gustavodemorais commented on code in PR #27603:
URL: https://github.com/apache/flink/pull/27603#discussion_r2811887442
##########
flink-table/flink-table-common/src/main/java/org/apache/flink/table/types/logical/utils/LogicalTypeCasts.java:
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@@ -293,6 +293,88 @@ public static boolean supportsExplicitCast(LogicalType
sourceType, LogicalType t
return supportsCasting(sourceType, targetType, true);
}
+ /**
+ * Returns whether the cast from source type to target type is injective
+ * (uniqueness-preserving).
+ *
+ * <p>An injective cast guarantees that every distinct input value maps to
a distinct output
+ * value. This property is useful for upsert key tracking through
projections: if a key column
+ * is cast using an injective conversion, the uniqueness of the key is
preserved.
+ *
+ * <p>This method returns {@code true} for casts that are either:
+ *
+ * <ul>
+ * <li>Implicit casts (handled by {@link #supportsImplicitCast}), which
are always safe type
+ * widenings
+ * <li>Explicit casts to STRING from types with deterministic string
representations: TINYINT,
+ * SMALLINT, INTEGER, BIGINT, FLOAT, DOUBLE, BOOLEAN, DATE, and
TIMESTAMP variants
+ * </ul>
+ *
+ * <p>Note: BYTES to STRING is explicitly excluded in this first version
because they are a
+ * special case and need additional testing (i.e., invalid UTF-8 bytes are
replaced with the
+ * Unicode replacement character).
+ *
+ * @param sourceType the source type
+ * @param targetType the target type
+ * @return {@code true} if the cast preserves uniqueness
+ */
+ public static boolean supportsInjectiveCast(LogicalType sourceType,
LogicalType targetType) {
+ // Implicit casts are always injective (safe widening)
+ if (supportsImplicitCast(sourceType, targetType)) {
Review Comment:
That is a good point.
Just to clarify, I understood where issue comes from: we were using
[supportsImplicitCast as a check for
isFidelityCheck](https://github.com/apache/flink/pull/27603/changes#diff-8be3389bbe106e1e8b09a8d33b8d0b803e320384dec6fb486de2867bd923e46cL224).
That is the current behaviour. That means, we have at least one bug for the
case you mentioned currently. If we're dropping this with my PR, my new
injective cast will have to cover all implicit cast cases that were previously
covered as well
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