Optimizer: IS NULL optimization

[liukun4515]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

The expr column is null can be optimize if the column is not null in the schema.

If a WHERE clause includes a col_name IS NULL condition for a column that is declared as NOT NULL, that expression is optimized away.

Describe the solution you'd like

In the PG database

1. create table

create table t1(c1 int not null, c2 int not null);

2. select with filter

select * from where c1 is null;

The expr c1 is null can be optimized as false away.

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Additional context
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[liukun4515]

@Dandandan please assign this to me.

[alamb]

@liukun4515 note this optimization is only valid prior to any OUTER joins (and maybe other operators) -- you would have to check

So like if x is non null,

SELECT ... FROM foo WHERE x IS NOT NULL

Can be rewritten to

SELECT ... FROM foo 

However,

SELECT ... FROM foo LEFT JOIN bar ON (a=b AND  x IS NOT NULL)

Can NOT be rewritten to

SELECT ... FROM foo LEFT JOIN bar ON (a=b)

The same general pattern applies in the select list

[alamb]

BTW I suggest adding this rule to https://github.com/apache/arrow-datafusion/blob/master/datafusion/src/optimizer/constant_folding.rs if possible

[liukun4515]

ote this optimization is only valid prior to any OUTER joins

@alamb

The idea is from is null optimize.
I think the is null optimization has no side effect if the column has not null constraint.
But the is not null optimization should be checked with more cases, I will do it in the followup pull request.