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Posted to issues@hive.apache.org by "Gopal V (JIRA)" <ji...@apache.org> on 2015/11/22 07:59:10 UTC
[jira] [Commented] (HIVE-12491) Statistics: 3 attribute join on a
2-source table is off
[ https://issues.apache.org/jira/browse/HIVE-12491?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15020875#comment-15020875 ]
Gopal V commented on HIVE-12491:
--------------------------------
band-aid fix.
{code}
// To avoid denominator getting larger and aggressively reducing
// number of rows, we will ease out denominator.
denom = getEasedOutDenominator(new ArrayList<Long>(new HashSet<Long>(distinctVals)));
{code}
> Statistics: 3 attribute join on a 2-source table is off
> -------------------------------------------------------
>
> Key: HIVE-12491
> URL: https://issues.apache.org/jira/browse/HIVE-12491
> Project: Hive
> Issue Type: Bug
> Reporter: Gopal V
> Assignee: Prasanth Jayachandran
>
> The eased out denominator has to detect duplicate row-stats from different attributes.
> {code}
> private Long getEasedOutDenominator(List<Long> distinctVals) {
> // Exponential back-off for NDVs.
> // 1) Descending order sort of NDVs
> // 2) denominator = NDV1 * (NDV2 ^ (1/2)) * (NDV3 ^ (1/4))) * ....
> Collections.sort(distinctVals, Collections.reverseOrder());
> long denom = distinctVals.get(0);
> for (int i = 1; i < distinctVals.size(); i++) {
> denom = (long) (denom * Math.pow(distinctVals.get(i), 1.0 / (1 << i)));
> }
> return denom;
> }
> {code}
> This gets {{[8007986, 821974390, 821974390]}}, which is actually 3 columns 2 of which are from the RHS table.
> So the eased out denominator is off by a factor of 30,000 or so, causing OOMs in map-joins.
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