Rewrite queries on unmapped fields as MatchNoDocsQueryBuilder

[jpountz]

It's probably more easily explained with an example: TermQueryBuilber#doToQuery looks like this today

    protected Query doToQuery(QueryShardContext context) throws IOException {
        Query query = null;
        MappedFieldType mapper = context.fieldMapper(this.fieldName);
        if (mapper != null) {
            query = mapper.termQuery(this.value, context);
        }
        if (query == null) {
            query = new TermQuery(new Term(this.fieldName, BytesRefs.toBytesRef(this.value)));
        }
        return query;
    }

So if the field is unmapped, we create a TermQuery on a field that doesn't exist, which is no different from returning a MatchNoDocsQuery.

It's becoming more and more common that multiple indices that have different mappings get queried together, e.g. as part of an alias or an index pattern, so a more efficient way to handle these cases would be to rewrite to a MatchNoDocsQueryBuilder in the rewrite() method. This would help figure out that the shard cannot possibly match in the can_match phase and skip the query phase completely.

[elasticmachine]

Pinging @elastic/es-search (:Search/Search)

[ejmiers]

Could I jump on this issue? Fairly new to elasticsearch.

[jpountz]

@ejmiers Sure, can you only focus on TermQueryBuilder for the first iteration of your PR, add me as a reviewer, and once I'm good with the approach for TermQueryBuilder we will generalize to other queries that need the same change like TermsQueryBuilder, MatchQueryBuilder and MultiMatchQueryBuilder?

[ejmiers]

Hi @jpountz, I am having difficulty locating the rewrite() function referenced above. Could you point me in the right direction?

[jpountz]

Apologies @ejmiers I had to work on this as part of another change, so this issue should soon no longer be relevant.

[singhprincejeet]

@jpountz Should this issue be closed then?