pgsql: Guard against input_rows == 0 in estimate_num_groups().

Guard against input_rows == 0 in estimate_num_groups().

This case doesn't normally happen, because the planner usually clamps
all row estimates to at least one row; but I found that it can arise
when dealing with relations excluded by constraints. Without a defense,
estimate_num_groups() can return zero, which leads to divisions by zero
inside the planner as well as assertion failures in the executor.

An alternative fix would be to change set_dummy_rel_pathlist() to make
the size estimate for a dummy relation 1 row instead of 0, but that seemed
pretty ugly; and probably someday we'll want to drop the convention that
the minimum rowcount estimate is 1 row.

Back-patch to 8.4, as the problem can be demonstrated that far back.

Branch
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REL9_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/dbd68df9b636d4b36469dab7281a2e7d38863570

Modified Files
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src/backend/utils/adt/selfuncs.c | 8 ++++++++
1 files changed, 8 insertions(+), 0 deletions(-)