Fix redundant assignment in QueryStringQueryBuilder

[ioanatia]

We have a redundant assignment in QueryStringQueryBuilder for extractedFields:

elasticsearch/server/src/main/java/org/elasticsearch/index/search/QueryStringQueryParser.java

Lines 268 to 282 in 82b1f2a

	if (allFields && this.field != null && this.field.equals(field)) {
	// "*" is the default field
	extractedFields = fieldsAndWeights;
	}
	boolean multiFields = Regex.isSimpleMatchPattern(field);
	// Filters unsupported fields if a pattern is requested
	// Filters metadata fields if all fields are requested
	extractedFields = resolveMappingField(
	context,
	field,
	1.0f,
	allFields == false,
	multiFields == false,
	quoted ? quoteFieldSuffix : null
	);

To fix this we can remove the extractedFields = fieldsAndWeights assignment altogether.
However fieldsAndWeights already contains the same result of the second assignment.
fieldsAndWeights is actually initialised with the exact same value (when the redundant assignment is executed) - see the call to resolveMappingField:

elasticsearch/server/src/main/java/org/elasticsearch/index/search/QueryStringQueryParser.java

Lines 132 to 134 in 82b1f2a

	public QueryStringQueryParser(SearchExecutionContext context, boolean lenient) {
	this(context, "*", resolveMappingField(context, "*", 1.0f, false, false, null), lenient);
	}

When matching all fields I think it was always the intention to just reuse fieldsAndWeights, but the redundant assignment and extra computation was added as a regression in #55158

In the context of a single query_string execution, extractMultiFields can be called multiple times. Some examples where the redundant assignment is triggered and we end up extracting the fields for wildcard queries multiple times:

• something OR nothing OR anything - 3 redundant assignments and recomputing the available fields, instead of relying on the stored fieldsAndWeights.
• *:something AND nothing - 2 redundant assignments

This is why I decided to keep the extractedFields = fieldsAndWeights assignment and just to make sure we don't call resolveMappingField unnecessarily.

[jimczi]

Good catch, LGTM

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[elasticsearchmachine]

💚 Backport successful

Status	Branch	Result
✅	8.x