sql: Improve error message when user's modified MR zone config

Previously we were returning an inaccurate error message when a user
modified zone config was present on a multi-region table. This was due
to the fact that we'd key off of num_replicas being set in a placeholder
zone config and think that that was the difference between the two zone
configs. In actuality, the difference was in the subzones.

This commit allows for the reporting of the true difference, the
presence of the index in the subzones.

Release note: None

pkg/ccl/logictestccl/testdata/logic_test/multi_region_zone_configs

@@ -730,7 +730,7 @@ SET override_multi_region_zone_config = true;
 ALTER INDEX tbl1@tbl1_i_idx CONFIGURE ZONE USING num_replicas=10;
 SET override_multi_region_zone_config = false
 
-statement error attempting to update zone configuration for table tbl1 which contains modified field "num_replicas"
+statement error attempting to update zone config which contains an extra zone configuration for index tbl1@tbl1_i_idx with field num_replicas populated
 ALTER TABLE tbl1 SET LOCALITY GLOBAL
 
 # This statement should wipe out the zone configuration we applied on the
@@ -816,20 +816,36 @@ CREATE TABLE tbl2 (
   pk INT PRIMARY KEY,
   i INT,
   INDEX(i)
-) LOCALITY REGIONAL BY ROW;
+) LOCALITY REGIONAL BY ROW
 
 statement ok
 SET override_multi_region_zone_config = true;
 ALTER INDEX tbl2@tbl2_i_idx CONFIGURE ZONE USING num_replicas=10;
 SET override_multi_region_zone_config = false
 
 statement error attempting to update zone config which contains an extra zone configuration for index tbl2@tbl2_i_idx with field num_replicas populated
+ALTER TABLE tbl2 SET LOCALITY GLOBAL
+
+statement ok
+SET override_multi_region_zone_config = true;
 ALTER TABLE tbl2 SET LOCALITY GLOBAL;
+SET override_multi_region_zone_config = false
+
+statement ok
+ALTER TABLE tbl2 SET LOCALITY REGIONAL BY ROW
+
+statement ok
+SET override_multi_region_zone_config = true;
+ALTER TABLE tbl2 CONFIGURE ZONE USING num_replicas=10;
+SET override_multi_region_zone_config = false
+
+statement error attempting to update zone configuration for table tbl2 which contains modified field "num_replicas"
+ALTER TABLE tbl2 SET LOCALITY GLOBAL
 
 statement ok
 SET override_multi_region_zone_config = true;
 ALTER TABLE tbl2 SET LOCALITY GLOBAL;
-SET override_multi_region_zone_config = false;
+SET override_multi_region_zone_config = false
 
 statement ok
-ALTER TABLE tbl2 SET LOCALITY REGIONAL BY TABLE IN PRIMARY REGION;
+ALTER TABLE tbl2 SET LOCALITY REGIONAL BY TABLE IN PRIMARY REGION

pkg/config/zonepb/zone.go

@@ -626,6 +626,7 @@ type DiffWithZoneMismatch struct {
 func (z *ZoneConfig) DiffWithZone(
 	other ZoneConfig, fieldList []tree.Name,
 ) (bool, DiffWithZoneMismatch, error) {
+	mismatchingNumReplicas := false
 	for _, fieldName := range fieldList {
 		switch fieldName {
 		case "num_replicas":
@@ -634,6 +635,13 @@ func (z *ZoneConfig) DiffWithZone(
 			}
 			if z.NumReplicas == nil || other.NumReplicas == nil ||
 				*z.NumReplicas != *other.NumReplicas {
+				// In cases where one of the zone configs are placeholders,
+				// defer the error reporting to below so that we can correctly
+				// report on a subzone difference, should one exist.
+				if z.IsSubzonePlaceholder() || other.IsSubzonePlaceholder() {
+					mismatchingNumReplicas = true
+					continue
+				}
 				return false, DiffWithZoneMismatch{
 					Field: "num_replicas",
 				}, nil
@@ -825,6 +833,13 @@ func (z *ZoneConfig) DiffWithZone(
 			}, nil
 		}
 	}
+	// If we've got a mismatch in the num_replicas field and we haven't found
+	// any other mismatch, report on num_replicas.
+	if mismatchingNumReplicas {
+		return false, DiffWithZoneMismatch{
+			Field: "num_replicas",
+		}, nil
+	}
 	return true, DiffWithZoneMismatch{}, nil
 }