Fix wrong hash function in Unordered2MapRecord

puzpuzpuz · puzpuzpuz · commit f52284aa1890 · 2025-11-22T23:18:38.000+02:00
diff --git a/core/src/main/java/io/questdb/cairo/map/Unordered2MapRecord.java b/core/src/main/java/io/questdb/cairo/map/Unordered2MapRecord.java
@@ -30,6 +30,7 @@
 import io.questdb.cairo.sql.RecordCursor;
 import io.questdb.std.Decimal128;
 import io.questdb.std.Decimal256;
+import io.questdb.std.Hash;
 import io.questdb.std.IntList;
 import io.questdb.std.Long256;
 import io.questdb.std.Long256Impl;
@@ -327,7 +328,10 @@ public MapValue getValue() {
 
     @Override
     public long keyHashCode() {
-        return 0; // no-op
+        // Although this map does not use hash codes, this method is implemented
+        // for the purpose of map sharding, i.e. to spread keys among multiple Unordered2Maps
+        // in case when group by functions, such as count_distinct(), have high cardinality.
+        return Hash.hashShort64(Unsafe.getUnsafe().getShort(startAddress));
     }
 
     public void of(long address) {
