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# Useful Tips
  This tutorial guides you to create CarbonData Tables and optimize performance.
  The following sections will elaborate on the below topics :

  * [Suggestions to create CarbonData Table](#suggestions-to-create-carbondata-table)
  * [Configuration for Optimizing Data Loading performance for Massive Data](#configuration-for-optimizing-data-loading-performance-for-massive-data)
  * [Optimizing Query Performance](#configurations-for-optimizing-carbondata-performance)
  * [Compaction Configurations for Optimizing CarbonData Query Performance](#compaction-configurations-for-optimizing-carbondata-query-performance)

## Suggestions to Create CarbonData Table

  For example, the results of the analysis for table creation with dimensions ranging from 10 thousand to 10 billion rows and 100 to 300 columns have been summarized below.
  The following table describes some of the columns from the table used.

  - **Table Column Description**

| Column Name | Data Type     | Cardinality | Attribution |
|-------------|---------------|-------------|-------------|
| msisdn      | String        | 30 million  | Dimension   |
| BEGIN_TIME  | BigInt        | 10 Thousand | Dimension   |
| HOST        | String        | 1 million   | Dimension   |
| Dime_1      | String        | 1 Thousand  | Dimension   |
| counter_1   | Decimal       | NA          | Measure     |
| counter_2   | Numeric(20,0) | NA          | Measure     |
| ...         | ...           | NA          | Measure     |
| counter_100 | Decimal       | NA          | Measure     |


  - **Put the frequently-used column filter in the beginning of SORT_COLUMNS**

  For example, MSISDN filter is used in most of the query then we must put the MSISDN as the first column in SORT_COLUMNS property.
  The create table command can be modified as suggested below :

  ```
  create table carbondata_table(
    msisdn String,
    BEGIN_TIME bigint,
    HOST String,
    Dime_1 String,
    counter_1 Decimal,
    ...
    
    )STORED AS carbondata
    TBLPROPERTIES ('SORT_COLUMNS'='msisdn, Dime_1')
  ```

  Now the query with MSISDN in the filter will be more efficient.

  - **Put the frequently-used columns in the order of low to high cardinality in SORT_COLUMNS**

  If the table in the specified query has multiple columns which are frequently used to filter the results, it is suggested to put
  the columns in the order of cardinality low to high in SORT_COLUMNS configuration. This ordering of frequently used columns improves the compression ratio and
  enhances the performance of queries with filter on these columns.

  For example, if MSISDN, HOST and Dime_1 are frequently-used columns, then the column order of table is suggested as
  Dime_1>HOST>MSISDN, because Dime_1 has the lowest cardinality.
  The create table command can be modified as suggested below :

  ```
  create table carbondata_table(
      msisdn String,
      BEGIN_TIME bigint,
      HOST String,
      Dime_1 String,
      counter_1 Decimal,
      ...
      
      )STORED AS carbondata
      TBLPROPERTIES ('SORT_COLUMNS'='Dime_1, HOST, MSISDN')
  ```

  - **For measure type columns with non high accuracy, replace Numeric(20,0) data type with Double data type**

  For columns of measure type, not requiring high accuracy, it is suggested to replace Numeric data type with Double to enhance query performance. 
  The create table command can be modified as below :

```
  create table carbondata_table(
    Dime_1 String,
    BEGIN_TIME bigint,
    END_TIME bigint,
    HOST String,
    MSISDN String,
    counter_1 decimal,
    counter_2 double,
    ...
    )STORED AS carbondata
    TBLPROPERTIES ('SORT_COLUMNS'='Dime_1, HOST, MSISDN')
```
  The result of performance analysis of test-case shows reduction in query execution time from 15 to 3 seconds, thereby improving performance by nearly 5 times.

 - **Columns of incremental character should be re-arranged at the end of dimensions**

  Consider the following scenario where data is loaded each day and the begin_time is incremental for each load, it is suggested to put begin_time at the end of dimensions.
  Incremental values are efficient in using min/max index. The create table command can be modified as below :

  ```
  create table carbondata_table(
    Dime_1 String,
    HOST String,
    MSISDN String,
    counter_1 double,
    counter_2 double,
    BEGIN_TIME bigint,
    END_TIME bigint,
    ...
    counter_100 double
    )STORED AS carbondata
    TBLPROPERTIES ('SORT_COLUMNS'='Dime_1, HOST, MSISDN')
  ```

  **NOTE:**
  + BloomFilter can be created to enhance performance for queries with precise equal/in conditions. You can find more information about it in BloomFilter index [document](./index/bloomfilter-index-guide.md).
  + Lucene index can be created on string columns which has content of more length to enhance the query performance. You can find more information about it in Lucene index [document](./index/lucene-index-guide.md).
  + Secondary index can be created based on the column position in main table(Recommended for right columns) and the queries should have filter on that column to improve the filter query performance. You can find more information about it in secondary index [document](./index/secondary-index-guide.md).
  + Materialized view can be created to improve query performance provided the storage requirements and loading time is acceptable. You can find more information about it in materialized view [document](./mv-guide.md).


## Configuration for Optimizing Data Loading performance for Massive Data


  CarbonData supports large data load, in this process sorting data while loading consumes a lot of memory and disk IO and
  this can result sometimes in "Out Of Memory" exception.
  If you do not have much memory to use, then you may prefer to slow the speed of data loading instead of data load failure.
  You can configure CarbonData by tuning following properties in carbon.properties file to get a better performance.

| Parameter | Default Value | Description/Tuning |
|-----------|-------------|--------|
|carbon.number.of.cores.while.loading|Default: 2. This value should be >= 2|Specifies the number of cores used for data processing during data loading in CarbonData. |
|carbon.sort.size|Default: 100000. The value should be >= 1000.|Threshold to write local file in sort step when loading data|
|carbon.sort.file.write.buffer.size|Default:  16384. The value should be >= 10240 and <= 10485760.|CarbonData sorts and writes data to intermediate files to limit the memory usage. This configuration determines the buffer size to be used for reading and writing such files. |
|carbon.merge.sort.reader.thread|Default: 3 |Specifies the number of cores used for temp file merging during data loading in CarbonData.|
|carbon.merge.sort.prefetch|Default: true | You may want set this value to false if you have not enough memory|

  For example, if there are 10 million records, and I have only 16 cores, 64 GB memory will be loaded to CarbonData table.
  Using the default configuration  always fail in sort step. Modify carbon.properties as suggested below:

  ```
  carbon.merge.sort.reader.thread=1
  carbon.sort.size=5000
  carbon.sort.file.write.buffer.size=5000
  carbon.merge.sort.prefetch=false
  ```

## Configurations for Optimizing CarbonData Performance

  Recently we did some performance POC on CarbonData for Finance and telecommunication Field. It involved detailed queries and aggregation
  scenarios. After the completion of POC, some of the configurations impacting the performance have been identified and tabulated below :

| Parameter | Location | Used For  | Description | Tuning |
|----------------------------------------------|-----------------------------------|---------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| |
| carbon.sort.intermediate.files.limit | spark/carbonlib/carbon.properties | Data loading | During the loading of data, local temp is used to sort the data. This number specifies the minimum number of intermediate files after which the  merge sort has to be initiated. | Increasing the parameter to a higher value will improve the load performance. For example, when we increase the value from 20 to 100, it increases the data load performance from 35MB/S to more than 50MB/S. Higher values of this parameter consumes  more memory during the load. |
| carbon.number.of.cores.while.loading | spark/carbonlib/carbon.properties | Data loading | Specifies the number of cores used for data processing during data loading in CarbonData. | If you have more number of CPUs, then you can increase the number of CPUs, which will increase the performance. For example if we increase the value from 2 to 4 then the CSV reading performance can increase about 1 times |
| carbon.compaction.level.threshold | spark/carbonlib/carbon.properties | Data loading and Querying | For minor compaction, specifies the number of segments to be merged in stage 1 and number of compacted segments to be merged in stage 2. | Each CarbonData load will create one segment, if every load is small in size it will generate many small files over a period of time impacting the query performance. Configuring this parameter will merge the small segment to one big segment which will sort the data and improve the performance. For Example in one telecommunication scenario, the performance improves about 2 times after minor compaction. |
| spark.sql.shuffle.partitions | spark/conf/spark-defaults.conf | Querying | The number of task started when spark shuffle. | The value can be 1 to 2 times as much as the executor cores. In an aggregation scenario, reducing the number from 200 to 32 reduced the query time from 17 to 9 seconds. |
| spark.executor.instances/spark.executor.cores/spark.executor.memory | spark/conf/spark-defaults.conf | Querying | The number of executors, CPU cores, and memory used for CarbonData query. | In the bank scenario, we provide the 4 CPUs cores and 15 GB for each executor which can get good performance. This 2 value does not mean more the better. It needs to be configured properly in case of limited resources. For example, In the bank scenario, it has enough CPU 32 cores each node but less memory 64 GB each node. So we cannot give more CPU but less memory. For example, when 4 cores and 12GB for each executor. It sometimes happens GC during the query which impact the query performance very much from the 3 second to more than 15 seconds. In this scenario need to increase the memory or decrease the CPU cores. |
| carbon.detail.batch.size | spark/carbonlib/carbon.properties | Querying | The buffer size to store records, returned from the block scan. | In limit scenario this parameter is very important. For example your query limit is 1000. But if we set this value to 3000 that means we get 3000 records from scan but spark will only take 1000 rows. So the 2000 remaining are useless. In one Finance test case after we set it to 100, in the limit 1000 scenario the performance increase about 2 times in comparison to if we set this value to 12000. |
| carbon.use.local.dir | spark/carbonlib/carbon.properties | Data loading | Whether use YARN local directories for multi-table load disk load balance | If this is set it to true CarbonData will use YARN local directories for multi-table load disk load balance, that will improve the data load performance. |
| carbon.sort.temp.compressor | spark/carbonlib/carbon.properties | Data loading | Specify the name of compressor to compress the intermediate sort temporary files during sort procedure in data loading. | The optional values are 'SNAPPY','GZIP','BZIP2','LZ4','ZSTD', and empty. Specially, empty means that Carbondata will not compress the sort temp files. This parameter will be useful if you encounter disk bottleneck. |
| carbon.load.skewedDataOptimization.enabled | spark/carbonlib/carbon.properties | Data loading | Whether to enable size based block allocation strategy for data loading. | When loading, carbondata will use file size based block allocation strategy for task distribution. It will make sure that all the executors process the same size of data -- It's useful if the size of your input data files varies widely, say 1MB to 1GB. |

  Note: If your CarbonData instance is provided only for query, you may specify the property 'spark.speculation=true' which is in conf directory of spark.

## Compaction Configurations for Optimizing CarbonData Query Performance

CarbonData provides many configurations to tune the compaction behavior so that query peformance is improved.



Based on the number of cores available in the node, it is recommended to tune the configuration 	***carbon.number.of.cores.while.compacting*** appropriately.Configuring a higher value will improve the overall compaction performance.

<p>&nbsp;</p>
<table style="width: 777px;">
<tbody>
<tr style="height: 23px;">
<td style="height: 23px; width: 95.375px;">No</td>
<td style="height: 23px; width: 299.625px;">&nbsp;Data Loading frequency</td>
<td style="height: 23px; width: 144px;">Data Size of each load</td>
<td style="height: 23px; width: 204px;">Minor Compaction configuration</td>
<td style="height: 23px; width: 197px;">&nbsp;Major compaction configuration</td>
</tr>
<tr style="height: 29.5px;">
<td style="height: 29.5px; width: 95.375px;">1</td>
<td style="height: 29.5px; width: 299.625px;">&nbsp;Batch(Once is several Hours)</td>
<td style="height: 29.5px; width: 144px;">Big</td>
<td style="height: 29.5px; width: 204px;">&nbsp;Not Suggested</td>
<td style="height: 29.5px; width: 197px;">Configure Major Compaction size of 3-4 load size.Perform Major compaction once in a day</td>
</tr>
<tr style="height: 23px;">
<td style="height: 23px; width: 95.375px;" rowspan="2">2</td>
<td style="height: 23px; width: 299.625px;" rowspan="2">&nbsp;Batch(Once in few minutes)&nbsp;</td>
<td style="height: 23px; width: 144px;">Big&nbsp;</td>
<td style="height: 23px; width: 204px;">
<p>&nbsp;Minor compaction (2,2).</p>
<p>Enable Auto compaction, if high rate data loading speed is not required or the time between loads is sufficient to run the compaction</p>
</td>
<td style="height: 23px; width: 197px;">Major compaction size of 10 load size.Perform Major compaction once in a day</td>
</tr>
<tr style="height: 23px;">
<td style="height: 23px; width: 144px;">Small</td>
<td style="height: 23px; width: 204px;">
<p>Minor compaction (6,6).</p>
<p>Enable Auto compaction, if high rate data loading speed is not required or the time between loads is sufficient to run the compaction</p>
</td>
<td style="height: 23px; width: 197px;">Major compaction size of 10 load size.Perform Major compaction once in a day</td>
</tr>
<tr style="height: 23px;">
<td style="height: 23px; width: 95.375px;">3</td>
<td style="height: 23px; width: 299.625px;">&nbsp;History data loaded as single load,incremental loads matches&nbsp;(1) or (2)</td>
<td style="height: 23px; width: 144px;">Big</td>
<td style="height: 23px; width: 204px;">
<p>&nbsp;Configure ALLOWED_COMPACTION_DAYS to exclude the History load.</p>
<p>Configure Minor compaction configuration based&nbsp;condition (1) or (2)</p>
</td>
<td style="height: 23px; width: 197px;">&nbsp;Configure Major compaction size smaller than the history load size.</td>
</tr>
<tr style="height: 23px;">
<td style="height: 23px; width: 95.375px;">4</td>
<td style="height: 23px; width: 299.625px;">&nbsp;There can be error in recent data loaded.Need reload sometimes</td>
<td style="height: 23px; width: 144px;">&nbsp;(1) or (2)</td>
<td style="height: 23px; width: 204px;">
<p>&nbsp;Configure COMPACTION_PRESERVE_SEGMENTS</p>
<p>to exclude the recent few segments from compacting.</p>
<p>Configure Minor compaction configuration based&nbsp;condition (1) or (2)</p>
</td>
<td style="height: 23px; width: 197px;">Same as (1) or (2)&nbsp;</td>
</tr>
</tbody>
</table>

