fix: all-fragments-pruned returns columns:[] + public API export gaps

- executeMultiFragment returned columns:[] when all fragments were
  pruned by partition/filter pushdown. Callers (pg-wire, show(),
  formatResultSummary) got empty column headers. Now returns actual
  schema from query.projections or first fragment's columns.
- Remove dead autoCoalesceGap export (superseded by optional maxGap
  in coalesceRanges).
- Export missing composability types from index.ts: buildPipeline,
  buildKeptPageIndices, canSkipPageMultiCol, FragmentSource,
  PipelineOptions, PipelineResult, SpillBackend.

Author: teamchong

src/coalesce.ts

@@ -33,13 +33,6 @@ function computeGap(sorted: Range[]): number {
   return Math.max(16 * 1024, Math.min(256 * 1024, medianGap * 2));
 }
 
-/** Compute optimal coalesce gap based on page density. Sorts a copy internally. */
-export function autoCoalesceGap(ranges: Range[]): number {
-  if (ranges.length < 2) return 64 * 1024;
-  const sorted = [...ranges].sort((a, b) => a.offset - b.offset);
-  return computeGap(sorted);
-}
-
 /** Merge nearby byte ranges into fewer R2 reads. Sorts by offset, merges if gap <= maxGap.
  *  If maxGap is omitted, auto-computes from page density (sorts once instead of twice). */
 export function coalesceRanges(ranges: Range[], maxGap?: number): CoalescedRange[] {

src/index.ts

@@ -23,14 +23,16 @@ export {
   ComputedColumnOperator, HashJoinOperator, ExternalSortOperator,
   InMemorySortOperator, WindowOperator, DistinctOperator, SetOperator,
   LimitOperator, SubqueryInOperator,
-  drainPipeline, buildEdgePipeline, FsSpillBackend,
+  drainPipeline, buildPipeline, buildEdgePipeline, FsSpillBackend,
+  buildKeptPageIndices, canSkipPageMultiCol,
 } from "./operators.js";
-export type { Operator, RowBatch } from "./operators.js";
+export type { Operator, RowBatch, FragmentSource, PipelineOptions, PipelineResult } from "./operators.js";
 export { QueryModeError } from "./errors.js";
 export type { ErrorCode } from "./errors.js";
 export { LocalExecutor } from "./local-executor.js";
 export { bigIntReplacer } from "./decode.js";
 export { R2SpillBackend } from "./r2-spill.js";
+export type { SpillBackend } from "./r2-spill.js";
 export { createFromJSON, createFromCSV, createDemo } from "./convenience.js";
 export { formatResultSummary, formatExplain, formatBytes } from "./format.js";
 export type { LocalTimingInfo } from "./format.js";

src/query-do.ts

@@ -1687,9 +1687,12 @@ export class QueryDO extends DurableObject<Env> {
     }
 
     if (fragments.length === 0) {
-      // All fragments pruned — return empty result
+      // All fragments pruned — return empty result with correct schema
+      const cols = query.projections.length > 0
+        ? query.projections
+        : (allFragments.length > 0 ? allFragments[0].columns.map(c => c.name) : []);
       return {
-        rows: [], rowCount: 0, columns: [],
+        rows: [], rowCount: 0, columns: cols,
         bytesRead: 0, pagesSkipped: 0, durationMs: Date.now() - t0,
         r2ReadMs: 0, wasmExecMs: 0, cacheHits: 0, cacheMisses: 0,
         edgeCacheHits: 0, edgeCacheMisses: 0,