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https://issues.apache.org/jira/browse/DRILL-5550?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16165009#comment-16165009
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ASF GitHub Bot commented on DRILL-5550:
---------------------------------------
Github user paul-rogers commented on a diff in the pull request:
https://github.com/apache/drill/pull/939#discussion_r138685942
--- Diff:
exec/java-exec/src/main/java/org/apache/drill/exec/store/easy/text/compliant/FieldVarCharOutput.java
---
@@ -45,11 +45,11 @@
static final String COL_NAME = "columns";
// array of output vector
- private final VarCharVector [] vectors;
+ private final NullableVarCharVector [] vectors;
--- End diff --
Let's think about this. CSV is defined as a fixed set of columns. For this
reason, the original code used non-nullable values for each column.
The question would be, what about this fix means that defined columns
should now become null? Are there other solutions to this bug?
Changing the column from required to nullable can have impact on the user
experience and on the client: where code once expected required columns, now
that code must change to handle nullable types.
> SELECT non-existent column produces empty required VARCHAR
> ----------------------------------------------------------
>
> Key: DRILL-5550
> URL: https://issues.apache.org/jira/browse/DRILL-5550
> Project: Apache Drill
> Issue Type: Bug
> Components: Storage - Text & CSV
> Affects Versions: 1.10.0
> Reporter: Paul Rogers
> Priority: Minor
>
> Drill's CSV column reader supports two forms of files:
> * Files with column headers as the first line of the file.
> * Files without column headers.
> The CSV storage plugin specifies which format to use for files accessed via
> that storage plugin config.
> Suppose we have a CSV file with headers:
> {code}
> a,b,c
> 10,foo,bar
> {code}
> Suppose we configure a storage plugin to use headers:
> {code}
> TextFormatConfig csvFormat = new TextFormatConfig();
> csvFormat.fieldDelimiter = ',';
> csvFormat.skipFirstLine = false;
> csvFormat.extractHeader = true;
> {code}
> (The above can also be done using JSON when running Drill as a server.)
> Execute the following query:
> {code}
> SELECT a, c, d FROM `dfs.data.example.csv`
> {code}
> Results:
> {code}
> a,c,d
> 10,bar,
> {code}
> The actual type of column {{d}} is non-nullable VARCHAR.
> This is inconsistent with other parts of Drill in two ways, one may be a bug.
> Most other parts of Drill use a nullable INT for "missing" columns.
> 1. For CSV it makes sense for the data type to be VARCHAR, since all CSV
> columns are of that type.
> 2. It may *not* make sense for the column to be non-nullable and blank rather
> than nullable and NULL. In SQL, NULL means that the data is unknown, which is
> the case here.
> In the future, we may want to use some other indication for a missing column.
> Until then, the requested change is to make the type of a missing CSV column
> a nullable VARCHAR set to value NULL.
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