Accessing an SQL Database (JDBC)

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.

Note: The JDBC connector does not guarantee consistency when writing to an external SQL database. Be aware that if an INSERT operation 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.

Prerequisites

Before you access an external SQL database using the PXF JDBC connector, ensure that:

• You can identify the PXF runtime configuration directory ($PXF_BASE).
• You have configured PXF, and PXF is running on each Greenplum Database host. See Configuring PXF for additional information.
• Connectivity exists between all Greenplum Database hosts and the external SQL database.
• You have configured your external SQL database for user access from all Greenplum Database hosts.
• You have registered any JDBC driver JAR dependencies.
• (Recommended) You have created one or more named PXF JDBC connector server configurations as described in Configuring the PXF JDBC Connector.

Data Types Supported

The PXF JDBC connector supports the following data types:

• INTEGER, BIGINT, SMALLINT
• REAL, FLOAT8
• NUMERIC
• BOOLEAN
• VARCHAR, BPCHAR, TEXT
• DATE
• TIMESTAMP
• BYTEA

Any data type not listed above is not supported by the PXF JDBC connector.

About Accessing Hive via JDBC

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
NUMERIC	2.3.0	HIVE-13614	Write
TIMESTAMP	2.0.0	HIVE-11748	Write
DATE	1.3.0, 2.0.0	HIVE-11024	Write
BYTEA	N/A	N/A	Read, Write

Accessing an External SQL Database

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.

Keyword	Value
<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 run in the remote SQL database.
PROFILE	The PROFILE keyword value must specify jdbc.
SERVER=<server_name>	The named server configuration that PXF uses to access the data. PXF uses the default server if not specified.
<custom‑option>=<value>	<custom-option> is profile-specific. jdbc profile-specific options are discussed in the next section.
FORMAT ‘CUSTOM’	The JDBC CUSTOM FORMAT supports the built-in 'pxfwritable_import' FORMATTER function for read operations and the built-in 'pxfwritable_export' function for write operations.

Note: You cannot use the HEADER option in your FORMAT specification when you create a PXF external table.

JDBC Custom Options

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:

Option Name	Operation	Description
BATCH_SIZE	Write	Integer that identifies the number of INSERT operations to batch to the external SQL database. Write batching is activated by default; the default value is 100.
FETCH_SIZE	Read	Integer that identifies the number of rows to buffer when reading from an external SQL database. Read row batching is activated by default. The default read fetch size for MySQL is -2147483648 (Integer.MIN_VALUE). The default read fetch size for all other databases is 1000.
QUERY_TIMEOUT	Read/Write	Integer that identifies the amount of time (in seconds) that the JDBC driver waits for a statement to run. The default wait time is infinite.
POOL_SIZE	Write	Activate thread pooling on INSERT operations and identify the number of threads in the pool. Thread pooling is deactivated by default.
PARTITION_BY	Read	Activates read partitioning. The partition column, <column-name>:<column-type>. You may specify only one partition column. The JDBC connector supports date, int, and enum <column-type> values, where int represents any JDBC integral type. If you do not identify a PARTITION_BY column, a single PXF instance services the read request.
RANGE	Read	Required when PARTITION_BY is specified. The query range; used as a hint to aid the creation of partitions. The RANGE format is dependent upon the data type of the partition column. When the partition column is an enum type, RANGE must specify a list of values, <value>:<value>[:<value>[…]], each of which forms its own fragment. If the partition column is an int or date type, RANGE must specify <start-value>:<end-value> and represents the interval from <start-value> through <end-value>, inclusive. The RANGE for an int partition column may span any 64-bit signed integer values. If the partition column is a date type, use the yyyy-MM-dd date format.
INTERVAL	Read	Required when PARTITION_BY is specified and of the int, bigint, or date type. The interval, <interval-value>[:<interval-unit>], of one fragment. Used with RANGE as a hint to aid the creation of partitions. Specify the size of the fragment in <interval-value>. If the partition column is a date type, use the <interval-unit> to specify year, month, or day. PXF ignores INTERVAL when the PARTITION_BY column is of the enum type.
QUOTE_COLUMNS	Read	Controls whether PXF should quote column names when constructing an SQL query to the external database. Specify true to force PXF to quote all column names; PXF does not quote column names if any other value is provided. If QUOTE_COLUMNS is not specified (the default), PXF automatically quotes all column names in the query when any column name: - includes special characters, or - is mixed case and the external database does not support unquoted mixed case identifiers.

Batching Insert Operations (Write)

When the JDBC driver of the external SQL database supports it, batching of INSERT operations may significantly increase performance.

Write batching is activated by default, and the default batch size is 100. To deactivate batching or to modify the default batch size value, create the PXF external table with a BATCH_SIZE setting:

• BATCH_SIZE=0 or BATCH_SIZE=1 - deactivates batching
• BATCH_SIZE=(n>1) - sets the BATCH_SIZE to n

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_SIZE omitted - The JDBC connector inserts without batching.
• BATCH_SIZE=(n>1) - The INSERT operation fails and the connector returns an error.

Batching on Read Operations

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:

FETCH_SIZE=5000

If the external database JDBC driver does not support batching on read, you must explicitly deactivate read row batching by setting FETCH_SIZE=0.

Thread Pooling (Write)

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 deactivate or activate 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 - deactivate thread pooling
• POOL_SIZE=(n>1)- set the POOL_SIZE to n

Partitioning (Read)

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 activated by default. To activate read partitioning, set the PARTITION_BY, RANGE, and INTERVAL custom options when you create the PXF external table.

PXF uses the RANGE and 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 activate 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:

• Fragment 1: WHERE (id < 1) - implicitly-generated fragment for RANGE start-bounded interval
• Fragment 2: WHERE (id >= 1) AND (id < 3) - fragment specified by partition settings
• Fragment 3: WHERE (id >= 3) AND (id < 5) - fragment specified by partition settings
• Fragment 4: WHERE (id >= 5) - implicitly-generated fragment for RANGE end-bounded interval
• Fragment 5: WHERE (id IS NULL) - implicitly-generated fragment

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.

Examples

Refer to the following topics for examples on how to use PXF to read data from and write data to specific SQL databases:

• Reading From and Writing to a PostgreSQL Table
• Reading From and Writing to a MySQL Table
• Reading From and Writing to an Oracle Table
• Reading From and Writing to a Trino Table

About Using Named Queries

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:

• You need to join several tables that all reside in the same external database.
• You want to perform complex aggregation closer to the data source.
• You would use, but are not allowed to create, a VIEW in the external database.
• You would rather consume computational resources in the external system to minimize utilization of Greenplum Database resources.
• You want to run a HIVE query and control resource utilization via YARN.

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 order_rpt:

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.

Overriding the JDBC Server Configuration with DDL

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 LOCATION clause:

Custom Option Name	jdbc-site.xml Property Name
JDBC_DRIVER	jdbc.driver
DB_URL	jdbc.url
USER	jdbc.user
PASS	jdbc.password
BATCH_SIZE	jdbc.statement.batchSize
FETCH_SIZE	jdbc.statement.fetchSize
QUERY_TIMEOUT	jdbc.statement.queryTimeout

Example JDBC connection strings specified via custom options:

&JDBC_DRIVER=org.postgresql.Driver&DB_URL=jdbc:postgresql://pgserverhost:5432/pgtestdb&USER=pguser1&PASS=changeme
&JDBC_DRIVER=com.mysql.jdbc.Driver&DB_URL=jdbc:mysql://mysqlhost:3306/testdb&USER=user1&PASS=changeme

For example:

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');

Warning: Credentials that you provide in this manner are visible as part of the external table definition. Do not use this method of passing credentials in a production environment.

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.