suibianwanwank commented on code in PR #4403:
URL: https://github.com/apache/calcite/pull/4403#discussion_r2123210385
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core/src/main/java/org/apache/calcite/sql/fun/SqlInternalOperators.java:
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@@ -224,6 +224,40 @@ private SqlInternalOperators() {
public static final SqlAggFunction LITERAL_AGG =
SqlLiteralAggFunction.INSTANCE;
+ /** CAST NOT NULL operator used for cast expression to make it non-nullable
in SqlNode.
+ *
+ * <p>For example:
+ * <pre>{@code COALESCE(a,b)
+ * }</pre>
+ *
+ * <p>is converted to
+ *
+ * <pre>{@code CASE WHEN a is not null THEN a ELSE b
+ * }</pre>
+ * by {@code SqlCoalesceFunction#rewriteCall}.
+ *
+ * <p>When a is nullable and b is non-nullable, the {@code COALESCE(a,b)}
data type will be
+ * non-nullable and {@code CASE WHEN a is not null THEN a ELSE b} data type
will be nullable.
Review Comment:
For example, a user wants to customize a special `SUBSTRING` function that
returns an empty string when the input str is null (the default implementation
should return null).
For convenience, let's assume the substring function takes the form
`substring(x, y, z)`.
To avoid implementing specific execution logic, I want to rewrite it in
`RewriteCall` as:
`case when [x] is null then '' else substring(x, y, z)`.
We want this function to return `not null` to enable more optimizations, but
the standard library's `substring(x, y, z)` will return nullable.
That is, sometimes nullable type inference cannot account for all complex
case. So the user should have a way to enforce the type.
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