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https://issues.apache.org/jira/browse/NIFI-5143?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Matt Burgess updated NIFI-5143:
-------------------------------
Description:
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 this case however (and perhaps in a more general sense), the customer has 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.
> 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
> 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 this case however (and perhaps in a more general sense), the customer has
> 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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