Обсуждение: btree gist indices, null and open-ended tsranges
Hi All, 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 &&) ); 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? 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? cheers, Chris
Hi Chris:
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, BTW:
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.
Francisco Olarte.
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 ;-) > 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
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
On 12/12/2016 14:33, Adrian Klaver wrote: > 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) Sure, so perhaps the default should change? Of course, no-one has yet offered anything on the question 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