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https://issues.apache.org/jira/browse/ORC-742?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Pavan Lanka updated ORC-742:
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Attachment: (was: image-2021-01-25-14-12-59-308.png)
> LazyIO of non-filter columns in the presence of filters
> -------------------------------------------------------
>
> Key: ORC-742
> URL: https://issues.apache.org/jira/browse/ORC-742
> Project: ORC
> Issue Type: Improvement
> Components: Reader
> Reporter: Pavan Lanka
> Assignee: Pavan Lanka
> Priority: Major
> Attachments: image-2021-01-25-14-14-20-912.png,
> image-2021-01-25-14-14-48-327.png, image-2021-01-25-14-15-19-262.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
> ORC-577 introduced filters during the read process. However the optimization
> did not include IO skips of non-filter columns.
> We propose that the following take place during the read on a high level:
> * Read the filter columns
> * Apply the filter
> * On the first hit read the non-filter columns
> h3. Read
> The read process has the following changes:
> !image-2021-01-25-14-15-19-262.png!
>
> The read process changes:
> * *Read Stripe* used to plan the read of all (search + select) columns. This
> is enhanced to plan and fetch only the search columns. The rest of the stripe
> planning process optimizations remain unchanged e.g. partial read planning of
> the stripe based on RowGroup statistics.
> * *Next Batch* identifies the processing that takes place when
> {{RecordReader.nextBatch}} is invoked.
> ** *Read Search Columns* takes place instead of reading all the selected
> columns. This is in sync with the planning that has taken place during *Read
> Stripe* where only the search columns have been planned.
> ** *Apply Filter* on the batch that at this point only includes search
> columns. Evaluate the result of the filter:
> *** *Size = 0* indicates all records have been filtered out. Given this we
> proceed to the next batch of search columns.
> *** *Size > 0* indicates that at least one record accepted by the filter.
> This record needs to be substantiated with other columns.
> ** *Plan Select Columns* is invoked to perform read of the select columns.
> The planning happens as follows:
> *** Determine the current position of the read within the stripe and plan
> the read for the select columns from this point forward to the end of the
> stripe.
> *** The Read planning of select columns respects the row groups filtered out
> as a result of the stripe planning.
> *** Fetch the select columns using the above plan.
> ** *Read Select Columns* into the vectorized row batch
> ** Return this batch.
> The current implementation performs a single read for the select columns in a
> stripe.
> !image-2021-01-25-14-14-48-327.png!
> The above diagram depicts a stripe with 7 Row Groups out of which *RG2* and
> *RG5* are selected by the filter. The current implementation does the
> following:
> * Start the read planning process from the first match RG2
> * Read to the end of the stripe that includes RG6
> * Based on the above fetch skips RG0 and RG1 subject to compression block
> boundaries
> The above logic could be enhanced to perform say *2 or n* reads before
> reading to the end of stripe. The current implementation allows 0 reads
> before reading to the end of the stripe. The value of *n* could be
> configurable but should avoid too many short reads.
> The read behavior changes as follows with multiple reads being allowed within
> a stripe for select columns:
> !image-2021-01-25-14-14-20-912.png!
> The figure shows that we could read significantly fewer bytes by performing
> an additional read before reading to the end of stripe. This shall be
> included as a subsequent enhancement to this patch.
> h2. Tests
> This offers significant savings on IO and CPU in scans that result in very
> rows as a result of the predicates.
> We have seen the following from our tests:
> * 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:
> ||Test||IO Reduction %||CPU Reduction %||
> |SELECT 16 fields|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 IOs compared to a single IO
> ** We do have a penalty as a result of the filter application on the
> selected records.
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