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impala/docs/topics/impala_alter_table.xml
John Russell eaefbb90ce IMPALA-1654: [DOCS] DDL for multiple partitions 
Syntax and usage notes for ALTER TABLE,
COMPUTE STATS, and SHOW FILES.

Mixed in a little bit with new Kudu syntax for
ALTER TABLE. Didn't include all new Kudu info
in this CR, the better to minimize merge conflicts.

Added note about performance/scalability of IMPALA-1654.

Added new Known Issue item for IMPALA-4106 under Performance category.

Change-Id: I2060552d5081e5f93b1b1f398414c52fa03f215b
Reviewed-on: http://gerrit.cloudera.org:8080/5726
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: Impala Public Jenkins
2017-01-25 18:39:12 +00:00

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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="alter_table">
<title>ALTER TABLE Statement</title>
<titlealts audience="PDF"><navtitle>ALTER TABLE</navtitle></titlealts>
<prolog>
<metadata>
<data name="Category" value="Impala"/>
<data name="Category" value="Impala Data Types"/>
<data name="Category" value="SQL"/>
<data name="Category" value="DDL"/>
<data name="Category" value="HDFS Caching"/>
<data name="Category" value="Tables"/>
<data name="Category" value="Schemas"/>
<data name="Category" value="S3"/>
<data name="Category" value="Developers"/>
<data name="Category" value="Data Analysts"/>
</metadata>
</prolog>
<conbody>
<p>
<indexterm audience="hidden">ALTER TABLE statement</indexterm>
The <codeph>ALTER TABLE</codeph> statement changes the structure or properties of an existing Impala table.
</p>
<p>
In Impala, this is primarily a logical operation that updates the table metadata in the metastore database that Impala
shares with Hive. Most <codeph>ALTER TABLE</codeph> operations do not actually rewrite, move, and so on the actual data
files. (The <codeph>RENAME TO</codeph> clause is the one exception; it can cause HDFS files to be moved to different paths.)
When you do an <codeph>ALTER TABLE</codeph> operation, you typically need to perform corresponding physical filesystem operations,
such as rewriting the data files to include extra fields, or converting them to a different file format.
</p>
<p conref="../shared/impala_common.xml#common/syntax_blurb"/>
<codeblock>ALTER TABLE [<varname>old_db_name</varname>.]<varname>old_table_name</varname> RENAME TO [<varname>new_db_name</varname>.]<varname>new_table_name</varname>
ALTER TABLE <varname>name</varname> ADD COLUMNS (<varname>col_spec</varname>[, <varname>col_spec</varname> ...])
ALTER TABLE <varname>name</varname> DROP [COLUMN] <varname>column_name</varname>
ALTER TABLE <varname>name</varname> CHANGE <varname>column_name</varname> <varname>new_name</varname> <varname>new_type</varname>
ALTER TABLE <varname>name</varname> REPLACE COLUMNS (<varname>col_spec</varname>[, <varname>col_spec</varname> ...])
ALTER TABLE <varname>name</varname> { ADD [IF NOT EXISTS] | DROP [IF EXISTS] } PARTITION (<varname>partition_spec</varname>) <ph rev="2.3.0">[PURGE]</ph>
<ph rev="2.3.0 IMPALA-1568 CDH-36799">ALTER TABLE <varname>name</varname> RECOVER PARTITIONS</ph>
ALTER TABLE <varname>name</varname> [PARTITION (<varname>partition_spec</varname>)]
SET { FILEFORMAT <varname>file_format</varname>
| LOCATION '<varname>hdfs_path_of_directory</varname>'
| TBLPROPERTIES (<varname>table_properties</varname>)
| SERDEPROPERTIES (<varname>serde_properties</varname>) }
<ph rev="2.6.0 IMPALA-3369">ALTER TABLE <varname>name</varname> <varname>colname</varname>
('<varname>statsKey</varname>'='<varname>val</varname>, ...)
statsKey ::= numDVs | numNulls | avgSize | maxSize</ph>
<ph rev="1.4.0">ALTER TABLE <varname>name</varname> [PARTITION (<varname>partition_spec</varname>)] SET { CACHED IN '<varname>pool_name</varname>' <ph rev="2.2.0">[WITH REPLICATION = <varname>integer</varname>]</ph> | UNCACHED }</ph>
<varname>new_name</varname> ::= [<varname>new_database</varname>.]<varname>new_table_name</varname>
<varname>col_spec</varname> ::= <varname>col_name</varname> <varname>type_name</varname>
<varname>partition_spec</varname> ::= <varname>simple_partition_spec</varname> | <ph rev="IMPALA-1654"><varname>complex_partition_spec</varname></ph> | <ph rev="kudu"><varname>kudu_partition_spec</varname></ph>
<varname>simple_partition_spec</varname> ::= <varname>partition_col</varname>=<varname>constant_value</varname>
<ph rev="IMPALA-1654"><varname>complex_partition_spec</varname> ::= <varname>comparison_expression_on_partition_col</varname></ph>
<varname>table_properties</varname> ::= '<varname>name</varname>'='<varname>value</varname>'[, '<varname>name</varname>'='<varname>value</varname>' ...]
<varname>serde_properties</varname> ::= '<varname>name</varname>'='<varname>value</varname>'[, '<varname>name</varname>'='<varname>value</varname>' ...]
<varname>file_format</varname> ::= { PARQUET | TEXTFILE | RCFILE | SEQUENCEFILE | AVRO }
</codeblock>
<p conref="../shared/impala_common.xml#common/ddl_blurb"/>
<p conref="../shared/impala_common.xml#common/complex_types_blurb"/>
<p rev="2.3.0">
In <keyword keyref="impala23_full"/> and higher, the <codeph>ALTER TABLE</codeph> statement can
change the metadata for tables containing complex types (<codeph>ARRAY</codeph>,
<codeph>STRUCT</codeph>, and <codeph>MAP</codeph>).
For example, you can use an <codeph>ADD COLUMNS</codeph>, <codeph>DROP COLUMN</codeph>, or <codeph>CHANGE</codeph>
clause to modify the table layout for complex type columns.
Although Impala queries only work for complex type columns in Parquet tables, the complex type support in the
<codeph>ALTER TABLE</codeph> statement applies to all file formats.
For example, you can use Impala to update metadata for a staging table in a non-Parquet file format where the
data is populated by Hive. Or you can use <codeph>ALTER TABLE SET FILEFORMAT</codeph> to change the format
of an existing table to Parquet so that Impala can query it. Remember that changing the file format for a table does
not convert the data files within the table; you must prepare any Parquet data files containing complex types
outside Impala, and bring them into the table using <codeph>LOAD DATA</codeph> or updating the table's
<codeph>LOCATION</codeph> property.
See <xref href="impala_complex_types.xml#complex_types"/> for details about using complex types.
</p>
<p conref="../shared/impala_common.xml#common/usage_notes_blurb"/>
<p>
Whenever you specify partitions in an <codeph>ALTER TABLE</codeph> statement, through the <codeph>PARTITION
(<varname>partition_spec</varname>)</codeph> clause, you must include all the partitioning columns in the
specification.
</p>
<p>
Most of the <codeph>ALTER TABLE</codeph> operations work the same for internal tables (managed by Impala) as
for external tables (with data files located in arbitrary locations). The exception is renaming a table; for
an external table, the underlying data directory is not renamed or moved.
</p>
<p>
<b>Dropping or altering multiple partitions:</b>
</p>
<p rev="IMPALA-1654">
In <keyword keyref="impala28_full"/> and higher,
the expression for the partition clause with a <codeph>DROP</codeph> or <codeph>SET</codeph>
operation can include comparison operators such as <codeph>&lt;</codeph>, <codeph>IN</codeph>,
or <codeph>BETWEEN</codeph>, and Boolean operators such as <codeph>AND</codeph>
and <codeph>OR</codeph>.
</p>
<p rev="IMPALA-1654">
For example, you might drop a group of partitions corresponding to a particular date
range after the data <q>ages out</q>:
</p>
<codeblock><![CDATA[
alter table historical_data drop partition (year < 1995);
alter table historical_data drop partition (year = 1996 and month between 1 and 6);
]]>
</codeblock>
<p rev="IMPALA-1654">
For tables with multiple partition keys columns, you can specify multiple
conditions separated by commas, and the operation only applies to the partitions
that match all the conditions (similar to using an <codeph>AND</codeph> clause):
</p>
<codeblock><![CDATA[
alter table historical_data drop partition (year < 1995, last_name like 'A%');
]]>
</codeblock>
<p rev="IMPALA-1654">
This technique can also be used to change the file format of groups of partitions,
as part of an ETL pipeline that periodically consolidates and rewrites the underlying
data files in a different file format:
</p>
<codeblock><![CDATA[
alter table fast_growing_data partition (year = 2016, month in (10,11,12)) set fileformat parquet;
]]>
</codeblock>
<note>
<p rev="IMPALA-1654">
The extended syntax involving comparison operators and multiple partitions
applies to the <codeph>SET FILEFORMAT</codeph>, <codeph>SET TBLPROPERTIES</codeph>,
<codeph>SET SERDEPROPERTIES</codeph>, and <codeph>SET [UN]CACHED</codeph> clauses.
You can also use this syntax with the <codeph>PARTITION</codeph> clause
in the <codeph>COMPUTE INCREMENTAL STATS</codeph> statement, and with the
<codeph>PARTITION</codeph> clause of the <codeph>SHOW FILES</codeph> statement.
Some forms of <codeph>ALTER TABLE</codeph> still only apply to one partition
at a time: the <codeph>SET LOCATION</codeph> and <codeph>ADD PARTITION</codeph>
clauses. The <codeph>PARTITION</codeph> clauses in the <codeph>LOAD DATA</codeph>
and <codeph>INSERT</codeph> statements also only apply to one partition at a time.
</p>
<p>
A DDL statement that applies to multiple partitions is considered successful
(resulting in no changes) even if no partitions match the conditions.
The results are the same as if the <codeph>IF EXISTS</codeph> clause was specified.
</p>
<p>
The performance and scalability of this technique is similar to
issuing a sequence of single-partition <codeph>ALTER TABLE</codeph>
statements in quick succession. To minimize bottlenecks due to
communication with the metastore database, or causing other
DDL operations on the same table to wait, test the effects of
performing <codeph>ALTER TABLE</codeph> statements that affect
large numbers of partitions.
</p>
</note>
<p conref="../shared/impala_common.xml#common/s3_blurb"/>
<p rev="2.6.0 CDH-39913 IMPALA-1878">
You can specify an <codeph>s3a://</codeph> prefix on the <codeph>LOCATION</codeph> attribute of a table or partition
to make Impala query data from the Amazon S3 filesystem. In <keyword keyref="impala26_full"/> and higher, Impala automatically
handles creating or removing the associated folders when you issue <codeph>ALTER TABLE</codeph> statements
with the <codeph>ADD PARTITION</codeph> or <codeph>DROP PARTITION</codeph> clauses.
</p>
<p conref="../shared/impala_common.xml#common/s3_ddl"/>
<p rev="1.4.0">
<b>HDFS caching (CACHED IN clause):</b>
</p>
<p rev="1.4.0">
If you specify the <codeph>CACHED IN</codeph> clause, any existing or future data files in the table
directory or the partition subdirectories are designated to be loaded into memory with the HDFS caching
mechanism. See <xref href="impala_perf_hdfs_caching.xml#hdfs_caching"/> for details about using the HDFS
caching feature.
</p>
<p conref="../shared/impala_common.xml#common/impala_cache_replication_factor"/>
<p conref="../shared/impala_common.xml#common/sync_ddl_blurb"/>
<p>
The following sections show examples of the use cases for various <codeph>ALTER TABLE</codeph> clauses.
</p>
<p>
<b>To rename a table (RENAME TO clause):</b>
</p>
<!-- Beefing up the syntax in its original location up to, don't need to repeat it here.
<codeblock>ALTER TABLE <varname>old_name</varname> RENAME TO <varname>new_name</varname>;</codeblock>
-->
<p>
The <codeph>RENAME TO</codeph> clause lets you change the name of an existing table, and optionally which
database it is located in.
</p>
<p>
For internal tables, this operation physically renames the directory within HDFS that contains the data files;
the original directory name no longer exists. By qualifying the table names with database names, you can use
this technique to move an internal table (and its associated data directory) from one database to another.
For example:
</p>
<codeblock>create database d1;
create database d2;
create database d3;
use d1;
create table mobile (x int);
use d2;
-- Move table from another database to the current one.
alter table d1.mobile rename to mobile;
use d1;
-- Move table from one database to another.
alter table d2.mobile rename to d3.mobile;</codeblock>
<p>
For external tables,
</p>
<p>
<b>To change the physical location where Impala looks for data files associated with a table or
partition:</b>
</p>
<codeblock>ALTER TABLE <varname>table_name</varname> [PARTITION (<varname>partition_spec</varname>)] SET LOCATION '<varname>hdfs_path_of_directory</varname>';</codeblock>
<p>
The path you specify is the full HDFS path where the data files reside, or will be created. Impala does not
create any additional subdirectory named after the table. Impala does not move any data files to this new
location or change any data files that might already exist in that directory.
</p>
<p>
To set the location for a single partition, include the <codeph>PARTITION</codeph> clause. Specify all the
same partitioning columns for the table, with a constant value for each, to precisely identify the single
partition affected by the statement:
</p>
<codeblock>create table p1 (s string) partitioned by (month int, day int);
-- Each ADD PARTITION clause creates a subdirectory in HDFS.
alter table p1 add partition (month=1, day=1);
alter table p1 add partition (month=1, day=2);
alter table p1 add partition (month=2, day=1);
alter table p1 add partition (month=2, day=2);
-- Redirect queries, INSERT, and LOAD DATA for one partition
-- to a specific different directory.
alter table p1 partition (month=1, day=1) set location '/usr/external_data/new_years_day';
</codeblock>
<note conref="../shared/impala_common.xml#common/add_partition_set_location"/>
<p rev="2.3.0 IMPALA-1568 CDH-36799">
<b>To automatically detect new partition directories added through Hive or HDFS operations:</b>
</p>
<p rev="2.3.0 IMPALA-1568 CDH-36799">
In <keyword keyref="impala23_full"/> and higher, the <codeph>RECOVER PARTITIONS</codeph> clause scans
a partitioned table to detect if any new partition directories were added outside of Impala,
such as by Hive <codeph>ALTER TABLE</codeph> statements or by <cmdname>hdfs dfs</cmdname>
or <cmdname>hadoop fs</cmdname> commands. The <codeph>RECOVER PARTITIONS</codeph> clause
automatically recognizes any data files present in these new directories, the same as
the <codeph>REFRESH</codeph> statement does.
</p>
<p rev="2.3.0 IMPALA-1568 CDH-36799">
For example, here is a sequence of examples showing how you might create a partitioned table in Impala,
create new partitions through Hive, copy data files into the new partitions with the <cmdname>hdfs</cmdname>
command, and have Impala recognize the new partitions and new data:
</p>
<p rev="2.3.0 IMPALA-1568 CDH-36799">
In Impala, create the table, and a single partition for demonstration purposes:
</p>
<codeblock rev="2.3.0 IMPALA-1568 CDH-36799">
<![CDATA[
create database recover_partitions;
use recover_partitions;
create table t1 (s string) partitioned by (yy int, mm int);
insert into t1 partition (yy = 2016, mm = 1) values ('Partition exists');
show files in t1;
+---------------------------------------------------------------------+------+--------------+
| Path | Size | Partition |
+---------------------------------------------------------------------+------+--------------+
| /user/hive/warehouse/recover_partitions.db/t1/yy=2016/mm=1/data.txt | 17B | yy=2016/mm=1 |
+---------------------------------------------------------------------+------+--------------+
quit;
]]>
</codeblock>
<p rev="2.3.0 IMPALA-1568 CDH-36799">
In Hive, create some new partitions. In a real use case, you might create the
partitions and populate them with data as the final stages of an ETL pipeline.
</p>
<codeblock rev="2.3.0 IMPALA-1568 CDH-36799">
<![CDATA[
hive> use recover_partitions;
OK
hive> alter table t1 add partition (yy = 2016, mm = 2);
OK
hive> alter table t1 add partition (yy = 2016, mm = 3);
OK
hive> quit;
]]>
</codeblock>
<p rev="2.3.0 IMPALA-1568 CDH-36799">
For demonstration purposes, manually copy data (a single row) into these
new partitions, using manual HDFS operations:
</p>
<codeblock rev="2.3.0 IMPALA-1568 CDH-36799">
<![CDATA[
$ hdfs dfs -ls /user/hive/warehouse/recover_partitions.db/t1/yy=2016/
Found 3 items
drwxr-xr-x - impala hive 0 2016-05-09 16:06 /user/hive/warehouse/recover_partitions.db/t1/yy=2016/mm=1
drwxr-xr-x - jrussell hive 0 2016-05-09 16:14 /user/hive/warehouse/recover_partitions.db/t1/yy=2016/mm=2
drwxr-xr-x - jrussell hive 0 2016-05-09 16:13 /user/hive/warehouse/recover_partitions.db/t1/yy=2016/mm=3
$ hdfs dfs -cp /user/hive/warehouse/recover_partitions.db/t1/yy=2016/mm=1/data.txt \
/user/hive/warehouse/recover_partitions.db/t1/yy=2016/mm=2/data.txt
$ hdfs dfs -cp /user/hive/warehouse/recover_partitions.db/t1/yy=2016/mm=1/data.txt \
/user/hive/warehouse/recover_partitions.db/t1/yy=2016/mm=3/data.txt
]]>
</codeblock>
<codeblock rev="2.3.0 IMPALA-1568 CDH-36799">
<![CDATA[
hive> select * from t1;
OK
Partition exists 2016 1
Partition exists 2016 2
Partition exists 2016 3
hive> quit;
]]>
</codeblock>
<p rev="2.3.0 IMPALA-1568 CDH-36799">
In Impala, initially the partitions and data are not visible.
Running <codeph>ALTER TABLE</codeph> with the <codeph>RECOVER PARTITIONS</codeph>
clause scans the table data directory to find any new partition directories, and
the data files inside them:
</p>
<codeblock rev="2.3.0 IMPALA-1568 CDH-36799">
<![CDATA[
select * from t1;
+------------------+------+----+
| s | yy | mm |
+------------------+------+----+
| Partition exists | 2016 | 1 |
+------------------+------+----+
alter table t1 recover partitions;
select * from t1;
+------------------+------+----+
| s | yy | mm |
+------------------+------+----+
| Partition exists | 2016 | 1 |
| Partition exists | 2016 | 3 |
| Partition exists | 2016 | 2 |
+------------------+------+----+
]]>
</codeblock>
<p rev="1.2">
<b>To change the key-value pairs of the TBLPROPERTIES and SERDEPROPERTIES fields:</b>
</p>
<codeblock>ALTER TABLE <varname>table_name</varname> SET TBLPROPERTIES ('<varname>key1</varname>'='<varname>value1</varname>', '<varname>key2</varname>'='<varname>value2</varname>'[, ...]);
ALTER TABLE <varname>table_name</varname> SET SERDEPROPERTIES ('<varname>key1</varname>'='<varname>value1</varname>', '<varname>key2</varname>'='<varname>value2</varname>'[, ...]);</codeblock>
<p>
The <codeph>TBLPROPERTIES</codeph> clause is primarily a way to associate arbitrary user-specified data items
with a particular table.
</p>
<p>
The <codeph>SERDEPROPERTIES</codeph> clause sets up metadata defining how tables are read or written, needed
in some cases by Hive but not used extensively by Impala. You would use this clause primarily to change the
delimiter in an existing text table or partition, by setting the <codeph>'serialization.format'</codeph> and
<codeph>'field.delim'</codeph> property values to the new delimiter character:
</p>
<codeblock>-- This table begins life as pipe-separated text format.
create table change_to_csv (s1 string, s2 string) row format delimited fields terminated by '|';
-- Then we change it to a CSV table.
alter table change_to_csv set SERDEPROPERTIES ('serialization.format'=',', 'field.delim'=',');
insert overwrite change_to_csv values ('stop','go'), ('yes','no');
!hdfs dfs -cat 'hdfs://<varname>hostname</varname>:8020/<varname>data_directory</varname>/<varname>dbname</varname>.db/change_to_csv/<varname>data_file</varname>';
stop,go
yes,no</codeblock>
<p>
Use the <codeph>DESCRIBE FORMATTED</codeph> statement to see the current values of these properties for an
existing table. See <xref href="impala_create_table.xml#create_table"/> for more details about these clauses.
See <xref href="impala_perf_stats.xml#perf_table_stats_manual"/> for an example of using table properties to
fine-tune the performance-related table statistics.
</p>
<p>
<b>To manually set or update table or column statistics:</b>
</p>
<p>
Although for most tables the <codeph>COMPUTE STATS</codeph> or <codeph>COMPUTE INCREMENTAL STATS</codeph>
statement is all you need to keep table and column statistics up to date for a table,
sometimes for a very large table or one that is updated frequently, the length of time to recompute
all the statistics might make it impractical to run those statements as often as needed.
As a workaround, you can use the <codeph>ALTER TABLE</codeph> statement to set table statistics
at the level of the entire table or a single partition, or column statistics at the level of
the entire table.
</p>
<p>
You can set the <codeph>numrows</codeph> value for table statistics by changing the
<codeph>TBLPROPERTIES</codeph> setting for a table or partition.
For example:
<codeblock conref="../shared/impala_common.xml#common/set_numrows_example"/>
<codeblock conref="../shared/impala_common.xml#common/set_numrows_partitioned_example"/>
See <xref href="impala_perf_stats.xml#perf_table_stats_manual"/> for details.
</p>
<p rev="2.6.0 IMPALA-3369">
In <keyword keyref="impala26_full"/> and higher, you can use the <codeph>SET COLUMN STATS</codeph> clause
to set a specific stats value for a particular column.
</p>
<p conref="../shared/impala_common.xml#common/set_column_stats_example"/>
<p>
<b>To reorganize columns for a table:</b>
</p>
<codeblock>ALTER TABLE <varname>table_name</varname> ADD COLUMNS (<varname>column_defs</varname>);
ALTER TABLE <varname>table_name</varname> REPLACE COLUMNS (<varname>column_defs</varname>);
ALTER TABLE <varname>table_name</varname> CHANGE <varname>column_name</varname> <varname>new_name</varname> <varname>new_type</varname>;
ALTER TABLE <varname>table_name</varname> DROP <varname>column_name</varname>;</codeblock>
<p>
The <varname>column_spec</varname> is the same as in the <codeph>CREATE TABLE</codeph> statement: the column
name, then its data type, then an optional comment. You can add multiple columns at a time. The parentheses
are required whether you add a single column or multiple columns. When you replace columns, all the original
column definitions are discarded. You might use this technique if you receive a new set of data files with
different data types or columns in a different order. (The data files are retained, so if the new columns are
incompatible with the old ones, use <codeph>INSERT OVERWRITE</codeph> or <codeph>LOAD DATA OVERWRITE</codeph>
to replace all the data before issuing any further queries.)
</p>
<p rev="CDH-37178">
For example, here is how you might add columns to an existing table.
The first <codeph>ALTER TABLE</codeph> adds two new columns, and the second
<codeph>ALTER TABLE</codeph> adds one new column.
A single Impala query reads both the old and new data files, containing different numbers of columns.
For any columns not present in a particular data file, all the column values are
considered to be <codeph>NULL</codeph>.
</p>
<codeblock rev="CDH-37178">
create table t1 (x int);
insert into t1 values (1), (2);
alter table t1 add columns (s string, t timestamp);
insert into t1 values (3, 'three', now());
alter table t1 add columns (b boolean);
insert into t1 values (4, 'four', now(), true);
select * from t1 order by x;
+---+-------+-------------------------------+------+
| x | s | t | b |
+---+-------+-------------------------------+------+
| 1 | NULL | NULL | NULL |
| 2 | NULL | NULL | NULL |
| 3 | three | 2016-05-11 11:19:45.054457000 | NULL |
| 4 | four | 2016-05-11 11:20:20.260733000 | true |
+---+-------+-------------------------------+------+
</codeblock>
<p>
You might use the <codeph>CHANGE</codeph> clause to rename a single column, or to treat an existing column as
a different type than before, such as to switch between treating a column as <codeph>STRING</codeph> and
<codeph>TIMESTAMP</codeph>, or between <codeph>INT</codeph> and <codeph>BIGINT</codeph>. You can only drop a
single column at a time; to drop multiple columns, issue multiple <codeph>ALTER TABLE</codeph> statements, or
define the new set of columns with a single <codeph>ALTER TABLE ... REPLACE COLUMNS</codeph> statement.
</p>
<p rev="CDH-37178">
The following examples show some safe operations to drop or change columns. Dropping the final column
in a table lets Impala ignore the data causing any disruption to existing data files. Changing the type
of a column works if existing data values can be safely converted to the new type. The type conversion
rules depend on the file format of the underlying table. For example, in a text table, the same value
can be interpreted as a <codeph>STRING</codeph> or a numeric value, while in a binary format such as
Parquet, the rules are stricter and type conversions only work between certain sizes of integers.
</p>
<codeblock rev="CDH-37178">
create table optional_columns (x int, y int, z int, a1 int, a2 int);
insert into optional_columns values (1,2,3,0,0), (2,3,4,100,100);
-- When the last column in the table is dropped, Impala ignores the
-- values that are no longer needed. (Dropping A1 but leaving A2
-- would cause problems, as we will see in a subsequent example.)
alter table optional_columns drop column a2;
alter table optional_columns drop column a1;
select * from optional_columns;
+---+---+---+
| x | y | z |
+---+---+---+
| 1 | 2 | 3 |
| 2 | 3 | 4 |
+---+---+---+
</codeblock>
<codeblock rev="CDH-37178">
create table int_to_string (s string, x int);
insert into int_to_string values ('one', 1), ('two', 2);
-- What was an INT column will now be interpreted as STRING.
-- This technique works for text tables but not other file formats.
-- The second X represents the new name of the column, which we keep the same.
alter table int_to_string change x x string;
-- Once the type is changed, we can insert non-integer values into the X column
-- and treat that column as a string, for example by uppercasing or concatenating.
insert into int_to_string values ('three', 'trois');
select s, upper(x) from int_to_string;
+-------+----------+
| s | upper(x) |
+-------+----------+
| one | 1 |
| two | 2 |
| three | TROIS |
+-------+----------+
</codeblock>
<p rev="CDH-37178">
Remember that Impala does not actually do any conversion for the underlying data files as a result of
<codeph>ALTER TABLE</codeph> statements. If you use <codeph>ALTER TABLE</codeph> to create a table
layout that does not agree with the contents of the underlying files, you must replace the files
yourself, such as using <codeph>LOAD DATA</codeph> to load a new set of data files, or
<codeph>INSERT OVERWRITE</codeph> to copy from another table and replace the original data.
</p>
<p rev="CDH-37178">
The following example shows what happens if you delete the middle column from a Parquet table containing three columns.
The underlying data files still contain three columns of data. Because the columns are interpreted based on their positions in
the data file instead of the specific column names, a <codeph>SELECT *</codeph> query now reads the first and second
columns from the data file, potentially leading to unexpected results or conversion errors.
For this reason, if you expect to someday drop a column, declare it as the last column in the table, where its data
can be ignored by queries after the column is dropped. Or, re-run your ETL process and create new data files
if you drop or change the type of a column in a way that causes problems with existing data files.
</p>
<codeblock rev="CDH-37178">
-- Parquet table showing how dropping a column can produce unexpected results.
create table p1 (s1 string, s2 string, s3 string) stored as parquet;
insert into p1 values ('one', 'un', 'uno'), ('two', 'deux', 'dos'),
('three', 'trois', 'tres');
select * from p1;
+-------+-------+------+
| s1 | s2 | s3 |
+-------+-------+------+
| one | un | uno |
| two | deux | dos |
| three | trois | tres |
+-------+-------+------+
alter table p1 drop column s2;
-- The S3 column contains unexpected results.
-- Because S2 and S3 have compatible types, the query reads
-- values from the dropped S2, because the existing data files
-- still contain those values as the second column.
select * from p1;
+-------+-------+
| s1 | s3 |
+-------+-------+
| one | un |
| two | deux |
| three | trois |
+-------+-------+
</codeblock>
<codeblock rev="CDH-37178">
-- Parquet table showing how dropping a column can produce conversion errors.
create table p2 (s1 string, x int, s3 string) stored as parquet;
insert into p2 values ('one', 1, 'uno'), ('two', 2, 'dos'), ('three', 3, 'tres');
select * from p2;
+-------+---+------+
| s1 | x | s3 |
+-------+---+------+
| one | 1 | uno |
| two | 2 | dos |
| three | 3 | tres |
+-------+---+------+
alter table p2 drop column x;
select * from p2;
WARNINGS:
File '<varname>hdfs_filename</varname>' has an incompatible Parquet schema for column 'add_columns.p2.s3'.
Column type: STRING, Parquet schema:
optional int32 x [i:1 d:1 r:0]
File '<varname>hdfs_filename</varname>' has an incompatible Parquet schema for column 'add_columns.p2.s3'.
Column type: STRING, Parquet schema:
optional int32 x [i:1 d:1 r:0]
</codeblock>
<p rev="IMPALA-3092">
In <keyword keyref="impala26_full"/> and higher, if an Avro table is created without column definitions in the
<codeph>CREATE TABLE</codeph> statement, and columns are later
added through <codeph>ALTER TABLE</codeph>, the resulting
table is now queryable. Missing values from the newly added
columns now default to <codeph>NULL</codeph>.
</p>
<p>
<b>To change the file format that Impala expects data to be in, for a table or partition:</b>
</p>
<p>
Use an <codeph>ALTER TABLE ... SET FILEFORMAT</codeph> clause. You can include an optional <codeph>PARTITION
(<varname>col1</varname>=<varname>val1</varname>, <varname>col2</varname>=<varname>val2</varname>,
...</codeph> clause so that the file format is changed for a specific partition rather than the entire table.
</p>
<p>
Because this operation only changes the table metadata, you must do any conversion of existing data using
regular Hadoop techniques outside of Impala. Any new data created by the Impala <codeph>INSERT</codeph>
statement will be in the new format. You cannot specify the delimiter for Text files; the data files must be
comma-delimited.
<!-- Although Impala can read Avro tables
created through Hive, you cannot specify the Avro file format in an Impala
<codeph>ALTER TABLE</codeph> statement. -->
</p>
<p>
To set the file format for a single partition, include the <codeph>PARTITION</codeph> clause. Specify all the
same partitioning columns for the table, with a constant value for each, to precisely identify the single
partition affected by the statement:
</p>
<codeblock>create table p1 (s string) partitioned by (month int, day int);
-- Each ADD PARTITION clause creates a subdirectory in HDFS.
alter table p1 add partition (month=1, day=1);
alter table p1 add partition (month=1, day=2);
alter table p1 add partition (month=2, day=1);
alter table p1 add partition (month=2, day=2);
-- Queries and INSERT statements will read and write files
-- in this format for this specific partition.
alter table p1 partition (month=2, day=2) set fileformat parquet;
</codeblock>
<p>
<b>To add or drop partitions for a table</b>, the table must already be partitioned (that is, created with a
<codeph>PARTITIONED BY</codeph> clause). The partition is a physical directory in HDFS, with a name that
encodes a particular column value (the <b>partition key</b>). The Impala <codeph>INSERT</codeph> statement
already creates the partition if necessary, so the <codeph>ALTER TABLE ... ADD PARTITION</codeph> is
primarily useful for importing data by moving or copying existing data files into the HDFS directory
corresponding to a partition. (You can use the <codeph>LOAD DATA</codeph> statement to move files into the
partition directory, or <codeph>ALTER TABLE ... PARTITION (...) SET LOCATION</codeph> to point a partition at
a directory that already contains data files.
</p>
<p>
The <codeph>DROP PARTITION</codeph> clause is used to remove the HDFS directory and associated data files for
a particular set of partition key values; for example, if you always analyze the last 3 months worth of data,
at the beginning of each month you might drop the oldest partition that is no longer needed. Removing
partitions reduces the amount of metadata associated with the table and the complexity of calculating the
optimal query plan, which can simplify and speed up queries on partitioned tables, particularly join queries.
Here is an example showing the <codeph>ADD PARTITION</codeph> and <codeph>DROP PARTITION</codeph> clauses.
</p>
<p>
To avoid errors while adding or dropping partitions whose existence is not certain,
add the optional <codeph>IF [NOT] EXISTS</codeph> clause between the <codeph>ADD</codeph> or
<codeph>DROP</codeph> keyword and the <codeph>PARTITION</codeph> keyword. That is, the entire
clause becomes <codeph>ADD IF NOT EXISTS PARTITION</codeph> or <codeph>DROP IF EXISTS PARTITION</codeph>.
The following example shows how partitions can be created automatically through <codeph>INSERT</codeph>
statements, or manually through <codeph>ALTER TABLE</codeph> statements. The <codeph>IF [NOT] EXISTS</codeph>
clauses let the <codeph>ALTER TABLE</codeph> statements succeed even if a new requested partition already
exists, or a partition to be dropped does not exist.
</p>
<p>
Inserting 2 year values creates 2 partitions:
</p>
<codeblock>
create table partition_t (s string) partitioned by (y int);
insert into partition_t (s,y) values ('two thousand',2000), ('nineteen ninety',1990);
show partitions partition_t;
+-------+-------+--------+------+--------------+-------------------+--------+-------------------+
| y | #Rows | #Files | Size | Bytes Cached | Cache Replication | Format | Incremental stats |
+-------+-------+--------+------+--------------+-------------------+--------+-------+
| 1990 | -1 | 1 | 16B | NOT CACHED | NOT CACHED | TEXT | false |
| 2000 | -1 | 1 | 13B | NOT CACHED | NOT CACHED | TEXT | false |
| Total | -1 | 2 | 29B | 0B | | | |
+-------+-------+--------+------+--------------+-------------------+--------+-------+
</codeblock>
<p>
Without the <codeph>IF NOT EXISTS</codeph> clause, an attempt to add a new partition might fail:
</p>
<codeblock>
alter table partition_t add partition (y=2000);
ERROR: AnalysisException: Partition spec already exists: (y=2000).
</codeblock>
<p>
The <codeph>IF NOT EXISTS</codeph> clause makes the statement succeed whether or not there was already a
partition with the specified key value:
</p>
<codeblock>
alter table partition_t add if not exists partition (y=2000);
alter table partition_t add if not exists partition (y=2010);
show partitions partition_t;
+-------+-------+--------+------+--------------+-------------------+--------+-------------------+
| y | #Rows | #Files | Size | Bytes Cached | Cache Replication | Format | Incremental stats |
+-------+-------+--------+------+--------------+-------------------+--------+-------+
| 1990 | -1 | 1 | 16B | NOT CACHED | NOT CACHED | TEXT | false |
| 2000 | -1 | 1 | 13B | NOT CACHED | NOT CACHED | TEXT | false |
| 2010 | -1 | 0 | 0B | NOT CACHED | NOT CACHED | TEXT | false |
| Total | -1 | 2 | 29B | 0B | | | |
+-------+-------+--------+------+--------------+-------------------+--------+-------+
</codeblock>
<p>
Likewise, the <codeph>IF EXISTS</codeph> clause lets <codeph>DROP PARTITION</codeph> succeed whether or not the partition is already
in the table:
</p>
<codeblock>
alter table partition_t drop if exists partition (y=2000);
alter table partition_t drop if exists partition (y=1950);
show partitions partition_t;
+-------+-------+--------+------+--------------+-------------------+--------+-------------------+
| y | #Rows | #Files | Size | Bytes Cached | Cache Replication | Format | Incremental stats |
+-------+-------+--------+------+--------------+-------------------+--------+-------+
| 1990 | -1 | 1 | 16B | NOT CACHED | NOT CACHED | TEXT | false |
| 2010 | -1 | 0 | 0B | NOT CACHED | NOT CACHED | TEXT | false |
| Total | -1 | 1 | 16B | 0B | | | |
+-------+-------+--------+------+--------------+-------------------+--------+-------+
</codeblock>
<p rev="2.3.0"> The optional <codeph>PURGE</codeph> keyword, available in
<keyword keyref="impala23_full"/> and higher, is used with the <codeph>DROP
PARTITION</codeph> clause to remove associated HDFS data files
immediately rather than going through the HDFS trashcan mechanism. Use
this keyword when dropping a partition if it is crucial to remove the data
as quickly as possible to free up space, or if there is a problem with the
trashcan, such as the trash cannot being configured or being in a
different HDFS encryption zone than the data files. </p>
<!--
To do: Make example more general by partitioning by year/month/day.
Then could show inserting into fixed year, variable month and day;
dropping particular year/month/day partition.
-->
<codeblock>-- Create an empty table and define the partitioning scheme.
create table part_t (x int) partitioned by (month int);
-- Create an empty partition into which you could copy data files from some other source.
alter table part_t add partition (month=1);
-- After changing the underlying data, issue a REFRESH statement to make the data visible in Impala.
refresh part_t;
-- Later, do the same for the next month.
alter table part_t add partition (month=2);
-- Now you no longer need the older data.
alter table part_t drop partition (month=1);
-- If the table was partitioned by month and year, you would issue a statement like:
-- alter table part_t drop partition (year=2003,month=1);
-- which would require 12 ALTER TABLE statements to remove a year's worth of data.
-- If the data files for subsequent months were in a different file format,
-- you could set a different file format for the new partition as you create it.
alter table part_t add partition (month=3) set fileformat=parquet;
</codeblock>
<p>
The value specified for a partition key can be an arbitrary constant expression, without any references to
columns. For example:
</p>
<codeblock>alter table time_data add partition (month=concat('Decem','ber'));
alter table sales_data add partition (zipcode = cast(9021 * 10 as string));</codeblock>
<note>
<p>
An alternative way to reorganize a table and its associated data files is to use <codeph>CREATE
TABLE</codeph> to create a variation of the original table, then use <codeph>INSERT</codeph> to copy the
transformed or reordered data to the new table. The advantage of <codeph>ALTER TABLE</codeph> is that it
avoids making a duplicate copy of the data files, allowing you to reorganize huge volumes of data in a
space-efficient way using familiar Hadoop techniques.
</p>
</note>
<p>
<b>To switch a table between internal and external:</b>
</p>
<p conref="../shared/impala_common.xml#common/switch_internal_external_table"/>
<p conref="../shared/impala_common.xml#common/cancel_blurb_no"/>
<p conref="../shared/impala_common.xml#common/permissions_blurb"/>
<p rev="CDH-19187">
Most <codeph>ALTER TABLE</codeph> clauses do not actually
read or write any HDFS files, and so do not depend on
specific HDFS permissions. For example, the <codeph>SET FILEFORMAT</codeph>
clause does not actually check the file format existing data files or
convert them to the new format, and the <codeph>SET LOCATION</codeph> clause
does not require any special permissions on the new location.
(Any permission-related failures would come later, when you
actually query or insert into the table.)
</p>
<!-- Haven't rigorously tested all the assertions in the following paragraph. -->
<!-- Most testing so far has been around RENAME TO clause. -->
<p>
In general, <codeph>ALTER TABLE</codeph> clauses that do touch
HDFS files and directories require the same HDFS permissions
as corresponding <codeph>CREATE</codeph>, <codeph>INSERT</codeph>,
or <codeph>SELECT</codeph> statements.
The permissions allow
the user ID that the <cmdname>impalad</cmdname> daemon runs under,
typically the <codeph>impala</codeph> user, to read or write
files or directories, or (in the case of the execute bit) descend into a directory.
The <codeph>RENAME TO</codeph> clause requires read, write, and execute permission in the
source and destination database directories and in the table data directory,
and read and write permission for the data files within the table.
The <codeph>ADD PARTITION</codeph> and <codeph>DROP PARTITION</codeph> clauses
require write and execute permissions for the associated partition directory.
</p>
<p conref="../shared/impala_common.xml#common/related_info"/>
<p>
<xref href="impala_tables.xml#tables"/>,
<xref href="impala_create_table.xml#create_table"/>, <xref href="impala_drop_table.xml#drop_table"/>,
<xref href="impala_partitioning.xml#partitioning"/>, <xref href="impala_tables.xml#internal_tables"/>,
<xref href="impala_tables.xml#external_tables"/>
</p>
</conbody>
</concept>
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