subquery performance and EXISTS

I am writing the subquery chapter of the book.  Tom, you once mentioned
you could get subqueries to use HASH joins, but you didn't because
someone said it may a run out of memory.

I would suggest that if it is that large a subquery, our nested loop
handling will take forever, so would never finish anyway, unless the
outer query has only a few rows.

Tom, does it still look easy to change, and if so, can you change it to
hash?  I know the real fix is for multiple target lists.

I would rather not have to go mention a serious performance workaround
in the book, because the limitation would be published perhaps for many
years.

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