LazyIO of non-filter columns

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.

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:

• 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:

1. ORC-741: Bug fix related to schema evolution of missing columns in the presence of filters
2. ORC-742: LazyIO of non-filter columns
3. ORC-743: Conversion of SArg to Filter

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:

• 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.