Github user arina-ielchiieva commented on a diff in the pull request:
https://github.com/apache/drill/pull/819#discussion_r113912681
--- 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 --
For limit 0 queries I can determine if we know precision or not. But for
regular queries, it's not always possible.
Example: `select substring(cast(id as int), 1, 5) from t`
Before length calculation, Drill implicitly casts int to varchar using max
varchar length [1].
Thus when it comes to substring length calculation, I receive varchar with
max varchar precision and I don't if this is real precision or just the one
used in implicit cast. I have treated unknown precision as max varchar length
to sync return type output between limit 0 and regular queries.
With this change we might not have good control of memory allocation for
varchar column, so I have excluded substring / substr / left / right functions
length calculation from this PR. Jira DRILL-5476 is created to address length
calculation logic for these functions later.
[1]
https://github.com/apache/drill/blob/master/exec/java-exec/src/main/java/org/apache/drill/exec/expr/ExpressionTreeMaterializer.java#L219
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