[teiid-issues] [JBoss JIRA] (TEIID-2892) OData buffers ALL rows from resultset before returning the first batch

[Ramesh Reddy (JIRA)]

    [ https://issues.jboss.org/browse/TEIID-2892?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12963265#comment-12963265 ] 

Ramesh Reddy commented on TEIID-2892:
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JDBC forward-only Vs scroll-intensive should give baseline numbers. Anything above is the odata overhead. Typically this involves marshaling of the resultset into OData output form and HTTP protocol weight. Never be equal to fully optimized binary JDBC protocol with forward-only cursor.

The choice to use the scroll intensive resultset was more to support "skip" and "batching" functionality than the row count. Since sending all the results on the first request is not feasible, and there is no "easier" way to keep track of session for next set of results. OData specification gives full details about the HTTP conversation, but never explicitly mentions how a connection/session can be maintained for duration of the query scrolling. Keeping the connection and resultset in cache per query is one option, but it was determined during the development time that it could lead memory exhaustion in situations like burst of in coming queries.  

One option could be we could by provide an option to turn on "forward-only" resultset through either 
* URL parameter (this will be custom extension) 
* Provide as configuration property (Will apply to all the vdbs)
* As HEADER in HTTP call (also will be a custom extension)
* Cookies? (also would be custom extension)

We may need to do some investigation as how others are solving this issue?

There may be other ways. Any input is welcome.

Steve: I do not think I followed your comment below
>We also may need to revisit the data flow into output buffers even with a transaction or a scrolling result, as >allowing unfettered buffering may be an general problem
                
> OData buffers ALL rows from resultset before returning the first batch
> ----------------------------------------------------------------------
>
>                 Key: TEIID-2892
>                 URL: https://issues.jboss.org/browse/TEIID-2892
>             Project: Teiid
>          Issue Type: Bug
>          Components: OData
>    Affects Versions: 8.4.1
>         Environment: Tested with Jboss DV 6.0.0. GA (enterprise edition) on Apple OSX 10.9.2 and Oracle Java VM 1.7.0_51.
>            Reporter: Patrick Deenen
>            Assignee: Steven Hawkins
>         Attachments: logfiles.zip
>
>
> OData doesn’t batch internally opposed to JDBC which does. E.g. when in JDBC a query is done with a large result, only the first 2048 rows are physically fetched from the source database and only the first 200 rows (depending on client application) are returned. But when the same query is executed by the use of Odata ALL rows in the result set are physically fetched by DV and stored in the buffer. Even with the default Odata fetch batch size of 256. This makes the Odata interface very inefficient for large query results where one only is interessed in the first 256 rows.
> Attached you can find two log files which show the problem. 
> The Odata query used is:
> http://localhost:8080/odata/PMA/EVENT_FACT?$filter=event_fact_id%20ge%20747000000%20and%20event_fact_id%20lt%20747200000
> Which is identical to the JDBC query used:
> select * from event_fact where event_fact_id between 747000000 and 747200000;
> In both cases the result contains 200.000 rows
> ODATA log information analysis (log file ’server start + odata batch 256.log’):
> row 4543 - 4657   - Start query
> row 4658 - 9030   - Read ALL results from result set and store them in buffer
> row 9031 - 9035   - Close DB connection
> row 9036 - 14647  - Clean buffers and create response?
> row 14648 - 14661 - return first batch and close connection
> JDBC log information analysis (log file ’server start + jdbc.log’):
> row 4925 - 5112   - Start query
> row 5113 - 5166   - Read ONLY the first 2048 results from result set and store them in buffer and return response
> row 5157 - 5214   - Close DB connection

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