Thread

Re: Some optimizations for COALESCE expressions during constant folding

Tender Wang <tndrwang@gmail.com> — 2025-11-25T12:07:17Z

Richard Guo <guofenglinux@gmail.com> 于2025年11月25日周二 18:51写道：

> Currently, we perform some simplification for Const arguments of a
> COALESCE expression.  For instance, if the first argument is a
> non-null constant, we use that constant as the result for the entire
> expression.  If a subsequent argument is a non-null constant, all
> following arguments are dropped since they will never be reached.
>
> We can extend this simplification to Var arguments since the NOT NULL
> attribute information is now available during constant folding.  0001
> implements this.
>

I took a quick look at the 0001. It seems correct to me.
One thing I want to confirm is that if var_is_nonnullable() returns true,
we can make sure that
the Var is 100% nonnullable, no matter what kind of join reorder happens.


> Another optimization that can be done for a COALESCE expression is
> when it is used in a NullTest.  We can determine that a COALESCE
> expression is non-nullable by checking if at least one of its
> arguments is proven non-null.  This information can then be used to
> reduce the NullTest qual to a constant true or false.  0002 implements
> this.  (I'm wondering whether it'd be better to consolidate the
> non-null check for Const, Var, and CoalesceExpr into one helper
> function to simplify the code in eval_const_expressions.)
>

I have no objections to the 0002 code logic.
But I wonder how often users write "COALECE()  is not null" in their query.
Before this patch, I didn't find the case in the regression test cases.


-- 
Thanks,
Tender Wang