Add _bucket_count option to buckets_path

This change adds a new special path to the buckets_path syntax
`_bucket_count`. This new option will return the number of buckets for a
multi-bucket aggregation, which can then be used in pipeline
aggregations.

Closes #19553

core/src/main/java/org/elasticsearch/search/aggregations/InternalMultiBucketAggregation.java

@@ -43,7 +43,7 @@ public abstract class InternalMultiBucketAggregation<A extends InternalMultiBuck
     /**
      * Create a new copy of this {@link Aggregation} with the same settings as
      * this {@link Aggregation} and contains the provided buckets.
-     * 
+     *
      * @param buckets
      *            the buckets to use in the new {@link Aggregation}
      * @return the new {@link Aggregation}
@@ -53,7 +53,7 @@ public abstract class InternalMultiBucketAggregation<A extends InternalMultiBuck
     /**
      * Create a new {@link InternalBucket} using the provided prototype bucket
      * and aggregations.
-     * 
+     *
      * @param aggregations
      *            the aggregations for the new bucket
      * @param prototype
@@ -66,6 +66,8 @@ public abstract class InternalMultiBucketAggregation<A extends InternalMultiBuck
     public Object getProperty(List<String> path) {
         if (path.isEmpty()) {
             return this;
+        } else if (path.get(0).equals("_bucket_count")) {
+            return getBuckets().size();
         } else {
             List<? extends Bucket> buckets = getBuckets();
             Object[] propertyArray = new Object[buckets.size()];

core/src/test/java/org/elasticsearch/search/aggregations/bucket/DateHistogramIT.java

@@ -380,6 +380,7 @@ public class DateHistogramIT extends ESIntegTestCase {
         assertThat(histo.getName(), equalTo("histo"));
         List<? extends Bucket> buckets = histo.getBuckets();
         assertThat(buckets.size(), equalTo(3));
+        assertThat(histo.getProperty("_bucket_count"), equalTo(3));
         Object[] propertiesKeys = (Object[]) histo.getProperty("_key");
         Object[] propertiesDocCounts = (Object[]) histo.getProperty("_count");
         Object[] propertiesCounts = (Object[]) histo.getProperty("sum.value");
@@ -600,7 +601,7 @@ public class DateHistogramIT extends ESIntegTestCase {
         assertThat(histo.getBuckets().size(), equalTo(4));
 
         // TODO: use diamond once JI-9019884 is fixed
-        List<Histogram.Bucket> buckets = new ArrayList<Histogram.Bucket>(histo.getBuckets());
+        List<Histogram.Bucket> buckets = new ArrayList<>(histo.getBuckets());
 
         Histogram.Bucket bucket = buckets.get(0);
         assertThat(bucket, notNullValue());

core/src/test/java/org/elasticsearch/search/aggregations/bucket/DateRangeIT.java

@@ -137,7 +137,7 @@ public class DateRangeIT extends ESIntegTestCase {
         assertThat(range.getBuckets().size(), equalTo(3));
 
         // TODO: use diamond once JI-9019884 is fixed
-        List<Range.Bucket> buckets = new ArrayList<Range.Bucket>(range.getBuckets());
+        List<Range.Bucket> buckets = new ArrayList<>(range.getBuckets());
 
         Range.Bucket bucket = buckets.get(0);
         assertThat((String) bucket.getKey(), equalTo("a long time ago"));
@@ -421,6 +421,7 @@ public class DateRangeIT extends ESIntegTestCase {
         assertThat(range.getName(), equalTo("range"));
         List<? extends Bucket> buckets = range.getBuckets();
         assertThat(buckets.size(), equalTo(3));
+        assertThat(range.getProperty("_bucket_count"), equalTo(3));
         Object[] propertiesKeys = (Object[]) range.getProperty("_key");
         Object[] propertiesDocCounts = (Object[]) range.getProperty("_count");
         Object[] propertiesCounts = (Object[]) range.getProperty("sum.value");
@@ -855,7 +856,7 @@ public class DateRangeIT extends ESIntegTestCase {
 
         Range dateRange = bucket.getAggregations().get("date_range");
         // TODO: use diamond once JI-9019884 is fixed
-        List<Range.Bucket> buckets = new ArrayList<Range.Bucket>(dateRange.getBuckets());
+        List<Range.Bucket> buckets = new ArrayList<>(dateRange.getBuckets());
         assertThat(dateRange, Matchers.notNullValue());
         assertThat(dateRange.getName(), equalTo("date_range"));
         assertThat(buckets.size(), is(1));

core/src/test/java/org/elasticsearch/search/aggregations/bucket/DoubleTermsIT.java

@@ -426,6 +426,7 @@ public class DoubleTermsIT extends AbstractTermsTestCase {
         assertThat(terms, notNullValue());
         assertThat(terms.getName(), equalTo("terms"));
         assertThat(terms.getBuckets().size(), equalTo(5));
+        assertThat(terms.getProperty("_bucket_count"), equalTo(5));
         Object[] propertiesKeys = (Object[]) terms.getProperty("_key");
         Object[] propertiesDocCounts = (Object[]) terms.getProperty("_count");
         Object[] propertiesCounts = (Object[]) terms.getProperty("sum.value");

core/src/test/java/org/elasticsearch/search/aggregations/bucket/FiltersIT.java

@@ -172,6 +172,7 @@ public class FiltersIT extends ESIntegTestCase {
         assertThat(filters.getName(), equalTo("tags"));
 
         assertThat(filters.getBuckets().size(), equalTo(2));
+        assertThat(filters.getProperty("_bucket_count"), equalTo(2));
         Object[] propertiesKeys = (Object[]) filters.getProperty("_key");
         Object[] propertiesDocCounts = (Object[]) filters.getProperty("_count");
         Object[] propertiesCounts = (Object[]) filters.getProperty("avg_value.value");
@@ -426,6 +427,7 @@ public class FiltersIT extends ESIntegTestCase {
         assertThat(filters.getName(), equalTo("tags"));
 
         assertThat(filters.getBuckets().size(), equalTo(3));
+        assertThat(filters.getProperty("_bucket_count"), equalTo(3));
         Object[] propertiesKeys = (Object[]) filters.getProperty("_key");
         Object[] propertiesDocCounts = (Object[]) filters.getProperty("_count");
         Object[] propertiesCounts = (Object[]) filters.getProperty("avg_value.value");

core/src/test/java/org/elasticsearch/search/aggregations/bucket/GeoDistanceIT.java

@@ -349,6 +349,7 @@ public class GeoDistanceIT extends ESIntegTestCase {
         assertThat(geoDist.getName(), equalTo("amsterdam_rings"));
         List<? extends Bucket> buckets = geoDist.getBuckets();
         assertThat(geoDist.getBuckets().size(), equalTo(3));
+        assertThat(geoDist.getProperty("_bucket_count"), equalTo(3));
         Object[] propertiesKeys = (Object[]) geoDist.getProperty("_key");
         Object[] propertiesDocCounts = (Object[]) geoDist.getProperty("_count");
         Object[] propertiesCities = (Object[]) geoDist.getProperty("cities");
@@ -429,7 +430,7 @@ public class GeoDistanceIT extends ESIntegTestCase {
 
         Range geoDistance = bucket.getAggregations().get("geo_dist");
         // TODO: use diamond once JI-9019884 is fixed
-        List<Range.Bucket> buckets = new ArrayList<Range.Bucket>(geoDistance.getBuckets());
+        List<Range.Bucket> buckets = new ArrayList<>(geoDistance.getBuckets());
         assertThat(geoDistance, Matchers.notNullValue());
         assertThat(geoDistance.getName(), equalTo("geo_dist"));
         assertThat(buckets.size(), is(1));

core/src/test/java/org/elasticsearch/search/aggregations/bucket/HistogramIT.java

@@ -26,7 +26,6 @@ import org.elasticsearch.plugins.Plugin;
 import org.elasticsearch.script.MockScriptPlugin;
 import org.elasticsearch.script.Script;
 import org.elasticsearch.script.ScriptService.ScriptType;
-import org.elasticsearch.search.aggregations.AggregationTestScriptsPlugin;
 import org.elasticsearch.search.aggregations.bucket.filter.Filter;
 import org.elasticsearch.search.aggregations.bucket.histogram.ExtendedBounds;
 import org.elasticsearch.search.aggregations.bucket.histogram.Histogram;
@@ -253,7 +252,7 @@ public class HistogramIT extends ESIntegTestCase {
         assertThat(histo.getBuckets().size(), equalTo(numValueBuckets));
 
         // TODO: use diamond once JI-9019884 is fixed
-        List<Histogram.Bucket> buckets = new ArrayList<Histogram.Bucket>(histo.getBuckets());
+        List<Histogram.Bucket> buckets = new ArrayList<>(histo.getBuckets());
         for (int i = 0; i < numValueBuckets; ++i) {
             Histogram.Bucket bucket = buckets.get(i);
             assertThat(bucket, notNullValue());
@@ -276,7 +275,7 @@ public class HistogramIT extends ESIntegTestCase {
         assertThat(histo.getBuckets().size(), equalTo(numValueBuckets));
 
         // TODO: use diamond once JI-9019884 is fixed
-        List<Histogram.Bucket> buckets = new ArrayList<Histogram.Bucket>(histo.getBuckets());
+        List<Histogram.Bucket> buckets = new ArrayList<>(histo.getBuckets());
         for (int i = 0; i < numValueBuckets; ++i) {
             Histogram.Bucket bucket = buckets.get(numValueBuckets - i - 1);
             assertThat(bucket, notNullValue());
@@ -300,7 +299,7 @@ public class HistogramIT extends ESIntegTestCase {
 
         LongHashSet buckets = new LongHashSet();
         // TODO: use diamond once JI-9019884 is fixed
-        List<Histogram.Bucket> histoBuckets = new ArrayList<Histogram.Bucket>(histo.getBuckets());
+        List<Histogram.Bucket> histoBuckets = new ArrayList<>(histo.getBuckets());
         long previousCount = Long.MIN_VALUE;
         for (int i = 0; i < numValueBuckets; ++i) {
             Histogram.Bucket bucket = histoBuckets.get(i);
@@ -329,7 +328,7 @@ public class HistogramIT extends ESIntegTestCase {
 
         LongHashSet buckets = new LongHashSet();
         // TODO: use diamond once JI-9019884 is fixed
-        List<Histogram.Bucket> histoBuckets = new ArrayList<Histogram.Bucket>(histo.getBuckets());
+        List<Histogram.Bucket> histoBuckets = new ArrayList<>(histo.getBuckets());
         long previousCount = Long.MAX_VALUE;
         for (int i = 0; i < numValueBuckets; ++i) {
             Histogram.Bucket bucket = histoBuckets.get(i);
@@ -356,12 +355,13 @@ public class HistogramIT extends ESIntegTestCase {
         assertThat(histo, notNullValue());
         assertThat(histo.getName(), equalTo("histo"));
         assertThat(histo.getBuckets().size(), equalTo(numValueBuckets));
+        assertThat(histo.getProperty("_bucket_count"), equalTo(numValueBuckets));
         Object[] propertiesKeys = (Object[]) histo.getProperty("_key");
         Object[] propertiesDocCounts = (Object[]) histo.getProperty("_count");
         Object[] propertiesCounts = (Object[]) histo.getProperty("sum.value");
 
         // TODO: use diamond once JI-9019884 is fixed
-        List<Histogram.Bucket> buckets = new ArrayList<Histogram.Bucket>(histo.getBuckets());
+        List<Histogram.Bucket> buckets = new ArrayList<>(histo.getBuckets());
         for (int i = 0; i < numValueBuckets; ++i) {
             Histogram.Bucket bucket = buckets.get(i);
             assertThat(bucket, notNullValue());
@@ -404,7 +404,7 @@ public class HistogramIT extends ESIntegTestCase {
         LongHashSet visited = new LongHashSet();
         double previousSum = Double.NEGATIVE_INFINITY;
         // TODO: use diamond once JI-9019884 is fixed
-        List<Histogram.Bucket> buckets = new ArrayList<Histogram.Bucket>(histo.getBuckets());
+        List<Histogram.Bucket> buckets = new ArrayList<>(histo.getBuckets());
         for (int i = 0; i < numValueBuckets; ++i) {
             Histogram.Bucket bucket = buckets.get(i);
             assertThat(bucket, notNullValue());
@@ -448,7 +448,7 @@ public class HistogramIT extends ESIntegTestCase {
         LongHashSet visited = new LongHashSet();
         double previousSum = Double.POSITIVE_INFINITY;
         // TODO: use diamond once JI-9019884 is fixed
-        List<Histogram.Bucket> buckets = new ArrayList<Histogram.Bucket>(histo.getBuckets());
+        List<Histogram.Bucket> buckets = new ArrayList<>(histo.getBuckets());
         for (int i = 0; i < numValueBuckets; ++i) {
             Histogram.Bucket bucket = buckets.get(i);
             assertThat(bucket, notNullValue());
@@ -492,7 +492,7 @@ public class HistogramIT extends ESIntegTestCase {
         LongHashSet visited = new LongHashSet();
         double previousSum = Double.POSITIVE_INFINITY;
         // TODO: use diamond once JI-9019884 is fixed
-        List<Histogram.Bucket> buckets = new ArrayList<Histogram.Bucket>(histo.getBuckets());
+        List<Histogram.Bucket> buckets = new ArrayList<>(histo.getBuckets());
         for (int i = 0; i < numValueBuckets; ++i) {
             Histogram.Bucket bucket = buckets.get(i);
             assertThat(bucket, notNullValue());
@@ -538,7 +538,7 @@ public class HistogramIT extends ESIntegTestCase {
         LongHashSet visited = new LongHashSet();
         double prevMax = asc ? Double.NEGATIVE_INFINITY : Double.POSITIVE_INFINITY;
         // TODO: use diamond once JI-9019884 is fixed
-        List<Histogram.Bucket> buckets = new ArrayList<Histogram.Bucket>(histo.getBuckets());
+        List<Histogram.Bucket> buckets = new ArrayList<>(histo.getBuckets());
         for (int i = 0; i < numValueBuckets; ++i) {
             Histogram.Bucket bucket = buckets.get(i);
             assertThat(bucket, notNullValue());
@@ -625,7 +625,7 @@ public class HistogramIT extends ESIntegTestCase {
         assertThat(histo.getBuckets().size(), equalTo(numValuesBuckets));
 
         // TODO: use diamond once JI-9019884 is fixed
-        List<Histogram.Bucket> buckets = new ArrayList<Histogram.Bucket>(histo.getBuckets());
+        List<Histogram.Bucket> buckets = new ArrayList<>(histo.getBuckets());
         for (int i = 0; i < numValuesBuckets; ++i) {
             Histogram.Bucket bucket = buckets.get(numValuesBuckets - i - 1);
             assertThat(bucket, notNullValue());

docs/reference/aggregations/pipeline.asciidoc

@@ -107,8 +107,7 @@ a metric embedded inside a sibling aggregation:
 === Special Paths
 
 Instead of pathing to a metric, `buckets_path` can use a special `"_count"` path.  This instructs
-the pipeline aggregation to use the document count as it's input.  For example, a moving average can be calculated on the document
-count of each bucket, instead of a specific metric:
+the pipeline aggregation to use the document count as it's input.  For example, a moving average can be calculated on the document count of each bucket, instead of a specific metric:
 
 [source,js]
 --------------------------------------------------
@@ -128,6 +127,44 @@ count of each bucket, instead of a specific metric:
 --------------------------------------------------
 <1> By using `_count` instead of a metric name, we can calculate the moving average of document counts in the histogram
 
+The `buckets_path` can also use `"_bucket_count"` and path to a multi-bucket aggregation to use the number of buckets
+returned by that aggregation in the pipeline aggregation instead of a metric. for example a `bucket_selector` can be
+used here to filter out buckets which contain no buckets for an inner terms aggregation:
+
+[source,js]
+--------------------------------------------------
+{
+  "size": 0,
+  "aggs": {
+    "histo": {
+      "date_histogram": {
+        "field": "date",
+        "interval": "day"
+      },
+      "aggs": {
+        "categories": {
+          "terms": {
+            "field": "category"
+          }
+        },
+        "min_bucket_selector": {
+          "bucket_selector": {
+            "buckets_path": {
+              "count": "categories._bucket_count"
+            },
+            "script": {
+              "inline": "count != 0"
+            }
+          }
+        }
+      }
+    }
+  }
+}
+--------------------------------------------------
+<1> By using `_bucket_count` instead of a metric name, we can filter out `histo` buckets where they contain no buckets
+for the `categories` aggregation
+
 [[dots-in-agg-names]]
 [float]
 === Dealing with dots in agg names