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https://issues.apache.org/jira/browse/NIFI-5727?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Valentin Nikotin updated NIFI-5727:
-----------------------------------
External issue URL: (was: https://github.com/apache/nifi/pull/3094)
> Optimize GenerateTableFetch to remove unnecessary COUNT(*)
> ----------------------------------------------------------
>
> Key: NIFI-5727
> URL: https://issues.apache.org/jira/browse/NIFI-5727
> Project: Apache NiFi
> Issue Type: Improvement
> Components: Core Framework
> Affects Versions: 1.7.1
> Reporter: Valentin Nikotin
> Priority: Minor
>
> When processor is configured to use *"Column for Value Partitioning"*
> processor property it still
> [calculates|https://github.com/apache/nifi/blob/master/nifi-nar-bundles/nifi-standard-bundle/nifi-standard-processors/src/main/java/org/apache/nifi/processors/standard/GenerateTableFetch.java#L293]
> {{COUNT(*)}} while it's not used further in code.
> At line 292 code adds an explicit {{SELECT COUNT(*)}} to the query that
> calculates the bounds of the partitions:
> {code:java}
> List<String> maxValueSelectColumns = new
> ArrayList<>(numMaxValueColumns + 1);
> maxValueSelectColumns.add("COUNT(*)");
> {code}
> At [line
> 414|https://github.com/apache/nifi/blob/master/nifi-nar-bundles/nifi-standard-bundle/nifi-standard-processors/src/main/java/org/apache/nifi/processors/standard/GenerateTableFetch.java#L414]
> we see that the count is only used if no columns have been set for paging:
> {code:java}
> if (useColumnValsForPaging) {
> final long valueRangeSize = maxValueForPartitioning ==
> null ? 0 : (maxValueForPartitioning - minValueForPartitioning + 1);
> numberOfFetches = (partitionSize == 0) ? 1 :
> (valueRangeSize / partitionSize) + (valueRangeSize % partitionSize == 0 ? 0 :
> 1);
> } else {
> numberOfFetches = (partitionSize == 0) ? 1 : (rowCount /
> partitionSize) + (rowCount % partitionSize == 0 ? 0 : 1);
> }
> {code}
> Since the {{SELECT COUNT(*)}} sometimes takes too long to complete, we want
> to optimize this code and perform the {{SELECT COUNT(*)}} only when property
> *"Column for Value Partitioning"* has not been set.
> The idea is to use *-1* value to minimize code changes:
> {code:java}
> if (useColumnValsForPaging) {
> maxValueSelectColumns.add("-1");
> } else {
> maxValueSelectColumns.add("COUNT(*)");
> }
> {code}
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