[jira] [Commented] (DRILL-6953) Merge row set-based JSON reader

["ASF GitHub Bot (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/DRILL-6953?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17014001#comment-17014001 ] 

ASF GitHub Bot commented on DRILL-6953:
---------------------------------------

paul-rogers commented on issue #1913: DRILL-6953: EVF-based version of the JSON reader
URL: https://github.com/apache/drill/pull/1913#issuecomment-573497529
 
 
   Turns out that the v1 JSON reader will ignore all-null columns. In the test `TestStarQueries.testSelStarOrderBy`, we execute the following query:
   
   ```
   select * from cp.`employee.json` order by last_name
   ```
   
   Against the Foodmart file `customer.json` which has records like this:
   
   ```
   {"employee_id":53,...","end_date":null,"salary":...}
   ```
   
   The test is written to exclude `end_date` from a `SELECT *` query.
   
   In V2, however, `end_date` is included in the query. One can argue that 'SELECT *' means "return all columns", not "return all columns except those that happen to be null in the first batch."
   
   See DRILL-7522.
   
   Modified the test to expect the old result for the V1 reader, the correct result for the V2. reader.
 
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> Merge row set-based JSON reader
> -------------------------------
>
>                 Key: DRILL-6953
>                 URL: https://issues.apache.org/jira/browse/DRILL-6953
>             Project: Apache Drill
>          Issue Type: Improvement
>    Affects Versions: 1.15.0
>            Reporter: Paul Rogers
>            Assignee: Paul Rogers
>            Priority: Major
>              Labels: doc-impacting
>             Fix For: 1.18.0
>
>
> The final step in the ongoing "result set loader" saga is to merge the revised JSON reader into master. This reader does two key things:
> * Demonstrates the prototypical "late schema" style of data reading (discover schema while reading).
> * Implements many tricks and hacks to handle schema changes while loading.
> * Shows that, even with all these tricks, the only true solution is to actually have a schema.
> The new JSON reader:
> * Uses an expanded state machine when parsing rather than the complex set of if-statements in the current version.
> * Handles reading a run of nulls before seeing the first data value (as long as the data value shows up in the first record batch).
> * Uses the result-set loader to generate fixed-size batches regardless of the complexity, depth of structure, or width of variable-length fields.
> While the JSON reader itself is helpful, the key contribution is that it shows how to use the entire kit of parts: result set loader, projection framework, and so on. Since the projection framework can handle an external schema, it is also a handy foundation for the ongoing schema project.
> Key work to complete after this merger will be to reconcile actual data with the external schema. For example, if we know a column is supposed to be a VarChar, then read the column as a VarChar regardless of the type JSON itself picks. Or, if a column is supposed to be a Double, then convert Int and String JSON values into Doubles.
> The Row Set framework was designed to allow inserting custom column writers. This would be a great opportunity to do the work needed to create them. Then, use the new JSON framework to allow parsing a JSON field as a specified Drill type.



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