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https://issues.apache.org/jira/browse/ARROW-2199?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16386694#comment-16386694
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ASF GitHub Bot commented on ARROW-2199:
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icexelloss commented on a change in pull request #1646: ARROW-2199: [JAVA]
Control the memory allocated for inner vectors in containers.
URL: https://github.com/apache/arrow/pull/1646#discussion_r172316594
##########
File path:
java/vector/src/main/java/org/apache/arrow/vector/DensityAwareVector.java
##########
@@ -0,0 +1,57 @@
+/**
+ * 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
+ * <p>
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * <p>
+ * 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.arrow.vector;
+
+/**
+ * Vector that support density aware initial capacity settings.
+ * We use this for ListVector and VarCharVector as of now to
+ * control the memory allocated.
+ *
+ * For ListVector, we have been using a multiplier of 5
+ * to compute the initial capacity of the inner data vector.
+ * For deeply nested lists and lists with lots of NULL values,
+ * this is over-allocation upfront. So density helps to be
+ * conservative when computing the value capacity of the
+ * inner vector.
+ *
+ * For example, a density value of 10 implies each position in the
+ * list vector has a list of 10 values. So we will provision
+ * an initial capacity of (valuecount * 10) for the inner vector.
+ * A density value of 0.1 implies out of 10 positions in the list vector,
+ * 1 position has a list of size 1 and remaining positions are
Review comment:
I see. Thanks for the explanation.
> Density is the average size of list per position in the List vector
This is fine.
> density value of 10 implies each position in the list vector has a list
of 10 values.
If I understand correctly, a density value of 10 can be either:
* 10 sub list of 10 values
* 1 sub list 100 values, 9 null sublists
* ...
As long as the average size of sub lists equals density.
Is that correct? If so, can we make it clear in the doc?
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> [JAVA] Follow up fixes for ARROW-2019. Ensure density driven capacity is
> never less than 1 and propagate density throughout the vector tree
> -------------------------------------------------------------------------------------------------------------------------------------------
>
> Key: ARROW-2199
> URL: https://issues.apache.org/jira/browse/ARROW-2199
> Project: Apache Arrow
> Issue Type: Improvement
> Components: Java - Vectors
> Reporter: Siddharth Teotia
> Assignee: Siddharth Teotia
> Priority: Major
> Labels: pull-request-available
> Fix For: 0.9.0
>
>
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