opt: Synthesize equality conditions during AssociateJoin rule

[andy-kimball]

When 3 or more tables are joined, it is not uncommon for 3 or more columns to be equivalent. For example:

SELECT * FROM a, b, c WHERE a.x=b.y AND b.y=c.z

The optimizer will consider these join orders:

1. (a join b) join c
2. a join (b join c)

But it will not consider (a join c) join b, because there is no explicit join condition between a and c. This could cause it to miss the best plan (in fact, it does miss the best plan for TPC-H Q9 for this reason).

The optimizer knows that columns (x, y, z) are all equivalent to one another. It can synthesize any missing equality conditions during the AssociateJoin rule, thus opening up new ordering possibilities.

For TPC-H Q9, adding the missing equality condition reduces execution time from ~120s to ~20s.

[justinj]

I think this is a dup of #36226 that Radu found, I've been meaning to look into it but haven't yet.

[rytaft]

I'm working on this now