>
> However the thing becomes complex with the below 2 cases.
>
> 1. SELECT * FROM t INNER JOIN j on t.nullable = q.b;
> We know t.b will be not null **finally**. But the current plan may
> something
> like this:
>
> QUERY PLAN
> ------------------------------------------
> Merge Join
> Merge Cond: (t.nullable = j.something)
> -> Sort
> Sort Key: t.nullable
> -> Seq Scan on t
> -> Sort
> Sort Key: j.something
> -> Seq Scan on j
> (8 rows)
>
> which means the Path "Seq Scan on t" still contains some null values. At
> least,
> we should not assume t.nullable is "not nullable" the base relation stage.
>
> 2. SELECT t.a FROM j LEFT JOIN t ON t.b = t.a;
> Even the t.a is not null by definition, but it may have null **finally**
> due to
> the outer join.
>
The above 2 cases have been addressed by defining the notnullattrs on
every RelOptInfo, and maintaining them on every join. However, per offline
discussion with David, IIUC, there is a more case to think about.
CREATE TABLE t (a INT, b INT);
SELECT * FROM t WHERE a = 1 and b = 2;
We know b is not null after we evaluate the qual b = 2, but it may still
nullable when we just evaluate a = 1;
I prefer to not handle it by saying the semantics of notnullattrs is correct
after we evaluate all the quals on its RelOptInfo.
> It would be good to agree on the correct representation for Vars that
>> cannot produce NULLs so that we don't shut the door on classes of
>> optimisation that require something other than what you need for your
>> case.
>>
>>
> Looks we have to maintain not null on the general RelOptInfo level rather
> than Base
> RelOptInfo. But I don't want to teach Var about the notnull so far. The
> reasons are: 1).
> We need to maintain the Planner version and Parser version due to the VIEW
> case.
> 2). We have to ignore the extra part for equal(Var, Var) . 3). Var is
> usually shared among
> different RelOptInfo. which means we have to maintain different copies for
> this purpose IIUC.
>
> I assume we want to know if a Var is nullable with a function like.
> is_var_notnullable(Var *var, Relids relids). If so, we can define the
> data as below:
>
> struct RelOptInfo {
>
> Bitmapset** notnullattrs;
> ..
> };
>
> After this we can implement the function as:
>
/*
* is_var_notnullable
* Check if the var is nullable for a given RelOptIno after
* all the quals on it have been evaluated.
*
* var is the var to check, relids is the ids of a RelOptInfo
* we will check on.
*/
bool
is_var_notnullable(Var* var, Relids relids)
{
RelOptInfo *rel = find_rel_by_relids(reldis);
return bms_is_member(var->varattno, rel->notnullattrs[var->varno]);
}
Do you think this is a reasonable solution?
>
bool
> is_var_notnullable(Var* var, Relids relids)
> {
> RelOptInfo *rel = find_rel_by_relids(reldis);
> return bms_is_member(var->varattno, rel->notnullattrs[var->varno]);
> }
>
> Probably we can make some hackers to reduce the notnullattrs's memory usage
> overhead.
>
> --
Best Regards
Andy Fan (https://www.aliyun.com/)
