Issue duckdb#9396: AsOf Inequality Optimisation

Don't remove the inequality condition for AsOf joins
even if the optimiser can tell it is always true.

fixes: duckdb#9396
fixes: duckdblabs/duckdb-internal#517

src/optimizer/statistics/operator/propagate_join.cpp

@@ -67,6 +67,21 @@ void StatisticsPropagator::PropagateStatistics(LogicalComparisonJoin &join, uniq
 				break;
 			case FilterPropagateResult::FILTER_ALWAYS_TRUE:
 				// filter is always true
+				// If this is the inequality for an AsOf join,
+				// then we must leave it in because it also flags
+				// the semantics of restricting to a single match
+				// so we can't replace it with an equi-join on the remaining conditions.
+				if (join.type == LogicalOperatorType::LOGICAL_ASOF_JOIN) {
+					switch (condition.comparison) {
+					case ExpressionType::COMPARE_GREATERTHAN:
+					case ExpressionType::COMPARE_GREATERTHANOREQUALTO:
+					case ExpressionType::COMPARE_LESSTHAN:
+					case ExpressionType::COMPARE_LESSTHANOREQUALTO:
+						continue;
+					default:
+						break;
+					}
+				}
 				if (join.conditions.size() > 1) {
 					// there are multiple conditions: erase this condition
 					join.conditions.erase(join.conditions.begin() + i);

test/sql/join/asof/test_asof_join.test

@@ -17,6 +17,26 @@ INSERT INTO events0 VALUES
 	(8, 3)
 ;
 
+# Prevent optimiser from removing true inequalities
+statement ok
+create table prices("when" timestamp, symbol int, price int);
+
+statement ok
+insert into prices values ('2020-01-01 00:00:00', 1, 42);
+
+statement ok
+create table trades("when" timestamp, symbol int);
+
+statement ok
+insert into trades values ('2020-01-01 00:00:03', 1);
+
+query III
+SELECT t.*, p.price
+FROM trades t ASOF JOIN prices p 
+  ON t.symbol = p.symbol AND t.when >= p.when;
+----
+2020-01-01 00:00:03	1	42
+
 # Use an ASOF join inside of a correlated subquery