[jira] [Commented] (ORC-744) LazyIO of non-filter columns

["Panagiotis Garefalakis (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ORC-744?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17300546#comment-17300546 ] 

Panagiotis Garefalakis commented on ORC-744:
--------------------------------------------

Hey [~planka] I can see a few open PRs here -- what the next one to look at? ORC-759?

> LazyIO of non-filter columns
> ----------------------------
>
>                 Key: ORC-744
>                 URL: https://issues.apache.org/jira/browse/ORC-744
>             Project: ORC
>          Issue Type: Improvement
>          Components: Reader
>            Reporter: Pavan Lanka
>            Assignee: Pavan Lanka
>            Priority: Major
>         Attachments: image-2021-01-25-14-34-45-375.png
>
>
> h2. Background
> This feature request started as a result of a large search that is performed with the following characteristics:
>  * The search fields are not part of partition, bucket or sort fields.
>  * The table is a very large table.
>  * The predicates result in very few rows compared to the scan size.
>  * The search columns are a significant subset of selection columns in the query.
> Initial analysis showed that we could have a significant benefit by lazily reading the non-search columns only when we have a match. We explore the design and some benchmarks in subsequent sections.
> h2. Design
> This builds further on ORC-577 which currently only restricts deserialization for some selected data types but does not improve on IO.
> On a high level the design includes the following components:
> !image-2021-01-25-14-34-45-375.png!
>  * *SArg to Filter*: Converts Search Arguments passed down into filters for efficient application during scans.
>  * *Read*: Performs the lazy read using the filters.
>  ** *Read Filter Columns*: Read the filter columns from the file.
>  ** *Apply Filter*: Apply the filter on the read filter columns.
>  ** *Read Select Columns*: If filter selects at least a row then read the remaining columns.
>  
> This issue has the following tasks that provides further details on the design of the respective components:
>  # ORC-741: Bug fix related to schema evolution of missing columns in the presence of filters
>  # ORC-742: LazyIO of non-filter columns
>  # ORC-743: Conversion of SArg to Filter
>  
> h2. Tests
> We evaluated this approach against a search job with the following stats:
>  * Table
>  ** Size: ~*420 TB*
>  ** Data fields: ~*120*
>  ** Partition fields: *3*
>  * Scan
>  ** Search fields: 3 data fields with large (~ 1000 value) IN clauses compounded by *OR*.
>  ** Select fields: 16 data fields (includes the 3 search fields), 1 partition field
>  ** Search:
>  *** Size: ~*180 TB*
>  *** Records: *3.99 T*
>  ** Selected:
>  *** Size: ~*100 MB*
>  *** Records: *1 M*
> We have observed the following reductions compared with the absence of the patch:
> ||Test||IO Reduction %||CPU Reduction %||
> |Select 16 columns|45|47|
> |SELECT *|70|87|
>  * The savings are more significant as you increase the number of select columns with respect to the search columns
>  * When the filter selects most data, no significant penalty observed as a result of 2 IO compared with a single IO
>  ** We do have a penalty as a result of the filter application on the selected records.



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