Re: [COMMITTERS] pgsql: pageinspect: Try to fix some bugs in previous commit.

On Fri, Feb 3, 2017 at 9:14 PM, Ashutosh Sharma <ashu(dot)coek88(at)gmail(dot)com> wrote:
> On Fri, Feb 3, 2017 at 8:29 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> On Fri, Feb 3, 2017 at 7:41 AM, Ashutosh Sharma <ashu(dot)coek88(at)gmail(dot)com> wrote:
>>> I think UInt32GetDatum(metad->hashm_procid) looks fine, the reason
>>> being 'hashm_procid' is defined as 32-bit unsigned integer but then we
>>> may have to define procid as int8 in SQL function.
>>
>> No, you're wrong. The GetDatum you choose macro has to match the SQL
>> type, not the type of the variable that you're passing to it. For
>> example, if you've got an "int" in the code and the SQL type is
>> "int8", you have to use Int64GetDatum, not Int32GetDatum. Otherwise,
>> stuff breaks, because on some systems 64-bit integers are passed by
>> reference, not by value, so the representation that Int32GetDatum
>> produces isn't valid when interpreted by DatumGetInt64 later on. The
>> latter is expecting a pointer, but the former didn't produce one.
>>
>
> Thank you very much for detailed information and explanation. It is
> really very helpful and understandable. But, As per your explanation,
> GetDatum we choose need to match the SQL type, not the type of the
> variable used in code and I do not see any unsigned integer SQL type
> in PostgreSQL then I am just wondering on why do we have
> UInt32GetDatum or UInt64GetDatum macros.

That's a pretty good question. UInt64GetDatum is used in exactly one
place (exec_stmt_getdiag) and there's really no reason why
Int64GetDatum wouldn't be more appropriate. UInt32GetDatum is used in
a few more places, and some of those are used for hash values which
are not exposed at the SQL level so they might be legitimate, but
others like the ones in pageinspect look like fuzzy thinking that has
only survived because it happens not to break anything. I suppose if
we wanted to be really rigorous about this sort of thing, we should
change UInt32GetDatum to do something tangibly different from
Int32GetDatum, like negate all the bits. Then if somebody picked the
wrong macro it would actually fail. I'm not sure that's really the
best place to spend our effort, though. The moral of this episode is
that it's important to at least get the right width. Currently,
getting the wrong signedness doesn't actually break anything.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company