Some of your data may already reside in an external SQL database. PXF provides access to this data via the PXF JDBC connector. The JDBC connector is a JDBC client. It can read data from and write data to SQL databases including MySQL, ORACLE, Microsoft SQL Server, DB2, PostgreSQL, Hive, and Apache Ignite.
This section describes how to use the PXF JDBC connector to access data in an external SQL database, including how to create and query or insert data into a PXF external table that references a table in an external database.
INSERToperation fails, some data may be written to the external database table. If you require consistency for writes, consider writing to a staging table in the external database, and loading to the target table only after verifying the write operation.
Before you access an external SQL database using the PXF JDBC connector, ensure that:
The PXF JDBC connector supports the following data types:
Any data type not listed above is not supported by the PXF JDBC connector.
PXF includes version 1.1.0 of the Hive JDBC driver. This version does not support the following data types when you use the PXF JDBC connector to operate on a Hive table:
|Data Type||Fixed in Hive JDBC Driver||Upstream Issue||Operations Not Supported|
The PXF JDBC connector supports a single profile named
jdbc. You can both read data from and write data to an external SQL database table with this profile. You can also use the connector to run a static, named query in external SQL database and read the results.
To access data in a remote SQL database, you create a readable or writable Greenplum Database external table that references the remote database table. The Greenplum Database external table and the remote database table or query result tuple must have the same definition; the column names and types must match.
Use the following syntax to create a Greenplum Database external table that references a remote SQL database table or a query result from the remote database:
CREATE [READABLE | WRITABLE] EXTERNAL TABLE <table_name> ( <column_name> <data_type> [, ...] | LIKE <other_table> ) LOCATION ('pxf://<external-table-name>|query:<query_name>?PROFILE=jdbc[&SERVER=<server_name>][&<custom-option>=<value>[...]]') FORMAT 'CUSTOM' (FORMATTER='pxfwritable_import'|'pxfwritable_export');
The specific keywords and values used in the Greenplum Database CREATE EXTERNAL TABLE command are described in the table below.
|<external‑table‑name>||The full name of the external table. Depends on the external SQL database, may include a schema name and a table name.|
|query:<query_name>||The name of the query to execute in the remote SQL database.|
|SERVER=<server_name>||The named server configuration that PXF uses to access the data. PXF uses the
|<custom‑option>=<value>||<custom-option> is profile-specific.
|FORMAT ‘CUSTOM’||The JDBC
Note: You cannot use the
HEADER option in your
FORMAT specification when you create a PXF external table.
You include JDBC connector custom options in the
LOCATION URI, prefacing each option with an ampersand
CREATE EXTERNAL TABLE <custom-option>s supported by the
jdbc profile include:
|BATCH_SIZE||Write||Integer that identifies the number of
|FETCH_SIZE||Read||Integer that identifies the number of rows to buffer when reading from an external SQL database. Read row batching is enabled by default. The default read fetch size for MySQL is
|QUERY_TIMEOUT||Read/Write||Integer that identifies the amount of time (in seconds) that the JDBC driver waits for a statement to execute. The default wait time is infinite.|
|POOL_SIZE||Write||Enable thread pooling on
|PARTITION_BY||Read||Enables read partitioning. The partition column, <column-name>:<column-type>. You may specify only one partition column. The JDBC connector supports
|QUOTE_COLUMNS||Read||Controls whether PXF should quote column names when constructing an SQL query to the external database. Specify
- includes special characters, or
- is mixed case and the external database does not support unquoted mixed case identifiers.
When the JDBC driver of the external SQL database supports it, batching of
INSERT operations may significantly increase performance.
Write batching is enabled by default, and the default batch size is 100. To disable batching or to modify the default batch size value, create the PXF external table with a
BATCH_SIZE=1- disables batching
BATCH_SIZE=(n>1)- sets the
When the external database JDBC driver does not support batching, the behaviour of the PXF JDBC connector depends on the
BATCH_SIZE setting as follows:
BATCH_SIZEomitted - The JDBC connector inserts without batching.
INSERToperation fails and the connector returns an error.
By default, the PXF JDBC connector automatically batches the rows it fetches from an external database table. The default row fetch size is 1000. To modify the default fetch size value, specify a
FETCH_SIZE when you create the PXF external table. For example:
If the external database JDBC driver does not support batching on read, you must explicitly disable read row batching by setting
The PXF JDBC connector can further increase write performance by processing
INSERT operations in multiple threads when threading is supported by the JDBC driver of the external SQL database.
Consider using batching together with a thread pool. When used together, each thread receives and processes one complete batch of data. If you use a thread pool without batching, each thread in the pool receives exactly one tuple.
The JDBC connector returns an error when any thread in the thread pool fails. Be aware that if an
INSERT operation fails, some data may be written to the external database table.
To disable or enable a thread pool and set the pool size, create the PXF external table with a
POOL_SIZE setting as follows:
POOL_SIZE=(n<1)- thread pool size is the number of CPUs in the system
POOL_SIZE=1- disable thread pooling
POOL_SIZE=(n>1)- set the
The PXF JDBC connector supports simultaneous read access from PXF instances running on multiple Greenplum Database hosts to an external SQL table. This feature is referred to as partitioning. Read partitioning is not enabled by default. To enable read partitioning, set the
INTERVAL custom options when you create the PXF external table.
PXF uses the
INTERVAL values and the
PARTITON_BY column that you specify to assign specific data rows in the external table to PXF instances running on the Greenplum Database segment hosts. This column selection is specific to PXF processing, and has no relationship to a partition column that you may have specified for the table in the external SQL database.
Example JDBC <custom-option> substrings that identify partitioning parameters:
&PARTITION_BY=id:int&RANGE=1:100&INTERVAL=5 &PARTITION_BY=year:int&RANGE=2011:2013&INTERVAL=1 &PARTITION_BY=createdate:date&RANGE=2013-01-01:2016-01-01&INTERVAL=1:month &PARTITION_BY=color:enum&RANGE=red:yellow:blue
When you enable partitioning, the PXF JDBC connector splits a
SELECT query into multiple subqueries that retrieve a subset of the data, each of which is called a fragment. The JDBC connector automatically adds extra query constraints (
WHERE expressions) to each fragment to guarantee that every tuple of data is retrieved from the external database exactly once.
For example, when a user queries a PXF external table created with a
LOCATION clause that specifies
&PARTITION_BY=id:int&RANGE=1:5&INTERVAL=2, PXF generates 5 fragments: two according to the partition settings and up to three implicitly generated fragments. The constraints associated with each fragment are as follows:
PXF distributes the fragments among Greenplum Database segments. A PXF instance running on a segment host spawns a thread for each segment on that host that services a fragment. If the number of fragments is less than or equal to the number of Greenplum segments configured on a segment host, a single PXF instance may service all of the fragments. Each PXF instance sends its results back to Greenplum Database, where they are collected and returned to the user.
When you specify the
PARTITION_BY option, tune the
INTERVAL value and unit based upon the optimal number of JDBC connections to the target database and the optimal distribution of external data across Greenplum Database segments. The
INTERVAL low boundary is driven by the number of Greenplum Database segments while the high boundary is driven by the acceptable number of JDBC connections to the target database. The
INTERVAL setting influences the number of fragments, and should ideally not be set too high nor too low. Testing with multiple values may help you select the optimal settings.
Refer to the following topics for examples on how to use PXF to read data from and write data to specific SQL databases:
The PXF JDBC Connector allows you to specify a statically-defined query to run against the remote SQL database. Consider using a named query when:
VIEWin the external database.
The Greenplum Database administrator defines a query and provides you with the query name to use when you create the external table. Instead of a table name, you specify
query:<query_name> in the
CREATE EXTERNAL TABLE
LOCATION clause to instruct the PXF JDBC connector to run the static query named
<query_name> in the remote SQL database.
PXF supports named queries only with readable external tables. You must create a unique Greenplum Database readable external table for each query that you want to run.
The names and types of the external table columns must exactly match the names, types, and order of the columns return by the query result. If the query returns the results of an aggregation or other function, be sure to use the
AS qualifier to specify a specific column name.
For example, suppose that you are working with PostgreSQL tables that have the following definitions:
CREATE TABLE customers(id int, name text, city text, state text); CREATE TABLE orders(customer_id int, amount int, month int, year int);
And this PostgreSQL query that the administrator named
SELECT c.name, sum(o.amount) AS total, o.month FROM customers c JOIN orders o ON c.id = o.customer_id WHERE c.state = 'CO' GROUP BY c.name, o.month
This query returns tuples of type
(name text, total int, month int). If the
order_rpt query is defined for the PXF JDBC server named
pgserver, you could create a Greenplum Database external table to read these query results as follows:
CREATE EXTERNAL TABLE orderrpt_frompg(name text, total int, month int) LOCATION ('pxf://query:order_rpt?PROFILE=jdbc&SERVER=pgserver&PARTITION_BY=month:int&RANGE=1:13&INTERVAL=3') FORMAT 'CUSTOM' (FORMATTER='pxfwritable_import');
This command references a query named
order_rpt defined in the
pgserver server configuration. It also specifies JDBC read partitioning options that provide PXF with the information that it uses to split/partition the query result data across its servers/segments.
For a more detailed example see Example: Using a Named Query with PostgreSQL.
The PXF JDBC connector automatically applies column projection and filter pushdown to external tables that reference named queries.
You can override certain properties in a JDBC server configuration for a specific external database table by directly specifying the custom option in the
CREATE EXTERNAL TABLE
|Custom Option Name||jdbc-site.xml Property Name|
Example JDBC connection strings specified via custom options:
CREATE EXTERNAL TABLE pxf_pgtbl(name text, orders int) LOCATION ('pxf://public.forpxf_table1?PROFILE=jdbc&JDBC_DRIVER=org.postgresql.Driver&DB_URL=jdbc:postgresql://pgserverhost:5432/pgtestdb&USER=pxfuser1&PASS=changeme') FORMAT 'CUSTOM' (FORMATTER='pxfwritable_export');
Refer to Configuration Property Precedence for detailed information about the precedence rules that PXF uses to obtain configuration property settings for a Greenplum Database user.