Re: Domains versus arrays versus typmods

On Tue, Oct 19, 2010 at 9:17 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> On Tue, Oct 19, 2010 at 6:14 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> I think that what we ought to do about it is to stop exposing typelem
>>> in domains' pg_type rows.  If you want to subscript a domain value, you
>>> should have to drill down to its base type with getBaseTypeAndTypmod,
>>> which would also give you the correct typmod to apply.  If we set
>>> typelem to zero in domain pg_type rows, it shouldn't take too long to
>>> find any places that are missing this consideration --- the breakage
>>> will be obvious rather than subtle.
>
>> I fear that this is going to degrade the performance of common cases
>> as a way of debugging rare cases.
>
> We've already accepted the cost of doing getBaseTypeAndTypmod() in a
> whole lot of performance-critical parsing paths, on the off chance that
> the target datatype might be a domain.  It's not apparent to me that
> array subscripting is so important as to deserve an exemption from that.
> Especially when not doing so doesn't work.

Hmm... so are there no cases where zeroing out the typelem will cost
us an otherwise-unnecessary syscache lookup?

--
Robert Haas
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