>
> I agree. As I said above, having the type overlayed into the lenght was
> and is a bad idea, I just haven't found a better one thats compatible
> yet.
> Except inventing typlen=-3 aka "toast2" or something. But even that
> wouldn't help getting rid of existing pg_upgraded tables. Besides being
> a maintenance nightmare.
>
> The only reasonable thing I can see us doing is renaming
> varattrib_1b_e.va_len_1be into va_type and redefine VARSIZE_1B_E into a
> switch that maps types into lengths. But I think I would put this off,
> except placing a comment somewhere, until its gets necessary.
I guess I wonder how hard we think it would be to insert such a thing
when it becomes necessary. How much stuff is there out there that
cares about the fact that that length is a byte?
>> But having said that, +1 on the general idea of getting something like
>> this done. We really need a better infrastructure to avoid copying
>> large values around repeatedly in memory - a gigabyte is a lot of data
>> to be slinging around.
>>
>> Of course, you will not be surprised to hear that I think this is 9.4 material.
>
> Yes, obviously. But I need time to actually propose a working patch (I
> already found 2 bugs in what I had submitted), thats why I brought it up
> now. No point in wasting time if there's an oviously better idea around.
Cool.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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