[jira] [Resolved] (IMPALA-8058) HBase scan cardinality division-by-zero leads to bogus cardinality

["Paul Rogers (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/IMPALA-8058?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Paul Rogers resolved IMPALA-8058.
---------------------------------
       Resolution: Fixed
         Assignee: Paul Rogers
    Fix Version/s: Impala 3.2.0

> HBase scan cardinality division-by-zero leads to bogus cardinality
> ------------------------------------------------------------------
>
>                 Key: IMPALA-8058
>                 URL: https://issues.apache.org/jira/browse/IMPALA-8058
>             Project: IMPALA
>          Issue Type: Bug
>          Components: Frontend
>    Affects Versions: Impala 3.1.0
>            Reporter: Paul Rogers
>            Assignee: Paul Rogers
>            Priority: Major
>             Fix For: Impala 3.2.0
>
>
> A particular HBase query has highly selective key filters and runs into code bugs that produce a bogus, huge cardinality value.
> {{HbaseScanNode.computeStats()}} attempts to compute table cardinality by calling {{HBaseTable.getEstimatedRowStats()}}. This then calls into (in the latest versions) {{FeHBaseTable.getEstimatedRowStats()}}.
> This code tries to estimate cardinality by:
> * Scanning a set of regions.
> * For each getting the size.
> * Averaging a bunch of rows to estimate row width.
> Once we know the size of the regions we need to scan, and the average row width, we can compute the scan cardinality.
> The problem in this particular query is that the predicates are so selective that no regions match. As a result, the average row width is zero. We divide (as a double) the region size by 0 and get INF. We cast that to a long and get Long.MAX_VALUE. We then use that as our (highly bogus) cardinality estimate.
> The code must:
> * Detect the division-by-zero (now sample rows) case.
> * Use an alternative estimate (such as multiplying total table row count from HMS by the filter selectivity.)



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