jpountz commented on code in PR #15083:
URL: https://github.com/apache/lucene/pull/15083#discussion_r2282258992


##########
lucene/core/src/java/org/apache/lucene/index/DocValuesSkipper.java:
##########
@@ -122,4 +123,61 @@ public final void advance(long minValue, long maxValue) 
throws IOException {
       advance(maxDocID + 1);
     }
   }
+
+  /**
+   * Returns the minimum value for a field across all segments, or {@link 
Long#MIN_VALUE} if not
+   * available
+   *
+   * @param searcher a searcher over the index
+   * @param field the field to retrieve values for
+   */
+  public static long globalMinValue(IndexSearcher searcher, String field) 
throws IOException {
+    long minValue = Long.MAX_VALUE;
+    for (LeafReaderContext ctx : searcher.getLeafContexts()) {
+      DocValuesSkipper skipper = ctx.reader().getDocValuesSkipper(field);
+      if (skipper == null) {
+        minValue = Long.MIN_VALUE;
+      } else {
+        minValue = Math.min(minValue, skipper.minValue());
+      }
+    }
+    return minValue;
+  }
+
+  /**
+   * Returns the maximum value for a field across all segments, or {@link 
Long#MIN_VALUE} if not
+   * available
+   *
+   * @param searcher a searcher over the index
+   * @param field the field to retrieve values for
+   */
+  public static long globalMaxValue(IndexSearcher searcher, String field) 
throws IOException {
+    long maxValue = Long.MIN_VALUE;
+    for (LeafReaderContext ctx : searcher.getLeafContexts()) {
+      DocValuesSkipper skipper = ctx.reader().getDocValuesSkipper(field);
+      if (skipper == null) {
+        maxValue = Long.MAX_VALUE;
+      } else {
+        maxValue = Math.max(maxValue, skipper.maxValue());
+      }
+    }
+    return maxValue;
+  }
+
+  /**
+   * Returns the total skipper document count for a field across all segments
+   *
+   * @param searcher a searcher over the index
+   * @param field the field to retrieve values for
+   */
+  public static int globalDocCount(IndexSearcher searcher, String field) 
throws IOException {
+    int docCount = 0;
+    for (LeafReaderContext ctx : searcher.getLeafContexts()) {
+      DocValuesSkipper skipper = ctx.reader().getDocValuesSkipper(field);
+      if (skipper != null) {
+        docCount += skipper.docCount();
+      }
+    }
+    return docCount;
+  }

Review Comment:
   IMO the contract would be a bit trappy if it returned -1 if the field exists 
in one segment (as per field infos) but has no skip index (e.g. `MultiReader` 
allows mixing segments that have a skip index on the field and segments that 
don't).



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