[I] feat: OFFSET pushdown with WHERE filters using fully-matched RG statistics [datafusion]

["zhuqi-lucas (via GitHub)" <gi...@apache.org>]

zhuqi-lucas opened a new issue, #21916:
URL: https://github.com/apache/datafusion/issues/21916

   ## Background
   
   #21828 implements OFFSET pushdown for parquet queries without filters. Queries with WHERE clauses still use `GlobalLimitExec` for offset handling because row counts may be inaccurate after filtering.
   
   ## Problem
   
   For queries like `SELECT * FROM table WHERE date >= '2020-01-01' LIMIT 5 OFFSET 1000000`, the offset is handled by `GlobalLimitExec` even when statistics prove all rows in some RGs satisfy the filter.
   
   ## Opportunity
   
   `prune_by_statistics` already marks RGs as `is_fully_matched` when column statistics prove ALL rows satisfy the predicate (e.g., `min(date) >= '2020-01-01'`). For these RGs, `num_rows` is the exact qualifying row count — safe to use for offset calculation.
   
   ## Proposed approach
   
   1. During `prune_by_offset`, skip leading **fully-matched** RGs whose cumulative rows fall within offset (already implemented in #21828's `prune_by_offset` with `has_predicate` flag)
   2. Stop at the first non-fully-matched RG (qualifying row count unknown)
   3. `GlobalLimitExec` handles the remaining offset (reduced by skipped rows)
   4. Need mechanism to communicate skipped row count from parquet opener back to `GlobalLimitExec` (reduce its skip)
   
   ## Challenge
   
   The key difficulty is coordinating between parquet-level RG skipping and `GlobalLimitExec`'s skip counter. The optimizer sets `GlobalLimitExec(skip=N)` at plan time, but the actual RG-level skipping happens at runtime. Options:
   - Shared counter between opener and GlobalLimitExec
   - Dynamic adjustment of GlobalLimitExec skip based on DataSourceExec's output
   
   ## Related
   
   - #21828 — Single-file no-filter OFFSET pushdown (parent PR)
   - #21915 — Multi-file OFFSET pushdown
   - #19654 — Original issue for OFFSET performance


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