Re: patch : Allow toast tables to be moved to a different tablespace

On 3 August 2015 at 01:35, Andreas Karlsson <andreas(at)proxel(dot)se> wrote:

> On 07/15/2015 09:30 PM, Robert Haas wrote:
>
>> On Tue, Jul 14, 2015 at 5:57 PM, Jim Nasby <Jim(dot)Nasby(at)bluetreble(dot)com>
>> wrote:
>>
>>> On 7/7/15 7:07 AM, Andres Freund wrote:
>>>
>>>> On 2015-07-03 18:03:58 -0400, Tom Lane wrote:
>>>>
>>>>> I have just looked through this thread, and TBH I think we should
>>>>> reject
>>>>> this patch altogether --- not RWF, but "no we don't want this". The
>>>>> use-case remains hypothetical: no performance numbers showing a
>>>>> real-world
>>>>> benefit have been exhibited AFAICS.
>>>>>
>>>>
>>>>
>>>> It's not that hard to imagine a performance benefit though? If the
>>>> toasted column is accessed infrequently/just after filtering on other
>>>> columns (not uncommon) it could very well be beneficial to put the main
>>>> table on fast storage (SSD) and the toast table on slow storage
>>>> (spinning rust).
>>>>
>>>> I've seen humoungous toast tables that are barely ever read for tables
>>>> that are constantly a couple times in the field.
>>>>
>>>
>>> +1. I know of one case where the heap was ~8GB and TOAST was over 400GB.
>>>
>>
>> Yeah, I think that's a good argument for this. I have to admit that
>> I'm a bit fatigued by this thread - it started out as a "learn
>> PostgreSQL" project, and we iterated through a few design problems
>> that made me kind of worried about what sort of state the patch was in
>> - and now this patch is more than 4 years old. But if some committer
>> still has the energy to go through it in detail and make sure that all
>> of the problems have been fixed and that the patch is, as Andreas
>> says, in good shape, then I don't see why we shouldn't take it.
>>
>
> I personally think the patch is in a decent shape, and a worthwhile
> feature. I agree though with Tom's objections about the pg_dump code.
>
> I do not have enough time or interest right now to fix up this patch (this
> is not a feature I personally have a lot of interest in), but if someone
> else wishes to I do not think it would be too much work.

To me the patch looks like it is in reasonable shape, caveats noted.

The patch doesn't really take us very far forwards in terms of
administration since workarounds exist which people use and understand.
Noting Tom's comments on additional complexity it seems like it could be a
source of bugs or production problems.

The deciding factor is that it brings TOAST as a keyword for us to support
forever. While reviewing this, I've come to realise that a better approach
to column stores and/or vertical partitioning is the more general way of
handling this, so creating a support legacy at this time doesn't make sense
to me personally.

On balance, the negatives seem to outweigh the positives so isn't the best
use of my time to fix it up.

I'm now returning this with feedback.

--
Simon Riggs http://www.2ndQuadrant.com/
<http://www.2ndquadrant.com/>
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