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From arina-ielchiieva <...@git.apache.org>
Subject [GitHub] drill pull request #819: DRILL-5419: Calculate return string length for lite...
Date Fri, 05 May 2017 12:30:53 GMT
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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