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Re: how is text-equality handled in postgresql?

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On 15/01/2014 12:36, Amit Langote wrote:
> On Wed, Jan 15, 2014 at 7:39 PM, Ivan Voras <ivoras@xxxxxxxxxxx> 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() -

>                 /*
>                  * 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);

That seems odd and inefficient. Why would it be necessary? I would think
indexing (and other collation-sensitive operations) don't care what the
actual collation result is for arbitrary blobs of strings, as long as
they are stable?

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