jprdonnelly/impala
1
0
Fork 0
mirror of https://github.com/apache/impala.git synced 2025-12-19 09:58:28 -05:00
Code Issues Projects Releases Wiki Activity
Files
master
impala/docs/topics/impala_txtfile.xml
Alex Rodoni 955868f88a IMPALA-9029: [DOCS] Impala 3.4 Release Notes 
-Added broadcast_bytes_limit query option

Change-Id: I4385749de35f8379ecf6566fe515ed500b42d6cc
Reviewed-on: http://gerrit.cloudera.org:8080/14863
Tested-by: Impala Public Jenkins <impala-public-jenkins@cloudera.com>
Reviewed-by: Joe McDonnell <joemcdonnell@cloudera.com>
2020-03-17 20:16:58 +00:00

721 lines
31 KiB
XML
Raw Permalink Blame History

<?xml version="1.0" encoding="UTF-8"?>
<!--
Licensed to the Apache Software Foundation (ASF) under one
or more contributor license agreements. See the NOTICE file
distributed with this work for additional information
regarding copyright ownership. The ASF licenses this file
to you under the Apache License, Version 2.0 (the
"License"); you may not use this file except in compliance
with the License. You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing,
software distributed under the License is distributed on an
"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
KIND, either express or implied. See the License for the
specific language governing permissions and limitations
under the License.
-->
<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="txtfile">
<title>Using Text Data Files with Impala Tables</title>
<titlealts audience="PDF"><navtitle>Text Data Files</navtitle></titlealts>
<prolog>
<metadata>
<data name="Category" value="Impala"/>
<data name="Category" value="File Formats"/>
<data name="Category" value="Tables"/>
<data name="Category" value="Developers"/>
<data name="Category" value="Data Analysts"/>
</metadata>
</prolog>
<conbody>
<p> Impala supports using text files as the storage format for input and
output. Text files are a convenient format to use for interchange with
other applications or scripts that produce or read delimited text files,
such as CSV or TSV with commas or tabs for delimiters. </p>
<p>
Text files are also very flexible in their column definitions. For example, a text file could have more
fields than the Impala table, and those extra fields are ignored during queries; or it could have fewer
fields than the Impala table, and those missing fields are treated as <codeph>NULL</codeph> values in
queries. You could have fields that were treated as numbers or timestamps in a table, then use <codeph>ALTER
TABLE ... REPLACE COLUMNS</codeph> to switch them to strings, or the reverse.
</p>
<table>
<title>Text Format Support in Impala</title>
<tgroup cols="5">
<colspec colname="1" colwidth="10*"/>
<colspec colname="2" colwidth="10*"/>
<colspec colname="3" colwidth="20*"/>
<colspec colname="4" colwidth="30*"/>
<colspec colname="5" colwidth="30*"/>
<thead>
<row>
<entry>
File Type
</entry>
<entry>
Format
</entry>
<entry>
Compression Codecs
</entry>
<entry>
Impala Can CREATE?
</entry>
<entry>
Impala Can INSERT?
</entry>
</row>
</thead>
<tbody>
<row conref="impala_file_formats.xml#file_formats/txtfile_support">
<entry/>
</row>
</tbody>
</tgroup>
</table>
<p outputclass="toc inpage"/>
</conbody>
<concept id="text_performance">
<title>Query Performance for Impala Text Tables</title>
<prolog>
<metadata>
<data name="Category" value="Performance"/>
</metadata>
</prolog>
<conbody>
<p>
Data stored in text format is relatively bulky, and not as efficient to query as binary formats such as
Parquet. You typically use text tables with Impala if that is the format you receive the data and you do
not have control over that process, or if you are a relatively new Hadoop user and not familiar with
techniques to generate files in other formats. (Because the default format for <codeph>CREATE
TABLE</codeph> is text, you might create your first Impala tables as text without giving performance much
thought.) Either way, look for opportunities to use more efficient file formats for the tables used in your
most performance-critical queries.
</p>
<p>
For frequently queried data, you might load the original text data files into one Impala table, then use an
<codeph>INSERT</codeph> statement to transfer the data to another table that uses the Parquet file format;
the data is converted automatically as it is stored in the destination table.
</p>
<p>
For more compact data, consider using LZO compression for the text files. LZO is the only compression codec
that Impala supports for text data, because the <q>splittable</q> nature of LZO data files lets different
nodes work on different parts of the same file in parallel. See <xref href="impala_txtfile.xml#lzo"/> for
details.
</p>
<p rev="2.0.0">
You can also use text data compressed in the bzip2, deflate, gzip, Snappy, or
zstd formats. Because these compressed formats are not <q>splittable</q> in the way that LZO
is, there is less opportunity for Impala to parallelize queries on them. Therefore, use
these types of compressed data only for convenience if that is the format in which you
receive the data. Prefer to use LZO compression for text data if you have the choice, or
convert the data to Parquet using an <codeph>INSERT ... SELECT</codeph> statement to copy
the original data into a Parquet table. </p>
<note rev="2.2.0">
<p>
Impala supports bzip files created by the <codeph>bzip2</codeph> command, but not bzip files with
multiple streams created by the <codeph>pbzip2</codeph> command. Impala decodes only the data from the
first part of such files, leading to incomplete results.
</p>
</note>
<p>
The maximum size that Impala can accommodate for an individual bzip file is 1 GB (after uncompression).
</p>
<p>
Impala supports zstd files created by the zstd command line tool.
</p>
<p conref="../shared/impala_common.xml#common/s3_block_splitting"/>
</conbody>
</concept>
<concept id="text_ddl">
<title>Creating Text Tables</title>
<conbody>
<p>
<b>To create a table using text data files:</b>
</p>
<p>
If the exact format of the text data files (such as the delimiter character) is not significant, use the
<codeph>CREATE TABLE</codeph> statement with no extra clauses at the end to create a text-format table. For
example:
</p>
<codeblock>create table my_table(id int, s string, n int, t timestamp, b boolean);
</codeblock>
<p>
The data files created by any <codeph>INSERT</codeph> statements will use the Ctrl-A character (hex 01) as
a separator between each column value.
</p>
<p>
A common use case is to import existing text files into an Impala table. The syntax is more verbose; the
significant part is the <codeph>FIELDS TERMINATED BY</codeph> clause, which must be preceded by the
<codeph>ROW FORMAT DELIMITED</codeph> clause. The statement can end with a <codeph>STORED AS
TEXTFILE</codeph> clause, but that clause is optional because text format tables are the default. For
example:
</p>
<codeblock>create table csv(id int, s string, n int, t timestamp, b boolean)
row format delimited
<ph id="csv">fields terminated by ',';</ph>
create table tsv(id int, s string, n int, t timestamp, b boolean)
row format delimited
<ph id="tsv">fields terminated by '\t';</ph>
create table pipe_separated(id int, s string, n int, t timestamp, b boolean)
row format delimited
<ph id="psv">fields terminated by '|'</ph>
stored as textfile;
</codeblock>
<p>
You can create tables with specific separator characters to import text files in familiar formats such as
CSV, TSV, or pipe-separated. You can also use these tables to produce output data files, by copying data
into them through the <codeph>INSERT ... SELECT</codeph> syntax and then extracting the data files from the
Impala data directory.
</p>
<p rev="1.3.1">
In Impala 1.3.1 and higher, you can specify a delimiter character <codeph>'\</codeph><codeph>0'</codeph> to
use the ASCII 0 (<codeph>nul</codeph>) character for text tables:
</p>
<codeblock rev="1.3.1">create table nul_separated(id int, s string, n int, t timestamp, b boolean)
row format delimited
fields terminated by '\0'
stored as textfile;
</codeblock>
<note>
<p>
Do not surround string values with quotation marks in text data files that you construct. If you need to
include the separator character inside a field value, for example to put a string value with a comma
inside a CSV-format data file, specify an escape character on the <codeph>CREATE TABLE</codeph> statement
with the <codeph>ESCAPED BY</codeph> clause, and insert that character immediately before any separator
characters that need escaping.
</p>
</note>
<p>
Issue a <codeph>DESCRIBE FORMATTED <varname>table_name</varname></codeph> statement to see the details of
how each table is represented internally in Impala.
</p>
<p conref="../shared/impala_common.xml#common/complex_types_unsupported_filetype"/>
</conbody>
</concept>
<concept id="text_data_files">
<title>Data Files for Text Tables</title>
<conbody>
<p>
When Impala queries a table with data in text format, it consults all the data files in the data directory
for that table, with some exceptions:
</p>
<ul rev="2.2.0">
<li>
<p>
Impala ignores any hidden files, that is, files whose names start with a dot or an underscore.
</p>
</li>
<li>
<p conref="../shared/impala_common.xml#common/ignore_file_extensions"/>
</li>
<li>
<p> Impala uses suffixes to recognize when text data files are compressed text. For Impala
to recognize the compressed text files, they must have the appropriate file extension
corresponding to the compression codec, either <codeph>.bz2</codeph>, <codeph>.deflate</codeph>
<codeph>.gz</codeph>, <codeph>.snappy</codeph>, or <codeph>.zst</codeph>.
The extensions can be in uppercase or lowercase. </p>
</li>
<li> Otherwise, the file names are not significant. When you put files
into an HDFS directory through ETL jobs, or point Impala to an
existing HDFS directory with the <codeph>CREATE EXTERNAL
TABLE</codeph> statement, or move data files under external control
with the <codeph>LOAD DATA</codeph> statement, Impala preserves the
original file names. </li>
</ul>
<p> File names for data produced through Impala <codeph>INSERT</codeph>
statements are given unique names to avoid file name conflicts. </p>
<p>
An <codeph>INSERT ... SELECT</codeph> statement produces one data file from each node that processes the
<codeph>SELECT</codeph> part of the statement. An <codeph>INSERT ... VALUES</codeph> statement produces a
separate data file for each statement; because Impala is more efficient querying a small number of huge
files than a large number of tiny files, the <codeph>INSERT ... VALUES</codeph> syntax is not recommended
for loading a substantial volume of data. If you find yourself with a table that is inefficient due to too
many small data files, reorganize the data into a few large files by doing <codeph>INSERT ...
SELECT</codeph> to transfer the data to a new table.
</p>
<p>
<b>Special values within text data files:</b>
</p>
<ul>
<li rev="1.4.0">
<p>
Impala recognizes the literal strings <codeph>inf</codeph> for infinity and <codeph>nan</codeph> for
<q>Not a Number</q>, for <codeph>FLOAT</codeph> and <codeph>DOUBLE</codeph> columns.
</p>
</li>
<li>
<p> Impala recognizes the literal string <codeph>\N</codeph> to
represent <codeph>NULL</codeph>. When using Sqoop, specify the
options <codeph>--null-non-string</codeph> and
<codeph>--null-string</codeph> to ensure all <codeph>NULL</codeph>
values are represented correctly in the Sqoop output files.
<codeph>\N</codeph> needs to be escaped as in the below example:
<codeblock>--null-string '\\N' --null-non-string '\\N'</codeblock>
</p>
</li>
<li>
<p>By default, Sqoop writes <codeph>NULL</codeph> values using the
string <codeph>null</codeph>, which causes a conversion error when
such rows are evaluated by Impala. A workaround for existing tables
and data files is to change the table properties through
<codeph>ALTER TABLE <varname>name</varname> SET
TBLPROPERTIES("serialization.null.format"="null")</codeph>.</p>
</li>
<li>
<p conref="../shared/impala_common.xml#common/skip_header_lines"/>
</li>
</ul>
</conbody>
</concept>
<concept id="text_etl">
<title>Loading Data into Impala Text Tables</title>
<prolog>
<metadata>
<data name="Category" value="ETL"/>
<data name="Category" value="Ingest"/>
</metadata>
</prolog>
<conbody>
<p>
To load an existing text file into an Impala text table, use the <codeph>LOAD DATA</codeph> statement and
specify the path of the file in HDFS. That file is moved into the appropriate Impala data directory.
</p>
<p>
To load multiple existing text files into an Impala text table, use the <codeph>LOAD DATA</codeph>
statement and specify the HDFS path of the directory containing the files. All non-hidden files are moved
into the appropriate Impala data directory.
</p>
<p>
To convert data to text from any other file format supported by Impala, use a SQL statement such as:
</p>
<codeblock>-- Text table with default delimiter, the hex 01 character.
CREATE TABLE text_table AS SELECT * FROM other_file_format_table;
-- Text table with user-specified delimiter. Currently, you cannot specify
-- the delimiter as part of CREATE TABLE LIKE or CREATE TABLE AS SELECT.
-- But you can change an existing text table to have a different delimiter.
CREATE TABLE csv LIKE other_file_format_table;
ALTER TABLE csv SET SERDEPROPERTIES ('serialization.format'=',', 'field.delim'=',');
INSERT INTO csv SELECT * FROM other_file_format_table;</codeblock>
<p>
This can be a useful technique to see how Impala represents special values within a text-format data file.
Use the <codeph>DESCRIBE FORMATTED</codeph> statement to see the HDFS directory where the data files are
stored, then use Linux commands such as <codeph>hdfs dfs -ls <varname>hdfs_directory</varname></codeph> and
<codeph>hdfs dfs -cat <varname>hdfs_file</varname></codeph> to display the contents of an Impala-created
text file.
</p>
<p>
To create a few rows in a text table for test purposes, you can use the <codeph>INSERT ... VALUES</codeph>
syntax:
</p>
<codeblock>INSERT INTO <varname>text_table</varname> VALUES ('string_literal',100,hex('hello world'));</codeblock>
<note>
Because Impala and the HDFS infrastructure are optimized for multi-megabyte files, avoid the <codeph>INSERT
... VALUES</codeph> notation when you are inserting many rows. Each <codeph>INSERT ... VALUES</codeph>
statement produces a new tiny file, leading to fragmentation and reduced performance. When creating any
substantial volume of new data, use one of the bulk loading techniques such as <codeph>LOAD DATA</codeph>
or <codeph>INSERT ... SELECT</codeph>. Or, <xref href="impala_hbase.xml#impala_hbase">use an HBase
table</xref> for single-row <codeph>INSERT</codeph> operations, because HBase tables are not subject to the
same fragmentation issues as tables stored on HDFS.
</note>
<p> When you create a text file for use with an Impala text table, specify
<codeph>\N</codeph> to represent a <codeph>NULL</codeph> value. For
the differences between <codeph>NULL</codeph> and empty strings, see
<xref href="impala_literals.xml#null"/>. </p>
<p>
If a text file has fewer fields than the columns in the corresponding Impala table, all the corresponding
columns are set to <codeph>NULL</codeph> when the data in that file is read by an Impala query.
</p>
<p>
If a text file has more fields than the columns in the corresponding Impala table, the extra fields are
ignored when the data in that file is read by an Impala query.
</p>
<p>
You can also use manual HDFS operations such as <codeph>hdfs dfs -put</codeph> or <codeph>hdfs dfs
-cp</codeph> to put data files in the data directory for an Impala table. When you copy or move new data
files into the HDFS directory for the Impala table, issue a <codeph>REFRESH
<varname>table_name</varname></codeph> statement in <cmdname>impala-shell</cmdname> before issuing the next
query against that table, to make Impala recognize the newly added files.
</p>
</conbody>
</concept>
<concept id="lzo">
<title>Using LZO-Compressed Text Files</title>
<prolog>
<metadata>
<data name="Category" value="LZO"/>
<data name="Category" value="Compression"/>
</metadata>
</prolog>
<conbody>
<p> Impala supports using text data files that employ LZO compression.
Where practical, apply compression to text data files. Impala queries
are usually I/O-bound; reducing the amount of data read from disk
typically speeds up a query, despite the extra CPU work to uncompress
the data in memory. </p>
<p>
Impala can work with LZO-compressed text files are preferable to files compressed by other codecs, because
LZO-compressed files are <q>splittable</q>, meaning that different portions of a file can be uncompressed
and processed independently by different nodes.
</p>
<p>
Impala does not currently support writing LZO-compressed text files.
</p>
<p>
Because Impala can query LZO-compressed files but currently cannot write them, you use Hive to do the
initial <codeph>CREATE TABLE</codeph> and load the data, then switch back to Impala to run queries. For
instructions on setting up LZO compression for Hive <codeph>CREATE TABLE</codeph> and
<codeph>INSERT</codeph> statements, see
<xref href="https://cwiki.apache.org/confluence/display/Hive/LanguageManual+LZO" scope="external" format="html">the
LZO page on the Hive wiki</xref>. Once you have created an LZO text table, you can also manually add
LZO-compressed text files to it, produced by the
<xref href="http://www.lzop.org/" scope="external" format="html"> <cmdname>lzop</cmdname></xref> command
or similar method.
</p>
<section id="lzo_setup">
<title>Preparing to Use LZO-Compressed Text Files</title>
<p>
Before using LZO-compressed tables in Impala, do the following one-time setup for each machine in the
cluster. Install the necessary packages using either the public repository, a private repository
you establish, or by using packages. You must do these steps manually, whether or not you
are using cluster management software.
</p>
<ol>
<li>
<b>Prepare your systems to work with LZO by downloading and installing the appropriate libraries:</b>
<p>
Download and install the appropriate file to each machine on which you intend to use LZO with Impala.
</p>
</li>
<li>
<b>Configure Impala to use LZO:</b>
<p>
Use <b>one</b> of the following sets of commands to refresh your package management system's
repository information, install the base LZO support for Hadoop, and install the LZO support for
Impala.
</p>
<note rev="1.2.0">
<p rev="1.2.0">
The name of the Hadoop LZO package changed in the distant past.
Currently, the package name is <codeph>hadoop-lzo</codeph>.
</p>
</note>
<p>
<b>For RHEL/CentOS systems:</b>
</p>
<codeblock>$ sudo yum update
$ sudo yum install hadoop-lzo
$ sudo yum install impala-lzo</codeblock>
<p>
<b>For SUSE systems:</b>
</p>
<codeblock rev="1.2">$ sudo apt-get update
$ sudo zypper install hadoop-lzo
$ sudo zypper install impala-lzo</codeblock>
<p>
<b>For Debian/Ubuntu systems:</b>
</p>
<codeblock>$ sudo zypper update
$ sudo apt-get install hadoop-lzo
$ sudo apt-get install impala-lzo</codeblock>
<note>
<p>
The level of the <codeph>impala-lzo</codeph> package is closely tied to the version of Impala
you use. Any time you upgrade Impala, re-do the installation command for
<codeph>impala-lzo</codeph> on each applicable machine to make sure you have the appropriate
version of that package.
</p>
</note>
</li>
<li>
For <codeph>core-site.xml</codeph> on the client <b>and</b> server (that is, in the configuration
directories for both Impala and Hadoop), append <codeph>com.hadoop.compression.lzo.LzopCodec</codeph>
to the comma-separated list of codecs. For example:
<codeblock>&lt;property&gt;
&lt;name&gt;io.compression.codecs&lt;/name&gt;
&lt;value&gt;org.apache.hadoop.io.compress.DefaultCodec,org.apache.hadoop.io.compress.GzipCodec,
org.apache.hadoop.io.compress.BZip2Codec,org.apache.hadoop.io.compress.DeflateCodec,
org.apache.hadoop.io.compress.SnappyCodec,com.hadoop.compression.lzo.LzopCodec&lt;/value&gt;
&lt;/property&gt;</codeblock>
<note>
<p>
If this is the first time you have edited the Hadoop <filepath>core-site.xml</filepath> file, note
that the <filepath>/etc/hadoop/conf</filepath> directory is typically a symbolic link, so the
canonical <filepath>core-site.xml</filepath> might reside in a different directory:
</p>
<codeblock>$ ls -l /etc/hadoop
total 8
lrwxrwxrwx. 1 root root 29 Feb 26 2013 conf -&gt; /etc/alternatives/hadoop-conf
lrwxrwxrwx. 1 root root 10 Feb 26 2013 conf.dist -&gt; conf.empty
drwxr-xr-x. 2 root root 4096 Feb 26 2013 conf.empty
drwxr-xr-x. 2 root root 4096 Oct 28 15:46 conf.pseudo</codeblock>
<p>
If the <codeph>io.compression.codecs</codeph> property is missing from
<filepath>core-site.xml</filepath>, only add <codeph>com.hadoop.compression.lzo.LzopCodec</codeph>
to the new property value, not all the names from the preceding example.
</p>
</note>
</li>
<li>
Restart the MapReduce and Impala services.
</li>
</ol>
</section>
<section id="lzo_create_table">
<title>Creating LZO Compressed Text Tables</title>
<p>
A table containing LZO-compressed text files must be created in Hive with the following storage clause:
</p>
<codeblock>STORED AS
INPUTFORMAT 'com.hadoop.mapred.DeprecatedLzoTextInputFormat'
OUTPUTFORMAT 'org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat'</codeblock>
<p>
Also, certain Hive settings need to be in effect. For example:
</p>
<codeblock>hive&gt; SET mapreduce.output.fileoutputformat.compress=true;
hive&gt; SET hive.exec.compress.output=true;
hive&gt; SET mapreduce.output.fileoutputformat.compress.codec=com.hadoop.compression.lzo.LzopCodec;
hive&gt; CREATE TABLE lzo_t (s string) STORED AS
&gt; INPUTFORMAT 'com.hadoop.mapred.DeprecatedLzoTextInputFormat'
&gt; OUTPUTFORMAT 'org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat';
hive&gt; INSERT INTO TABLE lzo_t SELECT col1, col2 FROM uncompressed_text_table;</codeblock>
<p>
Once you have created LZO-compressed text tables, you can convert data stored in other tables (regardless
of file format) by using the <codeph>INSERT ... SELECT</codeph> statement in Hive.
</p>
<p>
Files in an LZO-compressed table must use the <codeph>.lzo</codeph> extension. Examine the files in the
HDFS data directory after doing the <codeph>INSERT</codeph> in Hive, to make sure the files have the
right extension. If the required settings are not in place, you end up with regular uncompressed files,
and Impala cannot access the table because it finds data files with the wrong (uncompressed) format.
</p>
<p>
After loading data into an LZO-compressed text table, index the files so that they can be split. You
index the files by running a Java class,
<codeph>com.hadoop.compression.lzo.DistributedLzoIndexer</codeph>, through the Linux command line. This
Java class is included in the <codeph>hadoop-lzo</codeph> package.
</p>
<p>
Run the indexer using a command like the following:
</p>
<codeblock>$ hadoop jar /usr/lib/hadoop/lib/hadoop-lzo-<varname>version</varname>-gplextras.jar
com.hadoop.compression.lzo.DistributedLzoIndexer /<varname>hdfs_location_of_table</varname>/</codeblock>
<note>
If the path of the JAR file in the preceding example is not recognized, do a <cmdname>find</cmdname>
command to locate <filepath>hadoop-lzo-*-gplextras.jar</filepath> and use that path.
</note>
<p>
Indexed files have the same name as the file they index, with the <codeph>.index</codeph> extension. If
the data files are not indexed, Impala queries still work, but the queries read the data from remote
DataNodes, which is very inefficient.
</p>
<p>
Once the LZO-compressed tables are created, and data is loaded and indexed, you can query them through
Impala. As always, the first time you start <cmdname>impala-shell</cmdname> after creating a table in
Hive, issue an <codeph>INVALIDATE METADATA</codeph> statement so that Impala recognizes the new table.
(In Impala 1.2 and higher, you only have to run <codeph>INVALIDATE METADATA</codeph> on one node, rather
than on all the Impala nodes.)
</p>
</section>
</conbody>
</concept>
<concept rev="2.0.0" id="gzip">
<title>Using bzip2, deflate, gzip, Snappy, or zstd Text Files</title>
<prolog>
<metadata>
<data name="Category" value="Deflate"/>
<data name="Category" value="Snappy"/>
<data name="Category" value="Gzip"/>
<data name="Category" value="Zstd"/>
<data name="Category" value="Compression"/>
</metadata>
</prolog>
<conbody>
<p> Impala supports using text data files that employ bzip2, deflate, gzip, Snappy, or zstd
compression. These compression types are primarily for convenience within an existing ETL
pipeline rather than maximum performance. Although it requires less I/O to read compressed
text than the equivalent uncompressed text, files compressed by these codecs are not
<q>splittable</q> and therefore cannot take full advantage of the Impala parallel query
capability. Impala can read compressed text files written by Hive.</p>
<p> As each Snappy-compressed file is processed, the node doing the work reads the entire file
into memory and then decompresses it. Therefore, the node must have enough memory to hold
both the compressed and uncompressed data from the text file. The memory required to hold
the uncompressed data is difficult to estimate in advance, potentially causing problems on
systems with low memory limits or with resource management enabled. <ph rev="2.1.0">This
memory overhead is reduced for bzip2-, deflate-, gzip-, and zstd-compressed text files. The
compressed data is decompressed as it is read, rather than all at once.</ph>
</p>
<p> To create a table to hold compressed text, create a text table with no special compression
options. Specify the delimiter and escape character if required, using the <codeph>ROW
FORMAT</codeph> clause. </p>
<p>
Because Impala can query compressed text files but currently cannot write them, produce the compressed text
files outside Impala and use the <codeph>LOAD DATA</codeph> statement, manual HDFS commands to move them to
the appropriate Impala data directory. (Or, you can use <codeph>CREATE EXTERNAL TABLE</codeph> and point
the <codeph>LOCATION</codeph> attribute at a directory containing existing compressed text files.)
</p>
<p>
The following example shows how you can create a regular text table, put different kinds of compressed and
uncompressed files into it, and Impala automatically recognizes and decompresses each one based on their
file extensions:
</p>
<codeblock>create table csv_compressed (a string, b string, c string)
row format delimited fields terminated by ",";
insert into csv_compressed values
('one - uncompressed', 'two - uncompressed', 'three - uncompressed'),
('abc - uncompressed', 'xyz - uncompressed', '123 - uncompressed');
...make equivalent .bz2, .gz, .snappy, and .zst files and load them into same table directory...
select * from csv_compressed;
+--------------------+--------------------+----------------------+
| a | b | c |
+--------------------+--------------------+----------------------+
| one - snappy | two - snappy | three - snappy |
| one - uncompressed | two - uncompressed | three - uncompressed |
| abc - uncompressed | xyz - uncompressed | 123 - uncompressed |
| one - bz2 | two - bz2 | three - bz2 |
| abc - bz2 | xyz - bz2 | 123 - bz2 |
| one - gzip | two - gzip | three - gzip |
| abc - gzip | xyz - gzip | 123 - gzip |
| one - zstd | two - zstd | three - zstd |
| abc - zstd | xyz - zstd | 123 - zstd |
| one - deflate | two - deflate | three - deflate |
| abc - deflate | xyz - deflate | 123 - deflate |
+--------------------+--------------------+----------------------+
$ hdfs dfs -ls 'hdfs://127.0.0.1:8020/user/hive/warehouse/file_formats.db/csv_compressed/';
...truncated for readability...
75 hdfs://127.0.0.1:8020/user/hive/warehouse/file_formats.db/csv_compressed/csv_compressed.snappy
79 hdfs://127.0.0.1:8020/user/hive/warehouse/file_formats.db/csv_compressed/csv_compressed_bz2.csv.bz2
80 hdfs://127.0.0.1:8020/user/hive/warehouse/file_formats.db/csv_compressed/csv_compressed_gzip.csv.gz
58 hdfs://127.0.0.1:8020/user/hive/warehouse/file_formats.db/csv_compressed/csv_compressed_zstd.csv.zst
48 hdfs://127.0.0.1:8020/user/hive/warehouse/file_formats.db/csv_compressed/csv_compressed_deflate.csv.deflate
116 hdfs://127.0.0.1:8020/user/hive/warehouse/file_formats.db/csv_compressed/dd414df64d67d49b_data.0.
</codeblock>
</conbody>
</concept>
</concept>
Reference in New Issue View Git Blame Copy Permalink