gianm commented on a change in pull request #9488: Match GREATEST/LEAST
function behavior to other DBs
URL: https://github.com/apache/druid/pull/9488#discussion_r391092673
##########
File path: core/src/main/java/org/apache/druid/math/expr/Function.java
##########
@@ -976,6 +981,163 @@ protected ExprEval eval(double x, double y)
}
}
+ class GreatestFunc extends ReduceFunc
+ {
+ public static final String NAME = "greatest";
+
+ public GreatestFunc()
+ {
+ super(
+ Math::max,
+ Math::max,
+ BinaryOperator.maxBy(Comparator.naturalOrder())
+ );
+ }
+
+ @Override
+ public String name()
+ {
+ return NAME;
+ }
+ }
+
+ class LeastFunc extends ReduceFunc
+ {
+ public static final String NAME = "least";
+
+ public LeastFunc()
+ {
+ super(
+ Math::min,
+ Math::min,
+ BinaryOperator.minBy(Comparator.naturalOrder())
+ );
+ }
+
+ @Override
+ public String name()
+ {
+ return NAME;
+ }
+ }
+
+ abstract class ReduceFunc implements Function
+ {
+ private final DoubleBinaryOperator doubleReducer;
+ private final LongBinaryOperator longReducer;
+ private final BinaryOperator<String> stringReducer;
+
+ ReduceFunc(
+ DoubleBinaryOperator doubleReducer,
+ LongBinaryOperator longReducer,
+ BinaryOperator<String> stringReducer
+ )
+ {
+ this.doubleReducer = doubleReducer;
+ this.longReducer = longReducer;
+ this.stringReducer = stringReducer;
+ }
+
+ @Override
+ public void validateArguments(List<Expr> args)
+ {
+ // anything goes
+ }
+
+ @Override
+ public ExprEval apply(List<Expr> args, Expr.ObjectBinding bindings)
+ {
+ if (args.isEmpty()) {
+ return ExprEval.of(null);
+ }
+
+ ExprAnalysis exprAnalysis = analyzeExprs(args, bindings);
+ if (exprAnalysis == null) {
+ // The GREATEST/LEAST functions are not in the SQL standard, but most
(e.g., MySQL, Oracle) return NULL if any
+ // are NULL. Others (e.g., Postgres) only return NULL if all are NULL,
otherwise the NULLs are ignored.
Review comment:
Given it's not standard and we can do what we want, I would suggest going
with the Postgres behavior for these reasons:
- IMO the Postgres behavior is more likely to be useful: if I do
`GREATEST(x, y, z)` and `y` is null, I probably want that to be equivalent to
`GREATEST(x, z)`.
- Postgres behavior is used as a base for a wide variety of other databases
beyond the ones we're looking at here, so its behavior is influential.
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