liyafan82 commented on a change in pull request #2330:
URL: https://github.com/apache/calcite/pull/2330#discussion_r562487548
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File path: core/src/main/java/org/apache/calcite/rex/RexUtil.java
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@@ -2620,6 +2621,103 @@ private RexNode or(Iterable<? extends RexNode> nodes) {
return Util.transform(list, Object::toString);
}
+ /** Estimate the number of distinct values for the specified columns (if
possible),
+ * given the condition. */
+ public static @Nullable Double estimateColumnsNdv(
+ ImmutableBitSet columns, @Nullable RexNode condition) {
+ if (condition == null) {
+ return null;
+ }
+ if (condition.isAlwaysFalse()) {
+ return 0.0;
+ }
+ double ndv = 1.0;
+ List<RexNode> conditions = RelOptUtil.conjunctions(condition);
+ for (int col : columns) {
+ Double colNdv = estimateColumnNdv(col, conditions);
+ if (colNdv == null) {
+ // if one column's ndv cannot be estimated, we cannot
+ // estimate the ndv for the column set.
+ return null;
+ }
+ // ndv's should be multiplied.
+ // for example, NDV(x) <= a, and NDV(y) <= b;
+ // then we have NDV(x, y) <= a * b;
+ ndv *= colNdv;
+ }
+ return ndv;
+ }
+
+ /** Estimate the number of distinct values for a single column (if possible),
+ * given the condition. */
+ private static @Nullable Double estimateColumnNdv(int colIdx, List<RexNode>
conditions) {
+ Double ndv = null;
+ for (RexNode condition : conditions) {
+ Double singleNdv = estimateColumnNdvSingleCondition(colIdx, condition);
+ if (singleNdv != null) {
+ if (ndv == null) {
Review comment:
Accepted. Thanks for the suggestion.
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