SQL: use a calendar interval for histograms over 1 month intervals

docs/reference/sql/functions/grouping.asciidoc

@@ -87,8 +87,8 @@ actually used will be `INTERVAL '2' DAY`. If the interval specified is less than
 
 [IMPORTANT]
 All intervals specified for a date/time HISTOGRAM will use a <<search-aggregations-bucket-datehistogram-aggregation,fixed interval>>
-in their `date_histogram` aggregation definition, with the notable exception of `INTERVAL '1' YEAR` where a calendar interval is used.
-The choice for a calendar interval was made for having a more intuitive result for YEAR groupings. Calendar intervals consider a one year
+in their `date_histogram` aggregation definition, with the notable exceptions of `INTERVAL '1' YEAR` AND `INTERVAL '1' MONTH`  where a calendar interval is used.
+The choice for a calendar interval was made for having a more intuitive result for YEAR and MONTH groupings. In the case of YEAR, for example, the calendar intervals consider a one year
 bucket as the one starting on January 1st that specific year, whereas a fixed interval one-year-bucket considers one year as a number
 of milliseconds (for example, `31536000000ms` corresponding to 365 days, 24 hours per day, 60 minutes per hour etc.). With fixed intervals,
 the day of February 5th, 2019 for example, belongs to a bucket that starts on December 20th, 2018 and {es} (and implicitly {es-sql}) would

x-pack/plugin/sql/qa/src/main/resources/agg.csv-spec

@@ -503,6 +503,34 @@ SELECT HISTOGRAM(birth_date, INTERVAL 1 YEAR) AS h, COUNT(*) as c FROM test_emp
 null                    |10             
 ;
 
+histogramOneMonth
+schema::h:ts|c:l|birth_date:ts
+SELECT HISTOGRAM(birth_date, INTERVAL 1 MONTH) AS h, COUNT(*) as c, birth_date FROM test_emp GROUP BY h, birth_date HAVING c >= 1 ORDER BY h ASC LIMIT 20;
+
+           h            |       c       |       birth_date       
+------------------------+---------------+------------------------
+null                    |10             |null                    
+1952-02-01T00:00:00.000Z|1              |1952-02-27T00:00:00.000Z
+1952-04-01T00:00:00.000Z|1              |1952-04-19T00:00:00.000Z
+1952-05-01T00:00:00.000Z|1              |1952-05-15T00:00:00.000Z
+1952-06-01T00:00:00.000Z|1              |1952-06-13T00:00:00.000Z
+1952-07-01T00:00:00.000Z|1              |1952-07-08T00:00:00.000Z
+1952-08-01T00:00:00.000Z|1              |1952-08-06T00:00:00.000Z
+1952-11-01T00:00:00.000Z|1              |1952-11-13T00:00:00.000Z
+1952-12-01T00:00:00.000Z|1              |1952-12-24T00:00:00.000Z
+1953-01-01T00:00:00.000Z|1              |1953-01-07T00:00:00.000Z
+1953-01-01T00:00:00.000Z|1              |1953-01-23T00:00:00.000Z
+1953-02-01T00:00:00.000Z|1              |1953-02-08T00:00:00.000Z
+1953-04-01T00:00:00.000Z|1              |1953-04-03T00:00:00.000Z
+1953-04-01T00:00:00.000Z|1              |1953-04-20T00:00:00.000Z
+1953-04-01T00:00:00.000Z|1              |1953-04-21T00:00:00.000Z
+1953-07-01T00:00:00.000Z|1              |1953-07-28T00:00:00.000Z
+1953-09-01T00:00:00.000Z|1              |1953-09-02T00:00:00.000Z
+1953-09-01T00:00:00.000Z|1              |1953-09-19T00:00:00.000Z
+1953-09-01T00:00:00.000Z|1              |1953-09-29T00:00:00.000Z
+1953-11-01T00:00:00.000Z|1              |1953-11-07T00:00:00.000Z
+;
+
 histogramDateTimeWithMonthOnTop
 schema::h:i|c:l
 SELECT HISTOGRAM(MONTH(birth_date), 2) AS h, COUNT(*) as c FROM test_emp GROUP BY h ORDER BY h DESC;

x-pack/plugin/sql/src/main/java/org/elasticsearch/xpack/sql/expression/function/grouping/Histogram.java

@@ -6,6 +6,7 @@
 
 package org.elasticsearch.xpack.sql.expression.function.grouping;
 
+import org.elasticsearch.search.aggregations.bucket.histogram.DateHistogramInterval;
 import org.elasticsearch.xpack.ql.expression.Expression;
 import org.elasticsearch.xpack.ql.expression.Expressions.ParamOrdinal;
 import org.elasticsearch.xpack.ql.expression.Literal;
@@ -28,6 +29,8 @@ public class Histogram extends GroupingFunction {
 
     private final Literal interval;
     private final ZoneId zoneId;
+    public static String YEAR_INTERVAL = DateHistogramInterval.YEAR.toString();
+    public static String MONTH_INTERVAL = DateHistogramInterval.MONTH.toString();
 
     public Histogram(Source source, Expression field, Expression interval, ZoneId zoneId) {
         super(source, field, Collections.singletonList(interval));

x-pack/plugin/sql/src/main/java/org/elasticsearch/xpack/sql/expression/function/scalar/datetime/Year.java

@@ -5,10 +5,10 @@
  */
 package org.elasticsearch.xpack.sql.expression.function.scalar.datetime;
 
-import org.elasticsearch.search.aggregations.bucket.histogram.DateHistogramInterval;
 import org.elasticsearch.xpack.ql.expression.Expression;
 import org.elasticsearch.xpack.ql.tree.NodeInfo.NodeCtor2;
 import org.elasticsearch.xpack.ql.tree.Source;
+import org.elasticsearch.xpack.sql.expression.function.grouping.Histogram;
 import org.elasticsearch.xpack.sql.expression.function.scalar.datetime.DateTimeProcessor.DateTimeExtractor;
 
 import java.time.ZoneId;
@@ -18,8 +18,6 @@
  */
 public class Year extends DateTimeHistogramFunction {
 
-    public static String YEAR_INTERVAL = DateHistogramInterval.YEAR.toString();
-
     public Year(Source source, Expression field, ZoneId zoneId) {
         super(source, field, zoneId, DateTimeExtractor.YEAR);
     }
@@ -41,6 +39,6 @@ public String dateTimeFormat() {
 
     @Override
     public String calendarInterval() {
-        return YEAR_INTERVAL;
+        return Histogram.YEAR_INTERVAL;
     }
 }

x-pack/plugin/sql/src/main/java/org/elasticsearch/xpack/sql/planner/QueryFolder.java

@@ -41,7 +41,6 @@
 import org.elasticsearch.xpack.sql.expression.function.aggregate.TopHits;
 import org.elasticsearch.xpack.sql.expression.function.grouping.Histogram;
 import org.elasticsearch.xpack.sql.expression.function.scalar.datetime.DateTimeHistogramFunction;
-import org.elasticsearch.xpack.sql.expression.function.scalar.datetime.Year;
 import org.elasticsearch.xpack.sql.expression.literal.interval.IntervalYearMonth;
 import org.elasticsearch.xpack.sql.expression.literal.interval.Intervals;
 import org.elasticsearch.xpack.sql.plan.logical.Pivot;
@@ -91,6 +90,8 @@
 import java.util.concurrent.atomic.AtomicReference;
 
 import static org.elasticsearch.xpack.ql.util.CollectionUtils.combine;
+import static org.elasticsearch.xpack.sql.expression.function.grouping.Histogram.MONTH_INTERVAL;
+import static org.elasticsearch.xpack.sql.expression.function.grouping.Histogram.YEAR_INTERVAL;
 import static org.elasticsearch.xpack.sql.planner.QueryTranslator.toAgg;
 import static org.elasticsearch.xpack.sql.planner.QueryTranslator.toQuery;
 import static org.elasticsearch.xpack.sql.type.SqlDataTypes.DATE;
@@ -283,7 +284,6 @@ static GroupingContext groupBy(List<? extends Expression> groupings) {
                     field = field.exactAttribute();
                     key = new GroupByValue(aggId, field.name());
                 }
-
                 // handle functions
                 else if (exp instanceof Function) {
                     // dates are handled differently because of date histograms
@@ -322,13 +322,17 @@ else if (exp instanceof GroupingFunction) {
                             // date histogram
                             if (isDateBased(h.dataType())) {
                                 Object value = h.interval().value();
-                                // interval of exactly 1 year
-                                if (value instanceof IntervalYearMonth
-                                        && ((IntervalYearMonth) value).interval().equals(Period.ofYears(1))) {
-                                    String calendarInterval = Year.YEAR_INTERVAL;
 
-                                    // When the histogram is `INTERVAL '1' YEAR`, the interval used in the ES date_histogram will be
-                                    // a calendar_interval with value "1y". All other intervals will be fixed_intervals expressed in ms.
+                                // interval of exactly 1 year or 1 month
+                                if (value instanceof IntervalYearMonth &&
+                                        (((IntervalYearMonth) value).interval().equals(Period.ofYears(1)) 
+                                        || ((IntervalYearMonth) value).interval().equals(Period.ofMonths(1)))) {
+                                    Period yearMonth = ((IntervalYearMonth) value).interval();
+                                    String calendarInterval = yearMonth.equals(Period.ofYears(1)) ? YEAR_INTERVAL : MONTH_INTERVAL;
+
+                                    // When the histogram is `INTERVAL '1' YEAR` or `INTERVAL '1' MONTH`, the interval used in 
+                                    // the ES date_histogram will be a calendar_interval with value "1y" or "1M" respectively.
+                                    // All other intervals will be fixed_intervals expressed in ms.
                                     if (field instanceof FieldAttribute) {
                                         key = new GroupByDateHistogram(aggId, QueryTranslator.nameOf(field), calendarInterval, h.zoneId());
                                     } else if (field instanceof Function) {

x-pack/plugin/sql/src/test/java/org/elasticsearch/xpack/sql/planner/QueryTranslatorTests.java

@@ -1053,6 +1053,30 @@ public void testGroupByYearQueryTranslator() {
             endsWith("\"date_histogram\":{\"field\":\"date\",\"missing_bucket\":true,\"value_type\":\"date\",\"order\":\"asc\","
                     + "\"calendar_interval\":\"1y\",\"time_zone\":\"Z\"}}}]}}}"));
     }
+
+    public void testGroupByOneMonthHistogramQueryTranslator() {
+        PhysicalPlan p = optimizeAndPlan("SELECT HISTOGRAM(date, INTERVAL 1 MONTH) AS h FROM test GROUP BY h");
+        assertEquals(EsQueryExec.class, p.getClass());
+        EsQueryExec eqe = (EsQueryExec) p;
+        assertEquals(1, eqe.output().size());
+        assertEquals("h", eqe.output().get(0).qualifiedName());
+        assertEquals(DATETIME, eqe.output().get(0).dataType());
+        assertThat(eqe.queryContainer().aggs().asAggBuilder().toString().replaceAll("\\s+", ""),
+            endsWith("\"date_histogram\":{\"field\":\"date\",\"missing_bucket\":true,\"value_type\":\"date\",\"order\":\"asc\","
+                    + "\"calendar_interval\":\"1M\",\"time_zone\":\"Z\"}}}]}}}"));
+    }
+
+    public void testGroupByMoreMonthsHistogramQueryTranslator() {
+        PhysicalPlan p = optimizeAndPlan("SELECT HISTOGRAM(date, INTERVAL 5 MONTH) AS h FROM test GROUP BY h");
+        assertEquals(EsQueryExec.class, p.getClass());
+        EsQueryExec eqe = (EsQueryExec) p;
+        assertEquals(1, eqe.output().size());
+        assertEquals("h", eqe.output().get(0).qualifiedName());
+        assertEquals(DATETIME, eqe.output().get(0).dataType());
+        assertThat(eqe.queryContainer().aggs().asAggBuilder().toString().replaceAll("\\s+", ""),
+            endsWith("\"date_histogram\":{\"field\":\"date\",\"missing_bucket\":true,\"value_type\":\"date\",\"order\":\"asc\","
+                    + "\"fixed_interval\":\"12960000000ms\",\"time_zone\":\"Z\"}}}]}}}"));
+    }
 
     public void testGroupByYearAndScalarsQueryTranslator() {
         PhysicalPlan p = optimizeAndPlan("SELECT YEAR(CAST(date + INTERVAL 5 months AS DATE)) FROM test GROUP BY 1");