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https://issues.apache.org/jira/browse/DRILL-5266?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15868627#comment-15868627
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Paul Rogers commented on DRILL-5266:
------------------------------------
The logic for determining field widths is confusing.
{code}
public int next() {
...
if (allFieldsFixedLength) {
...
} else { // variable length columns
long fixedRecordsToRead = varLengthReader.readFields(recordsToRead,
firstColumnStatus); // Read var
readAllFixedFields(fixedRecordsToRead); // Read fixed
}
{code}
The above claims that we call one method to read variable length fields, then
another to read fixed length fields. Fine, presumably we pack in the
variable-length fields, figure out how many records that is, then read the
fixed length data to match. Makes sense. But then:
{code}
public class VarLenBinaryReader {
public long readFields(long recordsToReadInThisPass, ColumnReader<?>
firstColumnStatus) throws IOException {
...
recordsReadInCurrentPass = determineSizesSerial(recordsToReadInThisPass);
...
}
private long determineSizesSerial(long recordsToReadInThisPass) throws
IOException {
...
// check that the next record will fit in the batch
if (exitLengthDeterminingLoop ||
(recordsReadInCurrentPass + 1) *
parentReader.getBitWidthAllFixedFields()
+ totalVariableLengthData + lengthVarFieldsInCurrentRecord >
parentReader.getBatchSize()) {
{code}
That is, the *variable* length reader is making its decision about when to stop
based, in part on *fixed* length fields. This is contradictory to the earlier
code, rendering the entire operational incoherent.
> Parquet Reader produces "low density" record batches
> ----------------------------------------------------
>
> Key: DRILL-5266
> URL: https://issues.apache.org/jira/browse/DRILL-5266
> Project: Apache Drill
> Issue Type: Bug
> Components: Storage - Parquet
> Affects Versions: 1.10
> Reporter: Paul Rogers
>
> Testing with the managed sort revealed that, for at least one file, Parquet
> produces "low-density" batches: batches in which only 5% of each value vector
> contains actual data, with the rest being unused space. When fed into the
> sort, we end up buffering 95% of wasted space, using only 5% of available
> memory to hold actual query data. The result is poor performance of the sort
> as it must spill far more frequently than expected.
> The managed sort analyzes incoming batches to prepare good memory use
> estimates. The following the the output from the Parquet file in question:
> {code}
> Actual batch schema & sizes {
> T1¦¦cs_sold_date_sk(std col. size: 4, actual col. size: 4, total size:
> 196608, vector size: 131072, data size: 4516, row capacity: 32768, density: 4)
> T1¦¦cs_sold_time_sk(std col. size: 4, actual col. size: 4, total size:
> 196608, vector size: 131072, data size: 4516, row capacity: 32768, density: 4)
> T1¦¦cs_ship_date_sk(std col. size: 4, actual col. size: 4, total size:
> 196608, vector size: 131072, data size: 4516, row capacity: 32768, density: 4)
> ...
> c_email_address(std col. size: 54, actual col. size: 27, total size: 53248,
> vector size: 49152, data size: 30327, row capacity: 4095, density: 62)
> Records: 1129, Total size: 32006144, Row width:28350, Density:5}
> {code}
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