CanMatch search phase and AliasFilter

yfful · April 10, 2020, 2:10pm

Hi,

If an aliased index contains a filter, the SearchService#canMatch method would rewrite it. However, that method doesn't check if the alias filter got rewritten to a match_none which would indicate that the shard could be skipped.
Can you clarify why the alias filter is not taken into account by canMatch ?

Thanks

github.com

elastic/elasticsearch/blob/f7809dd096f50b345bf49914702af80f0ac081d4/server/src/main/java/org/elasticsearch/search/SearchService.java#L1112




public AliasFilter buildAliasFilter(ClusterState state, String index, Set<String> resolvedExpressions) {
    return indicesService.buildAliasFilter(state, index, resolvedExpressions);
}


/**
 * This method does a very quick rewrite of the query and returns true if the query can potentially match any documents.
 * This method can have false positives while if it returns <code>false</code> the query won't match any documents on the current
 * shard.
 */
public CanMatchResponse canMatch(ShardSearchRequest request) throws IOException {
    assert request.searchType() == SearchType.QUERY_THEN_FETCH : "unexpected search type: " + request.searchType();
    IndexService indexService = indicesService.indexServiceSafe(request.shardId().getIndex());
    IndexShard indexShard = indexService.getShard(request.shardId().getId());
    // we don't want to use the reader wrapper since it could run costly operations
    // and we can afford false positives.
    try (Engine.Searcher searcher = indexShard.acquireCanMatchSearcher()) {
        QueryShardContext context = indexService.newQueryShardContext(request.shardId().id(), searcher,
            request::nowInMillis, request.getClusterAlias());
        Rewriteable.rewrite(request.getRewriteable(), context, false);
        FieldSortBuilder sortBuilder = FieldSortBuilder.getPrimaryFieldSortOrNull(request.source());

jimczi · April 10, 2020, 3:19pm

It's not implemented but this is a good idea. I guess that you're using a range query in the alias filter ? I don't see any reason why we wouldn't do it so could you please open an issue in the public github ? If you're interested you can also open a pull request directly since the change should be straightforward.

yfful · April 10, 2020, 9:39pm

@jimczi This is the related issue https://github.com/elastic/elasticsearch/issues/55090

system · May 8, 2020, 9:39pm

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