Re: how is text-equality handled in postgresql?

On Wed, Jan 15, 2014 at 7:39 PM, Ivan Voras <ivoras(at)freebsd(dot)org> wrote:
> On 15/01/2014 10:10, Gábor Farkas wrote:
>> hi,
>>
>> when i create an unique-constraint on a varchar field, how exactly
>> does postgresql compare the texts? i'm asking because in UNICODE there
>> are a lot of complexities about this..
>>
>> or in other words, when are two varchars equal in postgres? when their
>> bytes are? or some algorithm is applied?
>
> By default, it is "whatever the operating system thinks it's right".
> PostgreSQL doesn't have its own collation code, it uses the OS's locale
> support for this.
>

Just to add to this, whenever strcoll() (a locale aware comparator)
says two strings are equal, postgres re-compares them using strcmp().
See following code snippet off
src/backend/utils/adt/varlena.c:varstr_cmp() -

#ifdef HAVE_LOCALE_T
if (mylocale)
result = strcoll_l(a1p, a2p, mylocale);
else
#endif
result = strcoll(a1p, a2p);

/*
* In some locales strcoll() can claim that
nonidentical strings are
* equal. Believing that would be bad news for a
number of reasons,
* so we follow Perl's lead and sort "equal" strings
according to
* strcmp().
*/
if (result == 0)
result = strcmp(a1p, a2p);

--
Amit Langote