CarbonData Lucene DataMap (Alpha feature in 1.4.0)

DataMap Management

Lucene DataMap can be created using following DDL

CREATE DATAMAP [IF NOT EXISTS] datamap_name
ON TABLE main_table
USING 'lucene'
DMPROPERTIES ('index_columns'='city, name', ...)

DataMap can be dropped using following DDL:

DROP DATAMAP [IF EXISTS] datamap_name
ON TABLE main_table

To show all DataMaps created, use:

SHOW DATAMAP 
ON TABLE main_table

It will show all DataMaps created on main table.

Lucene DataMap Introduction

Lucene is a high performance, full featured text search engine. Lucene is integrated to carbon as an index datamap and managed along with main tables by CarbonData.User can create lucene datamap to improve query performance on string columns which has content of more length. So, user can search tokenized word or pattern of it using lucene query on text content.

For instance, main table called datamap_test which is defined as:

CREATE TABLE datamap_test (
  name string,
  age int,
  city string,
  country string)
STORED BY 'carbondata'

User can create Lucene datamap using the Create DataMap DDL:

CREATE DATAMAP dm
ON TABLE datamap_test
USING 'lucene'
DMPROPERTIES ('INDEX_COLUMNS' = 'name, country',)

DMProperties

  1. INDEX_COLUMNS: The list of string columns on which lucene creates indexes.
  2. FLUSH_CACHE: size of the cache to maintain in Lucene writer, if specified then it tries to aggregate the unique data till the cache limit and flush to Lucene. It is best suitable for low cardinality dimensions.
  3. SPLIT_BLOCKLET: when made as true then store the data in blocklet wise in lucene , it means new folder will be created for each blocklet, thus, it eliminates storing blockletid in lucene and also it makes lucene small chunks of data.

Loading data

When loading data to main table, lucene index files will be generated for all the index_columns(String Columns) given in DMProperties which contains information about the data location of index_columns. These index files will be written inside a folder named with datamap name inside each segment folders.

A system level configuration carbon.lucene.compression.mode can be added for best compression of lucene index files. The default value is speed, where the index writing speed will be more. If the value is compression, the index file size will be compressed.

Querying data

As a technique for query acceleration, Lucene indexes cannot be queried directly. Queries are to be made on main table. when a query with TEXT_MATCH('name:c10') or TEXT_MATCH_WITH_LIMIT('name:n10',10)[the second parameter represents the number of result to be returned, if user does not specify this value, all results will be returned without any limit] is fired, two jobs are fired.The first job writes the temporary files in folder created at table level which contains lucene's seach results and these files will be read in second job to give faster results. These temporary files will be cleared once the query finishes.

User can verify whether a query can leverage Lucene datamap or not by executing EXPLAIN command, which will show the transformed logical plan, and thus user can check whether TEXT_MATCH() filter is applied on query or not.

Note:

  1. The filter columns in TEXT_MATCH or TEXT_MATCH_WITH_LIMIT must be always in lower case and filter condition like 'AND','OR' must be in upper case.

    Ex:

    select * from datamap_test where TEXT_MATCH('name:*10 AND name:*n*')
    
  2. Query supports only one TEXT_MATCH udf for filter condition and not multiple udfs.

    The following query is supported:

    select * from datamap_test where TEXT_MATCH('name:*10 AND name:*n*')
    

    The following query is not supported:

    select * from datamap_test where TEXT_MATCH('name:*10) AND TEXT_MATCH(name:*n*')
    

Below like queries can be converted to text_match queries as following:

select * from datamap_test where name='n10'

select * from datamap_test where name like 'n1%'

select * from datamap_test where name like '%10'

select * from datamap_test where name like '%n%'

select * from datamap_test where name like '%10' and name not like '%n%'

Lucene TEXT_MATCH Queries:

select * from datamap_test where TEXT_MATCH('name:n10')

select * from datamap_test where TEXT_MATCH('name:n1*')

select * from datamap_test where TEXT_MATCH('name:*10')

select * from datamap_test where TEXT_MATCH('name:*n*')

select * from datamap_test where TEXT_MATCH('name:*10 -name:*n*')

Note: For lucene queries and syntax, refer to lucene-syntax

Data Management with lucene datamap

Once there is lucene datamap is created on the main table, following command on the main table is not supported:

  1. Data management command: UPDATE/DELETE.
  2. Schema management command: ALTER TABLE DROP COLUMN, ALTER TABLE CHANGE DATATYPE, ALTER TABLE RENAME.

Note: Adding a new column is supported, and for dropping columns and change datatype command, CarbonData will check whether it will impact the lucene datamap, if not, the operation is allowed, otherwise operation will be rejected by throwing exception.

  1. Partition management command: ALTER TABLE ADD/DROP PARTITION.

However, there is still way to support these operations on main table, in current CarbonData release, user can do as following:

  1. Remove the lucene datamap by DROP DATAMAP command.
  2. Carry out the data management operation on main table.
  3. Create the lucene datamap again by CREATE DATAMAP command. Basically, user can manually trigger the operation by re-building the datamap.