Simplify the logic checking new range partition bounds.

The previous logic, whilst not actually wrong, was overly complex and
involved doing two binary searches, where only one was really
necessary. This simplifies that logic and improves the comments.

One visible change is that if the new partition overlaps multiple
existing partitions, the error message now always reports the overlap
with the first existing partition (the one with the lowest
bounds). The old code would sometimes report the clash with the first
partition and sometimes with the last one.

Original patch idea from Amit Langote, substantially rewritten by me.

Discussion: https://postgr.es/m/CAAJ_b947mowpLdxL3jo3YLKngRjrq9+Ej4ymduQTfYR+8=YAYQ@mail.gmail.com

src/backend/catalog/partition.c

@@ -745,78 +745,64 @@ check_new_partition_bound(char *relname, Relation parent,
 				if (partdesc->nparts > 0)
 				{
 					PartitionBoundInfo boundinfo = partdesc->boundinfo;
-					int			off1,
-								off2;
-					bool		equal = false;
+					int			offset;
+					bool		equal;
 
 					Assert(boundinfo && boundinfo->ndatums > 0 &&
 						   boundinfo->strategy == PARTITION_STRATEGY_RANGE);
 
 					/*
-					 * Firstly, find the greatest range bound that is less
-					 * than or equal to the new lower bound.
+					 * Test whether the new lower bound (which is treated
+					 * inclusively as part of the new partition) lies inside an
+					 * existing partition, or in a gap.
+					 *
+					 * If it's inside an existing partition, the bound at
+					 * offset + 1 will be the upper bound of that partition,
+					 * and its index will be >= 0.
+					 *
+					 * If it's in a gap, the bound at offset + 1 will be the
+					 * lower bound of the next partition, and its index will be
+					 * -1. This is also true if there is no next partition,
+					 * since the index array is initialised with an extra -1 at
+					 * the end.
 					 */
-					off1 = partition_bound_bsearch(key, boundinfo, lower, true,
-												   &equal);
+					offset = partition_bound_bsearch(key, boundinfo, lower,
+													 true, &equal);
 
+					if (boundinfo->indexes[offset + 1] < 0)
+					{
 						/*
-					 * off1 == -1 means that all existing bounds are greater
-					 * than the new lower bound.  In that case and the case
-					 * where no partition is defined between the bounds at
-					 * off1 and off1 + 1, we have a "gap" in the range that
-					 * could be occupied by the new partition.  We confirm if
-					 * so by checking whether the new upper bound is confined
-					 * within the gap.
+						 * Check that the new partition will fit in the gap.
+						 * For it to fit, the new upper bound must be less than
+						 * or equal to the lower bound of the next partition,
+						 * if there is one.
 						 */
-					if (!equal && boundinfo->indexes[off1 + 1] < 0)
+						if (offset + 1 < boundinfo->ndatums)
 						{
-						off2 = partition_bound_bsearch(key, boundinfo, upper,
-													   true, &equal);
+							int32		cmpval;
 
+							cmpval = partition_bound_cmp(key, boundinfo,
+														 offset + 1, upper,
+														 true);
+							if (cmpval < 0)
+							{
 								/*
-						 * If the new upper bound is returned to be equal to
-						 * the bound at off2, the latter must be the upper
-						 * bound of some partition with which the new
-						 * partition clearly overlaps.
-						 *
-						 * Also, if bound at off2 is not same as the one
-						 * returned for the new lower bound (IOW, off1 !=
-						 * off2), then the new partition overlaps at least one
-						 * partition.
+								 * The new partition overlaps with the existing
+								 * partition between offset + 1 and offset + 2.
 								 */
-						if (equal || off1 != off2)
-						{
 								overlap = true;
-
-							/*
-							 * The bound at off2 could be the lower bound of
-							 * the partition with which the new partition
-							 * overlaps.  In that case, use the upper bound
-							 * (that is, the bound at off2 + 1) to get the
-							 * index of that partition.
-							 */
-							if (boundinfo->indexes[off2] < 0)
-								with = boundinfo->indexes[off2 + 1];
-							else
-								with = boundinfo->indexes[off2];
+								with = boundinfo->indexes[offset + 2];
+							}
 						}
 					}
 					else
 					{
 						/*
-						 * Equal has been set to true and there is no "gap"
-						 * between the bound at off1 and that at off1 + 1, so
-						 * the new partition will overlap some partition. In
-						 * the former case, the new lower bound is found to be
-						 * equal to the bound at off1, which could only ever
-						 * be true if the latter is the lower bound of some
-						 * partition.  It's clear in such a case that the new
-						 * partition overlaps that partition, whose index we
-						 * get using its upper bound (that is, using the bound
-						 * at off1 + 1).
+						 * The new partition overlaps with the existing
+						 * partition between offset and offset + 1.
 						 */
 						overlap = true;
-						with = boundinfo->indexes[off1 + 1];
+						with = boundinfo->indexes[offset + 1];
 					}
 				}
 

src/test/regress/expected/create_table.out

@@ -589,7 +589,7 @@ CREATE TABLE part3 PARTITION OF range_parted2 FOR VALUES FROM (30) TO (40);
 CREATE TABLE fail_part PARTITION OF range_parted2 FOR VALUES FROM (10) TO (30);
 ERROR:  partition "fail_part" would overlap partition "part2"
 CREATE TABLE fail_part PARTITION OF range_parted2 FOR VALUES FROM (10) TO (50);
-ERROR:  partition "fail_part" would overlap partition "part3"
+ERROR:  partition "fail_part" would overlap partition "part2"
 -- now check for multi-column range partition key
 CREATE TABLE range_parted3 (
 	a int,