Fix GEQO to not assume its join order heuristic always works.

Back in commit 400e2c934457bef4bc3cc9a3e49b6289bd761bc0 I rewrote GEQO's
gimme_tree function to improve its heuristic for modifying the given tour
into a legal join order.  In what can only be called a fit of hubris,
I supposed that this new heuristic would *always* find a legal join order,
and ripped out the old logic that allowed gimme_tree to sometimes fail.

The folly of this is exposed by bug #12760, in which the "greedy" clumping
behavior of merge_clump() can lead it into a dead end which could only be
recovered from by un-clumping.  We have no code for that and wouldn't know
exactly what to do with it if we did.  Rather than try to improve the
heuristic rules still further, let's just recognize that it *is* a
heuristic and probably must always have failure cases.  So, put back the
code removed in the previous commit to allow for failure (but comment it
a bit better this time).

It's possible that this code was actually fully correct at the time and
has only been broken by the introduction of LATERAL.  But having seen this
example I no longer have much faith in that proposition, so back-patch to
all supported branches.

1 files changed, 25 insertions, 1 deletions

diff --git a/src/backend/optimizer/geqo/geqo_pool.c b/src/backend/optimizer/geqo/geqo_pool.c
index d2bbdce8047..9c14fa9dc22 100644
--- a/src/backend/optimizer/geqo/geqo_pool.c
+++ b/src/backend/optimizer/geqo/geqo_pool.c
@@ -92,13 +92,37 @@ random_init_pool(PlannerInfo *root, Pool *pool)
 {
 	Chromosome *chromo = (Chromosome *) pool->data;
 	int			i;
+	int			bad = 0;
 
-	for (i = 0; i < pool->size; i++)
+	/*
+	 * We immediately discard any invalid individuals (those that geqo_eval
+	 * returns DBL_MAX for), thereby not wasting pool space on them.
+	 *
+	 * If we fail to make any valid individuals after 10000 tries, give up;
+	 * this probably means something is broken, and we shouldn't just let
+	 * ourselves get stuck in an infinite loop.
+	 */
+	i = 0;
+	while (i < pool->size)
 	{
 		init_tour(root, chromo[i].string, pool->string_length);
 		pool->data[i].worth = geqo_eval(root, chromo[i].string,
 										pool->string_length);
+		if (pool->data[i].worth < DBL_MAX)
+			i++;
+		else
+		{
+			bad++;
+			if (i == 0 && bad >= 10000)
+				elog(ERROR, "geqo failed to make a valid plan");
+		}
 	}
+
+#ifdef GEQO_DEBUG
+	if (bad > 0)
+		elog(DEBUG1, "%d invalid tours found while selecting %d pool entries",
+			 bad, pool->size);
+#endif
 }
 
 /*