xuzifu666 commented on code in PR #5052:
URL: https://github.com/apache/calcite/pull/5052#discussion_r3487027033
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core/src/main/java/org/apache/calcite/sql/validate/SqlValidatorImpl.java:
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@@ -5658,6 +5665,277 @@ protected RelDataType validateSelectList(final
SqlNodeList selectItems,
return typeFactory.createStructType(fieldList);
}
+ /**
+ * Rewrites scalar sub-queries in the SELECT list whose single select item is
+ * an aggregate function whose arguments reference only outer columns. Per
the
+ * SQL standard, such aggregates belong to the outer query.
+ *
+ * <p>The algorithm is:
+ * <ol>
+ * <li>For each item in the SELECT list, check whether it is a scalar
+ * sub-query, optionally wrapped in {@code AS} or {@code WITH}.
+ * <li>Inside the sub-query, require the SELECT list to contain exactly one
+ * item, and that item to be an aggregate function call.
+ * <li>Require every argument of the aggregate to reference only columns from
+ * the outer query (no inner columns), and to contain no nested sub-queries.
+ * <li>If all conditions hold, lift the aggregate out of the sub-query: keep
+ * the aggregate in the outer SELECT list, and replace the original sub-query
+ * with {@code (SELECT 1 FROM ... LIMIT 1)}. The result is equivalent because
+ * the scalar sub-query contributes a factor of one per outer row, while the
+ * aggregate is evaluated over the outer rows.
+ * <li>If the outer query becomes an aggregate query as a result, upgrade its
+ * SELECT clause scope from {@link SelectScope} to
+ * {@link AggregatingSelectScope}.
+ * </ol>
+ *
+ * <p>For example,
+ * <blockquote><pre>
+ * WITH aa(a) AS (VALUES 1, 2, 3),
+ * t(x) AS (VALUES 10, 20, 30)
+ * SELECT (SELECT sum(a) FROM t LIMIT 1) FROM aa
+ * </pre></blockquote>
+ * is rewritten to
+ * <blockquote><pre>
+ * WITH aa(a) AS (VALUES 1, 2, 3),
+ * t(x) AS (VALUES 10, 20, 30)
+ * SELECT sum(a) * (SELECT 1 FROM t LIMIT 1) FROM aa
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