Add more tests

Ensure that the code path with pushdown is actually covered.

Author: alex-spies

x-pack/plugin/esql/qa/testFixtures/src/main/resources/union_types.csv-spec

@@ -393,7 +393,9 @@ count:long | @timestamp:date
          0 | 2023-10-23T13:50:00.000Z
 ;
 
-multiIndexIpStatsPushableCount
+multiIndexIpStatsNonPushableCount
+// Could be pushed to Lucene if we knew whether ip fields are single valued or not.
+// May still be pushed down on multi-node environments if a node has only the index where client_ip is keyword.
 required_capability: union_types
 required_capability: fix_count_pushdown_for_union_types
 
@@ -405,7 +407,9 @@ count:long
 14
 ;
 
-multiIndexIpStatsPushableCountEval
+multiIndexIpStatsNonPushableCountEval
+// Could be pushed to Lucene if we knew whether ip fields are single valued or not.
+// May still be pushed down on multi-node environments if a node has only the index where client_ip is keyword.
 required_capability: union_types
 required_capability: fix_count_pushdown_for_union_types
 
@@ -418,6 +422,45 @@ count:long
 14
 ;
 
+multiIndexIpStatsNonPushableCountWithFilter
+// Currently not pushable: has 2 aggs and we don't consider multi-typed fields aggregatable.
+required_capability: union_types
+required_capability: fix_count_pushdown_for_union_types
+
+FROM sample_data, sample_data_ts_long
+| STATS count_matching=count(@timestamp::long) WHERE @timestamp::long >= 1698069301543,
+        total_count=count(@timestamp::long)
+;
+
+count_matching:long | total_count:long
+2                   | 14
+;
+
+multiIndexIpStatsPushableCount
+required_capability: union_types
+required_capability: fix_count_pushdown_for_union_types
+
+FROM sample_data, sample_data_ts_long
+| STATS count=count(@timestamp::long)
+;
+
+count:long
+14
+;
+
+multiIndexIpStatsPushableCountEval
+required_capability: union_types
+required_capability: fix_count_pushdown_for_union_types
+
+FROM sample_data, sample_data_ts_long
+| EVAL @timestamp = @timestamp::long
+| STATS count=count(@timestamp)
+;
+
+count:long
+14
+;
+
 multiIndexIpStringStatsInline2
 required_capability: union_types
 required_capability: union_types_agg_cast

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/optimizer/rules/physical/local/PushStatsToSource.java

@@ -103,6 +103,10 @@ private Tuple<List<Attribute>, List<EsStatsQueryExec.Stat>> pushableStats(
                             }
                             if (fieldName != null) {
                                 if (count.hasFilter()) {
+                                    // Note: currently, it seems like we never actually perform stats pushdown if we reach here.
+                                    // That's because stats pushdown only works for 1 agg function (without BY); but in that case, filters
+                                    // are extracted into a separate filter node upstream from the aggregation (and hopefully pushed into
+                                    // the EsQueryExec separately).
                                     if (canPushToSource(count.filter()) == false) {
                                         return null; // can't push down
                                     }