Re: BUG #18468: CREATE TABLE ... LIKE leaves orphaned column reference in extended statistics

Tom Lane <tgl@sss.pgh.pa.us> 于2024年5月17日周五 02:45写道：

> Tender Wang <tndrwang@gmail.com> writes:
> > generateClonedExtStatsStmt just copy t0's ext_stat info to t1.
>
> So this is a complete mess.  The fundamental problem is that
> transformTableLikeClause believes it can process LIKE_STATISTICS
> requests immediately, because:
>
>      * We may copy extended statistics if requested, since the
> representation
>      * of CreateStatsStmt doesn't depend on column numbers.
>

The above comments in  transformTableLikeClause and below comments in
generateClonedExtStatsStmt:
   * Now handle expressions, if there are any. The order (with respect to
   * regular attributes) does not really matter for extended stats, so we
   * simply append them after simple column references.
now should do some revise.

That was true apparently in the original, limited implementation
> where we only supported CREATE STATISTICS on plain columns.
> It's completely wrong for statistics on expressions.  IMO what we
> need to do is make LIKE_STATISTICS work more like LIKE_INDEXES:
> postpone the transformation until expandTableLikeClause when we know
> the mapping from source columns to destination columns, and have a
> chance at converting the expression trees properly, comparably to
> generateClonedIndexStmt.

> Right now, there is nothing to do in transformExtendedStatistics.
> > Can we update the varattno to the right value here?
>
> We don't know the column mapping there, either.  What we need to have
> our hands on is the "attmap" computed in expandTableLikeClause, and
> then we can pass the stats expressions through map_variable_attnos().


This fix looks better than what I said in  transformExtendedStatistics.


>
I think this might not need to be a really large patch, but it
> definitely has to change where generateClonedExtStatsStmt is
> called from.  I can have a go at it if Tomas doesn't want to.
>
>                         regards, tom lane
>


-- 
Tender Wang
OpenPie:  https://en.openpie.com/