### Description

It'd be lovely is a `filters` or `filter` agg that touches a `constant_keyword` would properly rewrite itself to `match_all` and `match_none` and "go fast", at least when it's at the root of the aggs tree. Theoretically it should "just work" but it doesn't seem to. Also! The `terms` agg should be able to act on there being only a single possible value. Maybe it can even act on the `match_all`/`match_none` nature of the `constant_keyword`. I'm not sure how, but it does feel like there are optimizations in there. Particularly when `terms` is the top level agg.
server/src/test/java/org/elasticsearch/search/aggregations/bucket/filter/FiltersAggregatorTests.java
ft
            );
        });
    }
    /**
     * Tests a filter that needs the cache to be fast.
     */
    public void testPhraseFilter() throws IOException {
        MappedFieldType ft = new TextFieldMapper.TextFieldType("test", randomBoolean());
        AggregationBuilder builder = new FiltersAggregationBuilder(
            "test",
            new KeyedFilter("q1", new MatchPhraseQueryBuilder("test", "will find me").slop(0))
        );
        withIndex(iw -> {
            iw.addDocuments(
                List.of(
                    List.of(new Field("test", "will not find me", TextFieldMapper.Defaults.FIELD_TYPE)),
                    List.of(new Field("test", "will find me", TextFieldMapper.Defaults.FIELD_TYPE))
                )
            );
        }, searcher -> {
            searcher.setQueryCachingPolicy(new QueryCachingPolicy() {
                @Override
                public boolean shouldCache(Query query) throws IOException {
                    return true;
                }
                @Override
                public void onUse(Query query) {}
            });
            for (Matcher<Integer> segmentsCountedInConstantTime : List.of(equalTo(0), greaterThanOrEqualTo(1))) {