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From "Hadoop QA (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (PHOENIX-418) Support approximate COUNT DISTINCT
Date Sat, 26 Aug 2017 01:50:00 GMT

    [ https://issues.apache.org/jira/browse/PHOENIX-418?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16142533#comment-16142533
] 

Hadoop QA commented on PHOENIX-418:
-----------------------------------

{color:red}-1 overall{color}.  Here are the results of testing the latest attachment 
  http://issues.apache.org/jira/secure/attachment/12883785/PHOENIX-418-v7.patch
  against master branch at commit 435441ea8ba336e1967b03cf84f1868c5ef14790.
  ATTACHMENT ID: 12883785

    {color:green}+1 @author{color}.  The patch does not contain any @author tags.

    {color:red}-1 tests included{color}.  The patch doesn't appear to include any new or modified
tests.
                        Please justify why no new tests are needed for this patch.
                        Also please list what manual steps were performed to verify this patch.

    {color:green}+1 javac{color}.  The applied patch does not increase the total number of
javac compiler warnings.

    {color:red}-1 javadoc{color}.  The javadoc tool appears to have generated 56 warning messages.

    {color:green}+1 release audit{color}.  The applied patch does not increase the total number
of release audit warnings.

    {color:red}-1 lineLengths{color}.  The patch introduces the following lines longer than
100:
    +		String query = "SELECT APPROX_COUNT_DISTINCT(a.i1||a.i2||b.i2) FROM " + tableName +
" a, " + tableName
+	final private void prepareTableWithValues(final Connection conn, final int nRows) throws
Exception {
+		final PreparedStatement stmt = conn.prepareStatement("upsert into " + tableName + " VALUES
(?, ?)");
+@BuiltInFunction(name=DistinctCountHyperLogLogAggregateFunction.NAME, nodeClass=DistinctCountHyperLogLogAggregateParseNode.class,
args= {@Argument()} )
+    public DistinctCountHyperLogLogAggregateFunction(List<Expression> childExpressions,
CountAggregateFunction delegate){
+	private HyperLogLogPlus hll = new HyperLogLogPlus(DistinctCountHyperLogLogAggregateFunction.NormalSetPrecision,
DistinctCountHyperLogLogAggregateFunction.SparseSetPrecision);
+	private HyperLogLogPlus hll = new HyperLogLogPlus(DistinctCountHyperLogLogAggregateFunction.NormalSetPrecision,
DistinctCountHyperLogLogAggregateFunction.SparseSetPrecision);
+	protected final ImmutableBytesWritable valueByteArray = new ImmutableBytesWritable(ByteUtil.EMPTY_BYTE_ARRAY);
+    public DistinctCountHyperLogLogAggregateParseNode(String name, List<ParseNode>
children, BuiltInFunctionInfo info) {
+    public FunctionExpression create(List<Expression> children, StatementContext context)
throws SQLException {

     {color:red}-1 core tests{color}.  The patch failed these unit tests:
     ./phoenix-core/target/failsafe-reports/TEST-org.apache.phoenix.end2end.index.MutableIndexFailureIT

Test results: https://builds.apache.org/job/PreCommit-PHOENIX-Build/1305//testReport/
Javadoc warnings: https://builds.apache.org/job/PreCommit-PHOENIX-Build/1305//artifact/patchprocess/patchJavadocWarnings.txt
Console output: https://builds.apache.org/job/PreCommit-PHOENIX-Build/1305//console

This message is automatically generated.

> Support approximate COUNT DISTINCT
> ----------------------------------
>
>                 Key: PHOENIX-418
>                 URL: https://issues.apache.org/jira/browse/PHOENIX-418
>             Project: Phoenix
>          Issue Type: Task
>            Reporter: James Taylor
>            Assignee: Ethan Wang
>              Labels: gsoc2016
>         Attachments: PHOENIX-418-v1.patch, PHOENIX-418-v2.patch, PHOENIX-418-v3.patch,
PHOENIX-418-v4.patch, PHOENIX-418-v5.patch, PHOENIX-418-v6.patch, PHOENIX-418-v7.patch
>
>
> Support an "approximation" of count distinct to prevent having to hold on to all distinct
values (since this will not scale well when the number of distinct values is huge). The Apache
Drill folks have had some interesting discussions on this [here](http://mail-archives.apache.org/mod_mbox/incubator-drill-dev/201306.mbox/%3CJIRA.12650169.1369931282407.88049.1370645900553%40arcas%3E).
They recommend using  [Welford's method](http://en.wikipedia.org/wiki/Algorithms_for_calculating_variance_Online_algorithm).
I'm open to having a config option that uses exact versus approximate. I don't have experience
implementing an approximate implementation, so I'm not sure how much state is required to
keep on the server and return to the client (other than realizing it'd be much less that returning
all distinct values and their counts).



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