[jira] [Created] (KUDU-2731) Getting column schema information from KuduSchema requires copying a KuduColumnSchema object

["Will Berkeley (JIRA)" <ji...@apache.org>]

Will Berkeley created KUDU-2731:
-----------------------------------

             Summary: Getting column schema information from KuduSchema requires copying a KuduColumnSchema object
                 Key: KUDU-2731
                 URL: https://issues.apache.org/jira/browse/KUDU-2731
             Project: Kudu
          Issue Type: Improvement
    Affects Versions: 1.9.0
            Reporter: Will Berkeley


I'm looking at a CPU profile of Impala inserting into Kudu. {{KuduTableSink::Send}} has code that schematically does the following:

{noformat}
for each row in the batch
  for each column
    if (schema.Column(col_idx).isNullable()) {
      write->mutable_row()->SetNull(col);
    }
  }
}
{noformat}

See [kudu-table-sink.cc|https://github.com/apache/impala/blob/branch-3.1.0/be/src/exec/kudu-table-sink.cc#L236]. However, {{KuduSchema::Column}} copies the column schema and returns it by value, so the if statement constructs and destroys a column schema object just to check if the column is nullable.

This is by far the biggest user of CPU in the Impala process (35% or so). The workload might be I/O bound writing to Kudu anyway, though. Nevertheless, we should provide a way to avoid this copying in the API, either by adding a method like

{noformat}
class KuduSchema {
  const KuduColumnSchema& get_column(int idx);
}
{noformat}

or a method like

{noformat}
class KuduSchema {
  bool is_column_nullable(int idx);
}
{noformat}

The former is the most flexible while the latter frees the client from worrying about holding the ref longer than the KuduColumnSchema object lives. We might need to add a number of methods similar to the latter method to cover other potentially useful things like checking encoding, type, etc.



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