impala/docs/topics/impala_tablesample.xml

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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="tablesample" rev="IMPALA-5309">

  <title>TABLESAMPLE Clause</title>
  <prolog>
    <metadata>
      <data name="Category" value="Impala"/>
      <data name="Category" value="SQL"/>
      <data name="Category" value="Querying"/>
      <data name="Category" value="Developers"/>
      <data name="Category" value="Data Analysts"/>
    </metadata>
  </prolog>

  <conbody>

    <p>
      Specify the <codeph>TABLESAMPLE</codeph> clause in cases where you need
      to explore the data distribution within the table, the table is very large,
      and it is impractical or unnecessary to process all the data from the table
      or selected partitions.
    </p>

    <p>
      The clause makes the query process a randomized set of data files from the
      table, so that the total volume of data is greater than or equal to the specified
      percentage of data bytes within that table. (Or the data bytes within the set of
      partitions that remain after partition pruning is performed.)
    </p>

    <p conref="../shared/impala_common.xml#common/syntax_blurb"/>

<codeblock>
  <ph rev="IMPALA-5309">TABLESAMPLE SYSTEM(<varname>percentage</varname>) [REPEATABLE(<varname>seed</varname>)]</ph>
</codeblock>

    <p>
      The <codeph>TABLESAMPLE</codeph> clause comes immediately after a table name or table alias.
    </p>

    <p>
      The <codeph>SYSTEM</codeph> keyword represents the sampling method. Currently,
      Impala only supports a single sampling method named <codeph>SYSTEM</codeph>.
    </p>

    <p>
      The <varname>percentage</varname> argument is an integer literal from 0 to 100.
      A percentage of 0 produces an empty result set for a particular table reference,
      while a percentage of 100 uses the entire contents. Because the sampling works by
      selecting a random set of data files, the proportion of sampled data from the
      table may be greater than the specified percentage, based on the number and sizes
      of the underlying data files. See the usage notes for details.
    </p>

    <p>
      The optional <codeph>REPEATABLE</codeph> keyword lets you specify an arbitrary
      positive integer seed value that ensures that when the query is run again, the
      sampling selects the same set of data files each time. <codeph>REPEATABLE</codeph>
      does not have a default value. If you omit the <codeph>REPEATABLE</codeph> keyword,
      the random seed is derived from the current time.
    </p>

    <p conref="../shared/impala_common.xml#common/added_in_290"/>

    <p rev="2.12.0 IMPALA-5310">
      See <keyword keyref="compute_stats"/> for the
        <codeph>TABLESAMPLE</codeph> clause used in the <codeph>COMPUTE
        STATS</codeph> statement.
    </p>

    <p conref="../shared/impala_common.xml#common/usage_notes_blurb"/>

    <p>
      You might use this clause with aggregation queries, such as finding
      the approximate average, minimum, or maximum where exact precision
      is not required. You can use these findings to plan the most effective
      strategy for constructing queries against the full table or designing
      a partitioning strategy for the data.
    </p>

    <p>
      Some other database systems have a <codeph>TABLESAMPLE</codeph> clause.
      The Impala syntax for this clause is modeled on the syntax for popular
      relational databases, not the Hive <codeph>TABLESAMPLE</codeph> clause.
      For example, there is no <codeph>BUCKETS</codeph> keyword as in HiveQL.
    </p>

    <p>
      The precision of the <varname>percentage</varname> threshold depends on
      the number and sizes of the underlying data files. Impala brings in
      additional data files, one at a time, until the number of bytes exceeds
      the specified percentage based on the total number of bytes for the
      entire set of table data. The precision of the percentage threshold is higher
      when the table contains many data files with consistent sizes. See the
      code listings later in this section for examples.
    </p>

    <p>
      When you estimate characteristics of the data distribution based on sampling
      a percentage of the table data, be aware that the data might be unevenly distributed
      between different files. Do not assume that the percentage figure reflects the
      percentage of rows in the table. For example, one file might contain all blank values
      for a <codeph>STRING</codeph> column, while another file contains long strings
      in that column; therefore, one file could contain many more rows than another.
      Likewise, a table created with the <codeph>SORT BY</codeph> clause might
      contain narrow ranges of values for the sort columns, making it impractical to
      extrapolate the number of distinct values for those columns based on sampling
      only some of the data files.
    </p>

    <p>
      Because a sample of the table data might not contain all values for a particular
      column, if the <codeph>TABLESAMPLE</codeph> is used in a join query, the
      key relationships between the tables might produce incomplete result sets
      compared to joins using all the table data. For example, if you join 50%
      of table A with 50% of table B, some values in the join columns might
      not match between the two tables, even though overall there is a 1:1
      relationship between the tables.
    </p>

    <p>
      The <codeph>REPEATABLE</codeph> keyword makes identical queries use a
      consistent set of data files when the query is repeated. You specify an
      arbitrary integer key that acts as a seed value when Impala randomly
      selects the set of data files to use in the query. This technique
      lets you verify correctness, examine performance, and so on for queries
      using the <codeph>TABLESAMPLE</codeph> clause without the sampled data
      being different each time. The repeatable aspect is reset (that is, the
      set of selected data files may change) any time the contents of the table
      change. The statements or operations that can make sampling results
      non-repeatable are:
    </p>

    <ul>
      <li>
        <codeph>INSERT</codeph>.
      </li>
      <li>
        <codeph>TRUNCATE TABLE</codeph>.
      </li>
      <li>
        <codeph>LOAD DATA</codeph>.
      </li>
      <li>
        <codeph>REFRESH</codeph> or <codeph>INVALIDATE METADATA</codeph>
        after files are added or removed by a non-Impala mechanism.
      </li>
      <li>
      </li>
    </ul>

    <p>
      This clause is similar in some ways to the <codeph>LIMIT</codeph> clause,
      because both serve to limit the size of the intermediate data and final
      result set. <codeph>LIMIT 0</codeph> is more efficient than
      <codeph>TABLESAMPLE SYSTEM(0)</codeph> for verifying that a query can execute
      without producing any results. <codeph>TABLESAMPLE SYSTEM(<varname>n</varname>)</codeph>
      often makes query processing more efficient than using a <codeph>LIMIT</codeph> clause
      by itself, because all phases of query execution use less data overall.
      If the intent is to retrieve some representative values from the table
      in an efficient way, you might combine <codeph>TABLESAMPLE</codeph>,
      <codeph>ORDER BY</codeph>, and <codeph>LIMIT</codeph> clauses within a single query.
    </p>

    <p conref="../shared/impala_common.xml#common/partitioning_blurb"/>
    <p>
      When you query a partitioned table, any partition pruning happens
      before Impala selects the data files to sample. For example, in a
      table partitioned by year, a query with <codeph>WHERE year = 2017</codeph>
      and a <codeph>TABLESAMPLE SYSTEM(10)</codeph> clause would sample
      data files representing at least 10% of the bytes present in the
      2017 partition.
    </p>

    <p conref="../shared/impala_common.xml#common/s3_blurb"/>
    <p>
      This clause applies to S3 tables the same way as tables
      with data files stored on HDFS.
    </p>

    <p conref="../shared/impala_common.xml#common/adls_blurb"/>
    <p>
      This clause applies to ADLS tables the same way as tables
      with data files stored on HDFS.
    </p>

    <p conref="../shared/impala_common.xml#common/kudu_blurb"/>
    <p>
      This clause does not apply to Kudu tables.
    </p>

    <p conref="../shared/impala_common.xml#common/hbase_blurb"/>
    <p>
      This clause does not apply to HBase tables.
    </p>

    <p conref="../shared/impala_common.xml#common/performance_blurb"/>
    <p>
      From a performance perspective, the <codeph>TABLESAMPLE</codeph>
      clause is especially valuable for exploratory queries on
      text, Avro, or other file formats other than Parquet. Text-based
      or row-oriented file formats must process substantial amounts of
      redundant data for queries that derive aggregate results such as
      <codeph>MAX()</codeph>, <codeph>MIN()</codeph>, or <codeph>AVG()</codeph>
      for a single column. Therefore, you might use <codeph>TABLESAMPLE</codeph>
      early in the ETL pipeline, when data is still in raw text format
      and has not been converted to Parquet or moved into a partitioned
      table.
    </p>

    <p conref="../shared/impala_common.xml#common/restrictions_blurb"/>

    <p>
      This clause applies only to tables that use a storage layer
      with underlying raw data files, such as HDFS, Amazon S3,
      or Microsoft ADLS.
    </p>

    <p>
      This clause does not apply to table references that represent views.
      A query that applies the <codeph>TABLESAMPLE</codeph> clause to a
      view or a subquery fails with a semantic error.
    </p>

    <p>
      Because the sampling works at the level of entire data files, it
      is by nature coarse-grained. It is possible to specify a small
      sample percentage but still process a substantial portion of the
      table data if the table contains relatively few data files, if
      each data file is very large, or if the data files vary substantially
      in size. Be sure that you understand the data distribution and physical
      file layout so that you can verify if the results are suitable for
      extrapolation. For example, if the table contains only a single data file,
      the <q>sample</q> will consist of all the table data regardless of
      the percentage you specify. If the table contains data files of
      1 GiB, 1 GiB, and 1 KiB, when you specify a sampling percentage of
      50 you would either process slightly more than 50% of the table
      (1 GiB + 1 KiB) or almost the entire table (1 GiB + 1 GiB),
      depending on which data files were selected for sampling.
    </p>

    <p>
      If data files are added by a non-Impala mechanism, and the
      table metadata is not updated by a <codeph>REFRESH</codeph>
      or <codeph>INVALIDATE METADATA</codeph> statement, the
      <codeph>TABLESAMPLE</codeph> clause does not consider those
      new files when computing the number of bytes in the table
      or selecting which files to sample.
    </p>

    <p>
      If data files are removed by a non-Impala mechanism, and the
      table metadata is not updated by a <codeph>REFRESH</codeph>
      or <codeph>INVALIDATE METADATA</codeph> statement, the
      query fails if the <codeph>TABLESAMPLE</codeph> clause
      attempts to reference any of the missing files.
    </p>

    <p conref="../shared/impala_common.xml#common/example_blurb"/>

    <p>
      The following examples demonstrate the <codeph>TABLESAMPLE</codeph> clause.
      These examples intentionally use very small data sets to illustrate how
      the number of files, size of each file, and overall size of data in the table
      interact with the percentage specified in the clause.
    </p>

    <p>
      These examples use an unpartitioned table, containing several files of roughly
      the same size:
    </p>

<codeblock><![CDATA[
create table sample_demo (x int, s string);

insert into sample_demo values (1, 'one');
insert into sample_demo values (2, 'two');
insert into sample_demo values (3, 'three');
insert into sample_demo values (4, 'four');
insert into sample_demo values (5, 'five');

show files in sample_demo;
+---------------------+------+-----------+
| Path                | Size | Partition |
+---------------------+------+-----------+
| 991213608_data.0.   | 7B   |           |
| 982196806_data.0.   | 6B   |           |
| _2122096884_data.0. | 8B   |           |
| _586325431_data.0.  | 6B   |           |
| 1894746258_data.0.  | 7B   |           |
+---------------------+------+-----------+

show table stats sample_demo;
+-------+--------+------+--------+-------------------------+
| #Rows | #Files | Size | Format | Location                |
+-------+--------+------+--------+-------------------------+
| -1    | 5      | 34B  | TEXT   | /tsample.db/sample_demo |
+-------+--------+------+--------+-------------------------+
</codeblock>

    <p>
      A query that samples 50% of the table must process at least
      17 bytes of data. Based on the sizes of the data files,
      we can predict that each such query uses 3 arbitrary files.
      Any 1 or 2 files are not enough to reach 50% of the total
      data in the table (34 bytes), so the query adds more files
      until it passes the 50% threshold:
    </p>

<codeblock><![CDATA[
select distinct x from sample_demo tablesample system(50);
+---+
| x |
+---+
| 4 |
| 1 |
| 5 |
+---+

select distinct x from sample_demo tablesample system(50);
+---+
| x |
+---+
| 5 |
| 4 |
| 2 |
+---+

select distinct x from sample_demo tablesample system(50);
+---+
| x |
+---+
| 5 |
| 3 |
| 2 |
+---+
</codeblock>

    <p>
      To help run reproducible experiments, the <codeph>REPEATABLE</codeph>
      clause causes Impala to choose the same set of files for each query.
      Although the data set being considered is deterministic, the order
      of results varies (in the absence of an <codeph>ORDER BY</codeph>
      clause) because of the way distributed queries are processed:
    </p>

<codeblock><![CDATA[
select distinct x from sample_demo
  tablesample system(50) repeatable (12345);
+---+
| x |
+---+
| 3 |
| 2 |
| 1 |
+---+

select distinct x from sample_demo
  tablesample system(50) repeatable (12345);
+---+
| x |
+---+
| 2 |
| 1 |
| 3 |
+---+
</codeblock>

    <p>
      The following examples show how uneven data distribution affects
      which data is sampled. Adding another data file containing a long
      string value changes the threshold for 50% of the total data in
      the table:
    </p>

<codeblock><![CDATA[
insert into sample_demo values (1000, 'Boyhood is the longest time in li
fe for a boy. The last term of the school-year is made of decades, not o
f weeks, and living through them is like waiting for the millennium. Boo
th Tarkington');

show files in sample_demo;
+---------------------+------+-----------+
| Path                | Size | Partition |
+---------------------+------+-----------+
| 991213608_data.0.   | 7B   |           |
| 982196806_data.0.   | 6B   |           |
| _253317650_data.0.  | 196B |           |
| _2122096884_data.0. | 8B   |           |
| _586325431_data.0.  | 6B   |           |
| 1894746258_data.0.  | 7B   |           |
+---------------------+------+-----------+

show table stats sample_demo;
+-------+--------+------+--------+-------------------------+
| #Rows | #Files | Size | Format | Location                |
+-------+--------+------+--------+-------------------------+
| -1    | 6      | 230B | TEXT   | /tsample.db/sample_demo |
+-------+--------+------+--------+-------------------------+
</codeblock>

    <p>
      Even though the queries do not refer to the <codeph>S</codeph>
      column containing the long value, all the sampling queries include
      the data file containing the column value <codeph>X=1000</codeph>,
      because the query cannot reach the 50% threshold (115 bytes) without
      including that file. The large file might be considered first, in which
      case it is the only file processed by the query. Or an arbitrary
      set of other files might be considered first.
    </p>

<codeblock><![CDATA[
select distinct x from sample_demo tablesample system(50);
+------+
| x    |
+------+
| 1000 |
| 3    |
| 1    |
+------+

select distinct x from sample_demo tablesample system(50);
+------+
| x    |
+------+
| 1000 |
+------+

select distinct x from sample_demo tablesample system(50);
+------+
| x    |
+------+
| 1000 |
| 4    |
| 2    |
| 1    |
+------+
</codeblock>

    <p>
      The following examples demonstrate how the <codeph>TABLESAMPLE</codeph>
      clause interacts with other table aspects, such as partitioning and file
      format:
    </p>

<codeblock><![CDATA[
create table sample_demo_partitions (x int, s string) partitioned by (n int) stored as parquet;

insert into sample_demo_partitions partition (n = 1) select * from sample_demo;
insert into sample_demo_partitions partition (n = 2) select * from sample_demo;
insert into sample_demo_partitions partition (n = 3) select * from sample_demo;

show files in sample_demo_partitions;
+--------------------------------+--------+-----------+
| Path                           | Size   | Partition |
+--------------------------------+--------+-----------+
| 000000_364262785_data.0.parq   | 1.24KB | n=1       |
| 000001_973526736_data.0.parq   | 566B   | n=1       |
| 0000000_1300598134_data.0.parq | 1.24KB | n=2       |
| 0000001_689099063_data.0.parq  | 568B   | n=2       |
| 0000000_1861371709_data.0.parq | 1.24KB | n=3       |
| 0000001_1065507912_data.0.parq | 566B   | n=3       |
+--------------------------------+--------+-----------+

show table stats tablesample_demo_partitioned;
+-------+-------+--------+--------+---------+----------------------------------------------+
| n     | #Rows | #Files | Size   | Format  | Location                                     |
+-------+-------+--------+--------+---------+----------------------------------------------+
| 1     | -1    | 2      | 1.79KB | PARQUET | /tsample.db/tablesample_demo_partitioned/n=1 |
| 2     | -1    | 2      | 1.80KB | PARQUET | /tsample.db/tablesample_demo_partitioned/n=2 |
| 3     | -1    | 2      | 1.79KB | PARQUET | /tsample.db/tablesample_demo_partitioned/n=3 |
| Total | -1    | 6      | 5.39KB |         |                                              |
+-------+-------+--------+--------+---------+----------------------------------------------+
</codeblock>

    <p>
      If the query does not involve any partition pruning, the
      sampling applies to the data volume of the entire table:
    </p>

<codeblock><![CDATA[
-- 18 rows total.
select count(*) from sample_demo_partitions;
+----------+
| count(*) |
+----------+
| 18       |
+----------+

-- The number of rows per data file is not
-- perfectly balanced, therefore the count
-- is different depending on which set of files
-- is considered.
select count(*) from sample_demo_partitions
  tablesample system(75);
+----------+
| count(*) |
+----------+
| 14       |
+----------+

select count(*) from sample_demo_partitions
  tablesample system(75);
+----------+
| count(*) |
+----------+
| 16       |
+----------+
</codeblock>

    <p>
      If the query only processes certain partitions,
      the query computes the sampling threshold based on
      the data size and set of files only from the
      relevant partitions:
    </p>

<codeblock><![CDATA[
select count(*) from sample_demo_partitions
  tablesample system(50) where n = 1;
+----------+
| count(*) |
+----------+
| 6        |
+----------+

select count(*) from sample_demo_partitions
  tablesample system(50) where n = 1;
+----------+
| count(*) |
+----------+
| 2        |
+----------+
]]>
</codeblock>

    <p conref="../shared/impala_common.xml#common/related_info"/>
    <p>
      <xref href="impala_select.xml#select"/>
    </p>

  </conbody>
</concept>