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Reading a report

A text report has four parts. Take:

$ delta-explain ./users -w "country = 'DE' AND age > 40"

Predicate Analysis:
  partition-safe: country = 'DE'
  stats-safe:     age > 40
  unsplittable:   -
  confidence:     conservative

Files in snapshot: 6

Phase 1: Partition pruning [exact]
  predicate:       country = 'DE'
  files remaining: 2  (-4, 67% pruned)
Phase 2: Data skipping (min/max statistics) [conservative]
  predicate:       age > 40
  files remaining: 1  (-1, 50% pruned)

Total reduction: 6 -> 1 files (83% pruned)

Predicate Analysis

Each top-level AND conjunct is routed to the mechanism that can eliminate files on it:

  • partition-safe: references partition columns only; prunes at the directory level, exactly.
  • partition-exact: outside the kernel's language (an any-shape LIKE) but all its columns are partition columns, so it is evaluated directly against the literal partition values.
  • stats-safe: references non-partition columns; prunes on per-file min/max statistics.
  • unsplittable: mixes both axes, or contains a construct the tool cannot reason about; applied conservatively (keeps all files) with a diagnostic.

Confidence

Labels how precisely the elimination can be explained, never whether it is correct (it always is, conservatively):

  • exact: partition values compared directly; every dropped file provably has no match.
  • conservative: min/max ranges can overlap a bound without a match, so files may be kept in excess, never dropped in excess.
  • incomplete: part of the predicate could not be attributed to one phase; the totals stay sound, the attribution does not.

Phases

The pruning is decomposed into chained phases (partition pruning first, then data skipping), each showing the predicate fragment it honored and the files remaining. It is a decomposition of what the metadata makes possible, not an execution trace.

Per-file detail

Add --verbose to see each file kept or dropped and why; in JSON mode it emits a machine-readable files[] array. Cap large listings with --limit.

The precise guarantees behind every label are in What delta-explain guarantees.