Re: Wrapping a where clause to preserve rows with nulls

Well, there is a wrinkle: if the predicate returns `false` but one of the
columns is null then the whole thing ends up `true` when I'd want it to be
`false`. Say col_a = [1] and col_b = [null]:

WHERE (col_a < 1 AND col_b > 1) OR col_a IS NULL OR col_b IS NULL -> WHERE
(false AND null) OR false OR true -> WHERE false OR false OR true -> true.

That's still a pretty good solution for now.

On Wed, Dec 18, 2024 at 10:41 PM Adrian Garcia Badaracco <
adrian(at)adriangb(dot)com> wrote:

> Thank you for the great idea Tom. While yes I can't modify the original
> WHERE clause I do think I'll be able to introspect it or get the system
> generating it to tell me which columns it references and then add an OR x
> is NULL OR y is NULL ...
>
> For context, just in case it's interesting, I store Parquet statistics in
> a Postgres table and run the output of this thing on them:
> https://github.com/apache/datafusion/blob/f92442ea8e8944c78f8e40d6648d049ff8e335ec/datafusion/physical-optimizer/src/pruning.rs#L146-L456
> Hence why I can't really control the WHERE clause (at least not without
> re-implementing a bunch of finicky error prone code).
>
> On Wed, Dec 18, 2024 at 10:38 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
>> "David G. Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> writes:
>> > On Wednesday, December 18, 2024, Adrian Garcia Badaracco <
>> > adrian(at)adriangb(dot)com> wrote:
>> >> Is there any way to include the rows where the predicate evaluates to
>> null
>> >> while still using an index?
>>
>> > ... A btree index, which handles =, can’t be told to behave
>> > differently and so cannot fulfill your desire to produce rows where the
>> > stored value is null; it can only produce those equal to 5000.
>>
>> Not in a single scan, no. But multiple scans are possible:
>>
>> regression=# create table t (id int unique);
>> CREATE TABLE
>> regression=# explain select * from t where id = 5000 or id is null;
>> QUERY PLAN
>>
>>
>> ------------------------------------------------------------------------------
>> Bitmap Heap Scan on t (cost=8.42..18.98 rows=14 width=4)
>> Recheck Cond: ((id IS NULL) OR (id = 5000))
>> -> BitmapOr (cost=8.42..8.42 rows=14 width=0)
>> -> Bitmap Index Scan on t_id_key (cost=0.00..4.25 rows=13
>> width=0)
>> Index Cond: (id IS NULL)
>> -> Bitmap Index Scan on t_id_key (cost=0.00..4.16 rows=1
>> width=0)
>> Index Cond: (id = 5000)
>> (7 rows)
>>
>> The OP was quite unclear about what semantics he wants for
>> multiple-variable WHERE clauses, but maybe something like this
>> would work:
>>
>> WHERE (original-clause) OR x IS NULL OR y IS NULL OR ...
>>
>> where each variable mentioned in original-clause is allowed
>> to also be NULL. Or perhaps what is wanted is
>>
>> WHERE (original-clause) OR (x IS NULL AND y IS NULL AND ...)
>>
>> ??
>>
>> regards, tom lane
>>
>