WIP Using only 'synthetic' in union types removal to fix fork bug

Author: craigtaverner

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

@@ -1397,6 +1397,22 @@ event_duration:long | _index:keyword       | ts:date                   | ts_str:
 8268153             | sample_data_ts_nanos | 2023-10-23T13:52:55.015Z  | 2023-10-23T13:52:55.015123456Z  | 1698069175015123456 |  172.21.3.15   |  172.21.3.15
 ;
 
+multiIndexInlinestatsOfMultiTypedField
+required_capability: inlinestats_v11
+// https://github.com/elastic/elasticsearch/issues/133973
+FROM apps, apps_short
+| INLINESTATS s = sum(id::integer)
+| SORT name, version
+| LIMIT 5
+;
+
+id:unsupported | name:keyword | version:version | s:long       
+null           | aaaaa        | 1               | 210
+null           | aaaaa        | 1               | 210
+null           | aaaaa        | 1.2.3.4         | 210
+null           | aaaaa        | 1.2.3.4         | 210
+null           | bbbbb        | 2.1             | 210
+;
 
 inlineStatsUnionGroup-Ignore
 required_capability: union_types
@@ -1720,6 +1736,33 @@ id:integer | name:keyword | count:long
 14         | mmmmm        | 2
 ;
 
+shortIntegerWideningFork
+required_capability: union_types
+required_capability: casting_operator
+required_capability: union_types_numeric_widening
+required_capability: fork_v9
+
+FROM apps, apps_short
+| EVAL x = id::integer
+| FORK (WHERE true) (WHERE true)
+| DROP _fork
+| SORT x ASC, name ASC, version ASC
+| LIMIT 10
+;
+
+id:unsupported | name:keyword | version:version | x:integer
+null           | aaaaa        | 1               | 1
+null           | aaaaa        | 1               | 1
+null           | aaaaa        | 1               | 1
+null           | aaaaa        | 1               | 1
+null           | bbbbb        | 2.1             | 2
+null           | bbbbb        | 2.1             | 2
+null           | bbbbb        | 2.1             | 2
+null           | bbbbb        | 2.1             | 2
+null           | ccccc        | 2.3.4           | 3
+null           | ccccc        | 2.3.4           | 3
+;
+
 ImplicitCastingMultiTypedFieldsKeepSort
 required_capability: date_nanos_type
 required_capability: implicit_casting_date_and_date_nanos

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/analysis/Analyzer.java

@@ -812,7 +812,9 @@ private LogicalPlan resolveFork(Fork fork, AnalyzerContext context) {
             // We don't want to keep the same attributes that are outputted by the FORK branches.
             // Keeping the same attributes can have unintended side effects when applying optimizations like constant folding.
             for (Attribute attr : outputUnion) {
-                newOutput.add(new ReferenceAttribute(attr.source(), null, attr.name(), attr.dataType()));
+                newOutput.add(
+                    new ReferenceAttribute(attr.source(), null, attr.name(), attr.dataType(), Nullability.FALSE, null, attr.synthetic())
+                );
             }
 
             return changed ? new Fork(fork.source(), newSubPlans, newOutput) : fork;
@@ -1960,7 +1962,7 @@ private static LogicalPlan planWithoutSyntheticAttributes(LogicalPlan plan) {
 
             for (Attribute attr : output) {
                 // Do not let the synthetic union type field attributes end up in the final output.
-                if (attr.synthetic() && attr instanceof FieldAttribute) {
+                if (attr.synthetic()) {
                     continue;
                 }
                 newOutput.add(attr);