[GitHub] [druid] clintropolis commented on a change in pull request #10235: Add "offset" parameter to GroupBy query.

[GitBox <gi...@apache.org>]

clintropolis commented on a change in pull request #10235:
URL: https://github.com/apache/druid/pull/10235#discussion_r465346819



##########
File path: core/src/main/java/org/apache/druid/collections/StableLimitingSorter.java
##########
@@ -0,0 +1,134 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.druid.collections;
+
+import com.google.common.collect.MinMaxPriorityQueue;
+import com.google.common.collect.Ordering;
+
+import java.util.Comparator;
+import java.util.Iterator;
+import java.util.Objects;
+
+/**
+ * Simultaneously sorts and limits its input.
+ *
+ * The sort is stable, meaning that equal elements (as determined by the comparator) will not be reordered.
+ *
+ * Not thread-safe.
+ *
+ * Note: this class doesn't have its own unit tests. It is tested along with
+ * {@link org.apache.druid.java.util.common.guava.TopNSequence} in "TopNSequenceTest".
+ */
+public class StableLimitingSorter<T>
+{
+  private final MinMaxPriorityQueue<NumberedElement<T>> queue;
+
+  private long count = 0;
+
+  public StableLimitingSorter(final Comparator<T> comparator, final int limit)
+  {
+    this.queue = MinMaxPriorityQueue
+        .orderedBy(
+            Ordering.from(
+                Comparator.<NumberedElement<T>, T>comparing(NumberedElement::getElement, comparator)
+                    .thenComparing(NumberedElement::getNumber)
+            )
+        )
+        .maximumSize(limit)
+        .create();
+  }
+
+  /**
+   * Offer an element to the sorter.
+   */
+  public void add(T element)
+  {
+    queue.offer(new NumberedElement<>(element, count++));
+  }
+
+  /**
+   * Drain elements in sorted order (least first).

Review comment:
       What does 'least first' mean? Is this referring to `. thenComparing` by the number?

##########
File path: docs/querying/limitspec.md
##########
@@ -35,11 +35,23 @@ The default limit spec takes a limit and the list of columns to do an orderBy op
 ```json
 {
     "type"    : "default",
-    "limit"   : <integer_value>,
-    "columns" : [list of OrderByColumnSpec],
+    "limit"   : <optional integer>,
+    "offset"  : <optional integer>,
+    "columns" : [<optional list of OrderByColumnSpec>],
 }
 ```
 
+The "limit" parameter is the maximum number of rows to return.
+
+The "offset" parameter tells Druid to skip this many rows when returning results. If both "limit" and "offset" are
+provided, then "offset" will be applied first, followed by "limit". For example, a spec with limit 100 and offset 10
+will return 100 rows starting from row number 10. Skipped rows will still need to be generated internally and then
+discarded, meaning that raising offsets to high values can cause queries to use additional resources.

Review comment:
       Any idea on numbers that are too large, or is this a try it and find out what your cluster can handle kind of thing?

##########
File path: processing/src/test/java/org/apache/druid/query/groupby/GroupByQueryRunnerTest.java
##########
@@ -3700,7 +3702,7 @@ public void testGroupByWithOrderOnHyperUnique()
             query,
             "1970-01-01T00:00:00.000Z",
             "market",
-            "upfront",
+            "total_market",

Review comment:
       Are these changes from the stabilization?

##########
File path: sql/src/main/java/org/apache/druid/sql/calcite/rel/DruidQuery.java
##########
@@ -928,6 +928,7 @@ public GroupByQuery toGroupByQuery()
         sorting != null
         ? new DefaultLimitSpec(
             sorting.getOrderBys(),
+            0,

Review comment:
       Is the plan to update `Sorting` to store the offset, and `computeSorting` to set it when creating it `Sorting` from the calcite `Sort` in a follow-up so that this works in SQL?

##########
File path: sql/src/main/java/org/apache/druid/sql/calcite/rel/DruidQuery.java
##########
@@ -928,6 +928,7 @@ public GroupByQuery toGroupByQuery()
         sorting != null
         ? new DefaultLimitSpec(
             sorting.getOrderBys(),
+            0,

Review comment:
       Is the plan to update `Sorting` to store the offset, and `computeSorting` to set it when creating it from the calcite `Sort` in a follow-up so that this works in SQL?




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