Fix assertion failure in generate_orderedappend_paths()

In generate_orderedappend_paths(), there is an assumption that a child
relation's row estimate is always greater than zero.  There is an
Assert verifying this assumption, and the estimate is also used to
convert an absolute tuple count into a fraction.

However, this assumption is not always valid -- for example, upper
relations can have their row estimates unset, resulting in a value of
zero.  This can cause an assertion failure in debug builds or lead to
the tuple fraction being computed as infinity in production builds.

To fix, use the row estimate from the cheapest_total path to compute
the tuple fraction.  The row estimate in this path should already have
been forced to a valid value.

In passing, update the comment for generate_orderedappend_paths() to
note that the function also considers the cheapest-fractional case
when not all tuples need to be retrieved.  That is, it collects all
the cheapest fractional paths and builds an ordered append path for
each interesting ordering.

Backpatch to v18, where this issue was introduced.

Bug: #19102
Reported-by: Kuntal Ghosh <kuntalghosh.2007@gmail.com>
Author: Richard Guo <guofenglinux@gmail.com>
Reviewed-by: Kuntal Ghosh <kuntalghosh.2007@gmail.com>
Reviewed-by: Andrei Lepikhov <lepihov@gmail.com>
Discussion: https://postgr.es/m/19102-93480667e1200169@postgresql.org
Backpatch-through: 18

0 files changed, 0 insertions, 0 deletions