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_r391094852
##########
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.
+ return ExprEval.of(null);
+ }
+
+ Stream<ExprEval<?>> exprEvalStream = exprAnalysis.exprEvals.stream();
+ switch (exprAnalysis.comparisonType) {
+ case DOUBLE:
+ //noinspection OptionalGetWithoutIsPresent (empty list handled
earlier)
+ return
ExprEval.of(exprEvalStream.mapToDouble(ExprEval::asDouble).reduce(doubleReducer).getAsDouble());
+ case LONG:
+ //noinspection OptionalGetWithoutIsPresent (empty list handled
earlier)
+ return
ExprEval.of(exprEvalStream.mapToLong(ExprEval::asLong).reduce(longReducer).getAsLong());
+ default:
+ //noinspection OptionalGetWithoutIsPresent (empty list handled
earlier)
+ return
ExprEval.of(exprEvalStream.map(ExprEval::asString).reduce(stringReducer).get());
+ }
+ }
+
+ @Nullable
+ private ExprAnalysis analyzeExprs(List<Expr> exprs, Expr.ObjectBinding
bindings)
Review comment:
Even though this method is private, javadocs would be nice, especially for
explaining when `null` would be returned. The method is long enough that it is
not obvious what it does without studying it.
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