java获取列族的列,我们可以从HBase表中获取所有列名吗？

红太狼 2022-08-29 15:45 307阅读 0赞

![Image 1][]

Setup:

I have an HBase table, with 100M+ rows and 1 Million+ columns. Every row has data for only 2 to 5 columns. There is in just 1 Column Family.

Problem:

I want to find out all the distinct qualifiers (columns) in this column family. Is there a quick way to do that?

I can think of about scanning the whole table, then getting familyMap for each row, get qualifier and add it to a Set<>. But that would be awfully slow, as there are 100M+ rows.

Can we do any better?

解决方案

You can use a mapreduce for this. In this case you don't need to install a custom libs for hbase as in case for coprocessor.

Below a code for creating a mapreduce task.

Job setup

Job job = Job.getInstance(config);

job.setJobName("Distinct columns");

Scan scan = new Scan();

scan.setBatch(500);

scan.addFamily(YOU\_COLUMN\_FAMILY\_NAME);

scan.setFilter(new KeyOnlyFilter()); //scan only key part of KeyValue (raw, column family, column)

scan.setCacheBlocks(false); // don't set to true for MR jobs

TableMapReduceUtil.initTableMapperJob(

YOU\_TABLE\_NAME,

scan,

OnlyColumnNameMapper.class, // mapper

Text.class, // mapper output key

Text.class, // mapper output value

job);

job.setNumReduceTasks(1);

job.setReducerClass(OnlyColumnNameReducer.class);

Mapper

public class OnlyColumnNameMapper extends TableMapper \{

@Override

protected void map(ImmutableBytesWritable key, Result value, final Context context) throws IOException, InterruptedException \{

CellScanner cellScanner = value.cellScanner();

while (cellScanner.advance()) \{

Cell cell = cellScanner.current();

byte\[\] q = Bytes.copy(cell.getQualifierArray(),

cell.getQualifierOffset(),

cell.getQualifierLength());

context.write(new Text(q),new Text());

Reducer

public class OnlyColumnNameReducer extends Reducer \{

@Override

protected void reduce(Text key, Iterable values, Context context) throws IOException, InterruptedException \{

context.write(new Text(key), new Text());

[Image 1]:

java获取列族的列,我们可以从HBase表中获取所有列名吗？

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