Describe the bug

PushBuffers::clear_ranges performs an O(N*M) scan to release consumed buffers, where N is the number of buffered ranges and M is the number of ranges to clear. On wide schemas (10k+ columns), this produces quadratic overhead in PushDecoder row group construction.

Additionally, clear_ranges matches buffers by exact range equality. When the IO layer coalesces adjacent requested ranges into fewer, larger fetches, the coalesced buffer never exactly matches any individual requested range, so clear_ranges silently skips it. The buffer leaks in PushBuffers until the decoder finishes or the caller manually calls release_all_ranges, increasing peak RSS proportionally to the amount of data coalesced ahead of the current row group.

This puts coalescing in a bind: without it, buffer count scales with range count and the quadratic clear_ranges dominates. With it, memory is not reclaimed incrementally.

To Reproduce

Use PushDecoder on a Parquet file with a wide schema (10k+ columns). Push data without coalesced buffers. Observe row group construction time scaling quadratically with column count. Conversely if using coalescing, observe memory not being released by clear_ranges, growing RSS until the decoder finishes.

Expected behavior

Buffer release should scale with buffer count (not range count), and coalesced or arbitrarily-sized buffers should be released incrementally as the decoder progresses.

Additional context