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https://issues.apache.org/jira/browse/BEAM-5074?focusedWorklogId=131539&page=com.atlassian.jira.plugin.system.issuetabpanels:worklog-tabpanel#worklog-131539
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ASF GitHub Bot logged work on BEAM-5074:
----------------------------------------
Author: ASF GitHub Bot
Created on: 06/Aug/18 19:44
Start Date: 06/Aug/18 19:44
Worklog Time Spent: 10m
Work Description: amaliujia commented on issue #6145: [BEAM-5074][SQL]
improve BeamSqlRowComparator
URL: https://github.com/apache/beam/pull/6145#issuecomment-410830109
@apilloud
`orientation` tells what should be the order on a field. If the query has
`ASC` on a field, then `orientation` is true, and if the query has `DESC`, then
`orientation` is false.
However, if you observe what the code is written for `BeamSqlRowComparator`
([code
linik](https://github.com/apache/beam/blob/master/sdks/java/extensions/sql/src/main/java/org/apache/beam/sdk/extensions/sql/impl/rel/BeamSortRel.java#L320)),
you can find this line:
```
fieldRet *= (orientation.get(i) ? -1 : 1);
```
The comparator will reverse the comparison result. For example, if it is
`ASC` and `orientation` is true, and if fieldRet was `-1`, then fieldRet will
become 1, which means smaller value will be put after the bigger value (the
reversed order).
The reason this implementation is correct, as I tried to explain in the PR
description, is that `ORDER BY LIMIT` uses `Top.of`. `Top` transform returns
largest elements first (another reversed order), which finally made the SQL
queries' results correct (ASC gives increasing order result and DESC gives
decreasing order result).
The `isReversed`, in stead, allows we control the behavior of
`BeamSqlRowComparator`. The comparator will return -1 for `ASC` and
1.compareTo(2) if `isReversed = false`, and 1 otherwise. Right now, the
comparator returns 1 for `ASC` and 1.compareTo(2).
It will be useful if I want to implement `ORDER BY ASC/DESC`
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Issue Time Tracking
-------------------
Worklog Id: (was: 131539)
Time Spent: 40m (was: 0.5h)
> Make BeamSqlRowComparator more general
> --------------------------------------
>
> Key: BEAM-5074
> URL: https://issues.apache.org/jira/browse/BEAM-5074
> Project: Beam
> Issue Type: Improvement
> Components: dsl-sql
> Reporter: Rui Wang
> Assignee: Rui Wang
> Priority: Major
> Time Spent: 40m
> Remaining Estimate: 0h
>
> BeamSqlRowComparator returns reversed comparison results (if a < b in terms
> of value, BeamSqlRowComparator returns a > b).
>
> Add a flag to turn on/off this reversed effect. The reason of doing this is
> to help implement ORDER BY ASC/DESC in the normal way.
>
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