### 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/main/java/org/elasticsearch/search/aggregations/bucket/filter/FilterByFilterAggregator.java
/**
         * Subclasses should override this to adapt the
         * {@link FilterByFilterAggregator} into another sort of aggregator
         * if required.
         */
        protected abstract T adapt(CheckedFunction<AggregatorFactories, FilterByFilterAggregator, IOException> delegate) throws IOException;
        public final void add(String key, Query query) throws IOException {
            if (valid == false) {
                return;
            }
            if (query instanceof AbstractScriptFieldQuery) {
                /*
                 * We know that runtime fields aren't fast to query at all
                 * but we expect all other sorts of queries are at least as
                 * fast as the native aggregator.
                 */
                valid = false;
                return;
            }
            add(QueryToFilterAdapter.build(aggCtx.searcher(), key, query));
        }
        final void add(QueryToFilterAdapter filter) throws IOException {
            if (valid == false) {
                return;
            }
            QueryToFilterAdapter mergedFilter = filter.union(rewrittenTopLevelQuery);
            if (mergedFilter.isInefficientUnion()) {
                /*
                 * For now any complex union kicks us out of filter by filter
                 * mode. Its possible that this de-optimizes many "filters"