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@xxxxxxxxxxxx> 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-L456Hence 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@xxxxxxxxxxxxx> wrote:"David G. Johnston" <david.g.johnston@xxxxxxxxx> writes:
> On Wednesday, December 18, 2024, Adrian Garcia Badaracco <
> adrian@xxxxxxxxxxxx> 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
