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ESQL: Add syntax for FIRST and LAST #132474

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ESQL: Add syntax for FIRST and LAST #132474

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ESQL: Add syntax for FIRST and LAST
nik9000 Aug 5, 2025
2ec5d54
Fix return type
nik9000 Aug 5, 2025
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3 changes: 2 additions & 1 deletion x-pack/plugin/esql/src/main/antlr/parser/Expression.g4
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Original file line number Diff line number Diff line change
Expand Up @@ -49,7 +49,8 @@ primaryExpression
;

functionExpression
: functionName LP (ASTERISK | (booleanExpression (COMMA booleanExpression)* (COMMA mapExpression)?))? RP
: functionName LP (ASTERISK | (booleanExpression (COMMA booleanExpression)* (COMMA mapExpression)?))? RP #functionStandard
| (FIRST | LAST) LP value=booleanExpression BY by=booleanExpression RP #functionFirstLast
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@idegtiarenko idegtiarenko Aug 6, 2025 •
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Why value and by are boolean expressions? I would expect something about column identifier.

Also I am a bit worried about making a custom syntax for each function.
I understand the workaround for FIRST and LAST, however I am not sure if we should also have BY in it opposed to a simpler coma separated list of parameters.
I am imagining something like:

functionExpression
    : name=(functionName | FIRST | LAST) LP (ASTERISK | (booleanExpression (COMMA booleanExpression)* (COMMA mapExpression)?))? RP

I feel like #132469 could be a safer option

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I think it should work that we specifically allow (FIRST | LAST) in place of the function name.

I think we shouldn't introduce the BY part as it would change how our functions work; we normally don't have special keywords inside function arguments - normally that is reserved for operators.

I'd start with FIRST(v, @timestamp) to implement the function, and would put syntactic sugar (FIRST(v BY @timestamp)) into a separate PR, as we'd need to see how this fits into the language as a whole.

I think a straight-forward way would be to adjust functionName below and keep everything else the same:

functionName
    : identifierOrParameter
    | keywordsAllowedAsFunctionNames
    ;

keywordsAllowedAsFunctionNames:
    FIRST
    | LAST
    ;

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Once FIRST and LAST get enabled (in SNAPSHOT), we also best test that they can be properly used as a query parameter (although keywords do not clash with parameters):

curl -u elastic:password -H "Content-Type: application/json" "127.0.0.1:9200/_query?format=txt" -d '
{
  "query": "row x = 1 | stats y = ?foo(x, @timestamp)", "params": [{"foo": {"identifier": "first"}}]
}'

;

functionName
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2 changes: 1 addition & 1 deletion x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/parser/EsqlBaseParser.interp
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