On 12/11/2016 11:34 PM, Chris Withers wrote:
> On 01/12/2016 12:12, Francisco Olarte wrote:
>> On Thu, Dec 1, 2016 at 12:56 PM, Chris Withers
>> <chris@simplistix.co.uk> wrote:
>>> So, first observation: if I make room nullable, the exclude
>>> constraint does
>>> not apply for rows that have a room of null. I guess that's to be
>>> expected,
>>> right?
>>
>> I would expect it, given:
>>
>> n=> select null=null, null<>null, not (null=null);
>> ?column? | ?column? | ?column?
>> ----------+----------+----------
>> | |
>> (1 row)
>>
>> Those are nulls,
>
> Yes, it's a shame psql has the same repr for null and empty-string ;-)
test=# select NULL;
?column?
----------
(1 row)
test=# \pset null 'NULL'
Null display is "NULL".
test=# select NULL;
?column?
----------
NULL
(1 row)
>
>> n=> select (null=null) is null, (null<>null) is null, (not
>> (null=null)) is null;
>> ?column? | ?column? | ?column?
>> ----------+----------+----------
>> t | t | t
>> (1 row)
>>
>> I.e., the same happens with a nullable unique column, you can have one
>> of each not null values and as many nulls as you want.
>>
>> SQL null is a strange beast.
>
> Sure, I think that was the answer I was expecting but not hoping for...
>
> However, my "next question" was the one I was really hoping for help with:
>
> Working with the exclude constraint example from
> https://www.postgresql.org/docs/current/static/rangetypes.html:
>
> CREATE EXTENSION btree_gist;
> CREATE TABLE room_reservation (
> room text,
> during tsrange,
> EXCLUDE USING GIST (room WITH =, during WITH &&)
> );
>
> Next question: if lots of rows have open-ended periods
> (eg: [, 2010-01-01 15:00) or [2010-01-01 14:00,)), how does that affect
> the performance of the btree gist index backing the exclude constraint?
>
> Tom Lane made a comment on here but never followed up with a definitive
> answer. Can anyone else help?
>
> cheers,
>
> Chris
>
>
--
Adrian Klaver
adrian.klaver@aklaver.com