release-25.1: execbuilder: fix a silly bug around finding the most recent full stat

pkg/sql/opt/exec/execbuilder/relational.go

@@ -410,8 +410,9 @@ func (b *Builder) maybeAnnotateWithEstimates(node exec.Node, e memo.RelExpr) {
 				// The first stat is the most recent full one.
 				var first int
 				for first < tab.StatisticCount() &&
-					tab.Statistic(first).IsPartial() ||
-					(tab.Statistic(first).IsForecast() && !b.evalCtx.SessionData().OptimizerUseForecasts) {
+					(tab.Statistic(first).IsPartial() ||
+						(tab.Statistic(first).IsMerged() && !b.evalCtx.SessionData().OptimizerUseMergedPartialStatistics) ||
+						(tab.Statistic(first).IsForecast() && !b.evalCtx.SessionData().OptimizerUseForecasts)) {
 					first++
 				}
 

pkg/sql/opt/exec/execbuilder/testdata/partial_stats

@@ -1381,7 +1381,7 @@ distribution: local
 vectorized: true
 ·
 • scan
-  estimated row count: 4 (36% of the table; stats collected <hidden> ago)
+  estimated row count: 4 (40% of the table; stats collected <hidden> ago)
   table: ka@ka_a_idx
   spans: [/6 - ]
 
@@ -1420,3 +1420,38 @@ vectorized: true
   estimated row count: 1 (9.1% of the table; stats collected <hidden> ago)
   table: ka@ka_a_idx
   spans: [/10 - /10]
+
+subtest regression_148316
+
+# Ensure we can run DELETE statement on system.table_statistics.
+statement ok
+INSERT INTO system.users VALUES ('node', NULL, true, 3);
+
+statement ok
+GRANT node TO root;
+
+# Keep only partial stats on the target table.
+statement ok
+DELETE FROM system.table_statistics WHERE name NOT LIKE '%partial%' AND "tableID" = 'ka'::REGCLASS::OID;
+
+query TT
+SELECT statistics_name, column_names FROM [SHOW STATISTICS FOR TABLE ka] ORDER BY created
+----
+ka_partialstat  {a}
+
+# Ensure that the system table is read on the next query.
+statement ok
+SELECT crdb_internal.clear_table_stats_cache();
+
+query T
+EXPLAIN SELECT * FROM ka WHERE a = 10
+----
+distribution: local
+vectorized: true
+·
+• scan
+  missing stats
+  table: ka@ka_a_idx
+  spans: [/10 - /10]
+
+subtest end