Re: Performance improvement for joins where outer side is unique

dim 100k rows, fact 1M rows
---------------------------

         master     patched
        -------     -------

 med    280.994     261.406 (-7%)


dim 1M rows, fact 10M rows
--------------------------

         master     patched
       --------    --------

   
 med   4360.157    3648.333 (-17%)


dim 10M rows, fact 100M rows
----------------------------

         master     patched
       --------    --------

 med  45898.408   40810.203 (-10%)


So the gains seem quite solid - it's not something that would make the query an order of magnitude faster, but it's well above the noise.

Of course, in practice the queries will be more complicated, making the improvement less significant, but I don't think that's a reason not to apply it.

Two minor comments on the patch:

1) the 'subquery' variable in specialjoin_is_unique_join is unused

2) in the explain output, there should probably be a space before the
   '(inner unique)' text, so

     Hash Join (inner unique) ...

   instead of

     Hash Join(inner unique)

but that's just nitpicking at this point. Otherwise the patch seems quite solid to me.