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https://issues.apache.org/jira/browse/NIFI-5143?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16482816#comment-16482816
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ASF GitHub Bot commented on NIFI-5143:
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GitHub user mattyb149 opened a pull request:
https://github.com/apache/nifi/pull/2728
NIFI-5143: Initial work to support column values for paging results
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This closes #2728
----
commit bc614ce908addb3c961ae4aeb18879b4801c570a
Author: Matthew Burgess <mattyb149@...>
Date: 2018-05-21T17:45:16Z
NIFI-5143: Initial work to support column values for paging results
----
> Allow GenerateTableFetch to use column values for partitioning
> --------------------------------------------------------------
>
> Key: NIFI-5143
> URL: https://issues.apache.org/jira/browse/NIFI-5143
> Project: Apache NiFi
> Issue Type: Improvement
> Reporter: Matt Burgess
> Assignee: Matt Burgess
> Priority: Major
>
> GenerateTableFetch generates SQL statements for fetching "pages" of rows from
> a database, and the SQL it generates often leverages the concept of "row
> numbers" when doing things such as LIMIT and OFFSET.
> Oracle 11 is an example of a special case for GenerateTableFetch, because it
> doesn't make a "row number" available for doing paging ("get rows 10 through
> 19", e.g.). For that reason the row number is created as an extra column
> using a nested select with an ORDER BY clause. For large tables this can be
> very inefficient.
> In a general sense, the user may have a column that contains integer IDs and
> would like to have a more efficient query. I propose the following as a
> generic solution that would mostly benefit this kind of use case:
> - Add a property to GenerateTableFetch to "Use Column Values For
> Partitioning". This would be a boolean property defaulting to false to retain
> current behavior.
> - If the maximum value column type is numeric and "Use Column Values For
> Partitioning" is true, fetch the minimum value of the column (after the where
> clause is applied)
> - If "Use Column Values For Partitioning" is true, then determine the page
> offsets by taking the difference of the maximum value and minimum value,
> dividing by the Partition Size.
> These changes would use the "Max-value Columns" values rather than the result
> set row numbers for figuring out the paging. The documentation needs to be
> very clear on when this property should be used. For example, when "Use
> Column Values For Partitioning" is true, then the column values should be
> evenly distributed and not sparse, for best performance.
> As a counterexample, consider the following:
> - Use Column Values For Partitioning = true
> - Max Value Column = "id"
> - Table "myTable" has only 3 rows, one with id = 1, one with id = 50, one
> with id = 100
> - Partition Size = 5
> GenerateTableFetch would generate 20 flow files, of the following pattern:
> SELECT * from myTable where ID >= 1 AND ID < 5
> SELECT * from myTable where ID >= 5 AND ID < 10
> SELECT * from myTable where ID >= 10 AND ID < 15
> ...
> SELECT * from myTable where ID >= 50 AND ID < 55
> ...
> SELECT * from myTable where ID >= 100 AND ID < 105
> Note that only 3 flow files out of 20 will contain SQL statements that would
> retrieve any rows. The rest will return empty ResultSets. However when the
> columns are appropriate for use of their values, this improvement could be
> quite substantial.
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