Github user jinfengni commented on a diff in the pull request:
https://github.com/apache/drill/pull/819#discussion_r113795987
--- Diff:
exec/java-exec/src/main/java/org/apache/drill/exec/expr/fn/DrillSubstringFuncHolder.java
---
@@ -0,0 +1,94 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to you under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.drill.exec.expr.fn;
+
+import org.apache.drill.common.expression.LogicalExpression;
+import org.apache.drill.common.expression.ValueExpressions;
+import org.apache.drill.common.types.TypeProtos;
+import org.apache.drill.common.types.Types;
+import org.apache.drill.exec.expr.fn.impl.StringFunctionHelpers;
+
+import java.util.List;
+
+/**
+ * Function holder for functions with function scope set as
+ * {@link
org.apache.drill.exec.expr.annotations.FunctionTemplate.FunctionScope#SUBSTRING}.
+ */
+public class DrillSubstringFuncHolder extends DrillSimpleFuncHolder {
+
+ public DrillSubstringFuncHolder(FunctionAttributes functionAttributes,
FunctionInitializer initializer) {
+ super(functionAttributes, initializer);
+ }
+
+ /**
+ * Defines function return type and calculates output precision.
+ *
+ * <b>substring(source, regexp)<b/>
+ * <ul/>If input precision is known, output precision is max varchar
value {@link Types#MAX_VARCHAR_LENGTH}.<ul/>
+ *
+ * <b>substring(source, offset)<b/>
+ * <ul>If input precision is unknown then output precision is max
varchar value {@link Types#MAX_VARCHAR_LENGTH}.<ul/>
+ * <ul>If input precision is known, output precision is input precision
minus offset plus 1
+ * since offset starts from 1.<ul/>
+ * <ul>If offset value is greater than input precision or offset value
is corrupted (less then equals zero),
+ * output precision is zero.<ul/>
+ *
+ * <b>substring(source, offset, length)<b/>
+ * <ul>If offset value is greater than input precision or offset or
length values are corrupted (less then equals zero),
+ * output precision is zero.<ul/>
+ * <ul>If source length (including offset) is less than substring
length, output precision is source length (including offset).<ul/>
+ * <ul>If source length (including offset) is greater than substring
length, output precision is substring length.<ul/>
+ *
+ * @param logicalExpressions logical expressions
+ * @return return type
+ */
+ @Override
+ public TypeProtos.MajorType getReturnType(List<LogicalExpression>
logicalExpressions) {
+ TypeProtos.MajorType.Builder builder =
TypeProtos.MajorType.newBuilder()
+ .setMinorType(getReturnType().getMinorType())
+ .setMode(getReturnTypeDataMode(logicalExpressions));
+
+ int sourceLength =
logicalExpressions.get(0).getMajorType().hasPrecision() ?
+ logicalExpressions.get(0).getMajorType().getPrecision() :
Types.MAX_VARCHAR_LENGTH;
--- End diff --
Seems there are two cases, for precision == MAX_VARCHAR_LENGTH:
1) precision is not set for a varchar column. Hence we may return
MAX_VARCHAR_LENGTH, if it's asked for the maximum length of any value in the
column.
2) precision is explicitly set to MAX_VARCHAR_LENGTH, because of certain
calculation.
Do we have a way to differentiate the above two cases? The code here seems
to set "sourceLength = MAX_VARCHAR_LENGTH" if source does not have precision,
then sourceLength could be set to the return type. In other words, from a
varchar not knowning it's length ==> a varchar with MAX_VARCHAR_LENGTH.
I think it might make sense to differentiate if we want to have better
control of memory allocation for varchar column in the future: we may use
different way to estimate the memory size if we do not have any idea about
varchar lenght, from the case where we know the varchar's max length is
MAX_VARCHAR_LENGTH.
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