[GitHub] [druid] gianm commented on a change in pull request #11082: DataSchema: Improve duplicate-column error message.

[GitBox <gi...@apache.org>]

gianm commented on a change in pull request #11082:
URL: https://github.com/apache/druid/pull/11082#discussion_r610769060



##########
File path: server/src/main/java/org/apache/druid/segment/indexing/DataSchema.java
##########
@@ -192,6 +176,89 @@ private static DimensionsSpec computeDimensionsSpec(
     return dimensionsSpec.withDimensionExclusions(additionalDimensionExclusions);
   }
 
+  private static Set<String> computeInputFieldNames(
+      final TimestampSpec timestampSpec,
+      final DimensionsSpec dimensionsSpec,
+      final AggregatorFactory[] aggregators
+  )
+  {
+    final Set<String> fields = new HashSet<>();
+
+    fields.add(timestampSpec.getTimestampColumn());
+    fields.addAll(dimensionsSpec.getDimensionNames());
+    Arrays.stream(aggregators)
+          .flatMap(aggregator -> aggregator.requiredFields().stream())
+          .forEach(fields::add);
+
+    return fields;
+  }
+
+  /**
+   * Computes the set of field names that are specified by the provided dimensions and aggregator lists.
+   *
+   * If either list is null, it is ignored.
+   *
+   * @throws IllegalArgumentException if there are duplicate field names, or if any dimension or aggregator
+   *                                  has a null name
+   */
+  private static Set<String> computeAndValidateOutputFieldNames(
+      @Nullable final DimensionsSpec dimensionsSpec,
+      @Nullable final AggregatorFactory[] aggregators
+  )
+  {
+    // Field name -> where it was seen
+    final Map<String, Multiset<String>> fields = new TreeMap<>();
+
+    fields.computeIfAbsent(ColumnHolder.TIME_COLUMN_NAME, k -> TreeMultiset.create()).add(
+        StringUtils.format(
+            "primary timestamp (%s cannot appear as a dimension or metric)",
+            ColumnHolder.TIME_COLUMN_NAME
+        )
+    );
+
+    if (dimensionsSpec != null) {
+      for (int i = 0; i < dimensionsSpec.getDimensions().size(); i++) {
+        final String field = dimensionsSpec.getDimensions().get(i).getName();
+        if (Strings.isNullOrEmpty(field)) {
+          throw new IAE("Encountered dimension with null or empty name at position %d", i);
+        }
+
+        fields.computeIfAbsent(field, k -> TreeMultiset.create()).add("dimensions list");
+      }
+    }
+
+    if (aggregators != null) {
+      for (int i = 0; i < aggregators.length; i++) {
+        final String field = aggregators[i].getName();
+        if (Strings.isNullOrEmpty(field)) {
+          throw new IAE("Encountered metric with null or empty name at position %d", i);
+        }
+
+        fields.computeIfAbsent(field, k -> TreeMultiset.create()).add("metricsSpec list");
+      }
+    }
+
+    final List<String> errors = new ArrayList<>();
+
+    for (Map.Entry<String, Multiset<String>> entry : fields.entrySet()) {
+      if (entry.getValue().entrySet().stream().mapToInt(Multiset.Entry::getCount).sum() > 1) {
+        errors.add(
+            StringUtils.format(
+                "[%s] seen in %s",

Review comment:
       It _could_ happen, but I was thinking it would still be understandable. The message would be like:
   
   ```
   Cannot specify a column more than once: [metric1] seen in metricsSpec list
   ```
   
   I was thinking that the user would be able to look at the metricsSpec and see that `metric1` is there more than once. But I'm happy to take suggestions on better error messages.




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