ES|QL: Return columns for unsupported field types in FORK

[ioanatia]

When querying indices that have field types that are unsupported in ES|QL (e.g. ip_range, sparse_vector etc) with FORK, the query returns an error.

Steps to replicate:

Create an index with an unsupported field type

PUT my-index
{
    "mappings": {
        "properties": {
            "abc": {
                "type": "date_range"
            },
            "x": {
                "type": "keyword"
            }
        }
    }
}

Issue a query using FORK

POST /_query
{
    "query": """
        FROM my-index
        | FORK (limit 1) (limit 1)
        """
}

See error response:

{
  "error": {
    "root_cause": [
      {
        "type": "esql_illegal_argument_exception",
        "reason": "unexpected projection: !abc"
      }
    ],
    "type": "esql_illegal_argument_exception",
    "reason": "unexpected projection: !abc"
  },
  "status": 500
}

Stacktrace:

[2025-05-27T14:19:56,091][WARN ][r.suppressed             ] [runTask-0] path: /_query, params: {pretty=true}, status: 500 org.elasticsearch.xpack.esql.EsqlIllegalArgumentException: unexpected projection: !abc
        at org.elasticsearch.xpack.esql.analysis.Analyzer$ResolveRefs.resolveKeep(Analyzer.java:1063)
        at org.elasticsearch.xpack.esql.analysis.Analyzer$ResolveRefs.rule(Analyzer.java:502)
        at org.elasticsearch.xpack.esql.analysis.Analyzer$ResolveRefs.rule(Analyzer.java:470)
        at org.elasticsearch.xpack.esql.analysis.AnalyzerRules$ParameterizedAnalyzerRule.lambda$apply$0(AnalyzerRules.java:49)
        at org.elasticsearch.xpack.esql.core.tree.Node.lambda$transformUp$10(Node.java:209)
        at org.elasticsearch.xpack.esql.core.tree.Node.transformUp(Node.java:204)
        at org.elasticsearch.xpack.esql.core.tree.Node.lambda$transformUp$9(Node.java:202)
        at org.elasticsearch.xpack.esql.core.tree.Node.transformChildren(Node.java:227)
        at org.elasticsearch.xpack.esql.core.tree.Node.transformUp(Node.java:202)
        at org.elasticsearch.xpack.esql.core.tree.Node.transformUp(Node.java:209)
        at org.elasticsearch.xpack.esql.analysis.AnalyzerRules$ParameterizedAnalyzerRule.apply(AnalyzerRules.java:49)
        at org.elasticsearch.xpack.esql.analysis.AnalyzerRules$ParameterizedAnalyzerRule.apply(AnalyzerRules.java:41)
        at org.elasticsearch.xpack.esql.rule.ParameterizedRuleExecutor.lambda$transform$0(ParameterizedRuleExecutor.java:29)
        at org.elasticsearch.xpack.esql.rule.RuleExecutor$Transformation.<init>(RuleExecutor.java:92)
        at org.elasticsearch.xpack.esql.rule.RuleExecutor.executeWithInfo(RuleExecutor.java:171)
        at org.elasticsearch.xpack.esql.rule.RuleExecutor.execute(RuleExecutor.java:140)
        at org.elasticsearch.xpack.esql.analysis.Analyzer.analyze(Analyzer.java:192)
        at org.elasticsearch.xpack.esql.session.EsqlSession.lambda$analyzedPlan$7(EsqlSession.java:325)
        at org.elasticsearch.xpack.esql.session.EsqlSession.analyzeAndMaybeRetry(EsqlSession.java:524)
        at org.elasticsearch.xpack.esql.session.EsqlSession.lambda$analyzedPlan$15(EsqlSession.java:367)

Cause

The problem is that when we resolve FORK during analysis, we add KEEP plans to align the output of FORK branches.
If the output contains UnsupportedAttribute, these are not correctly handled when we resolve the Keep projections (in Analyzer.resolveKeep).

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)