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PostgreSQL connector

postgresql.png

The PostgreSQL connector allows querying and creating tables in an external PostgreSQL database. This can be used to join data between different systems like PostgreSQL and Hive, or between different PostgreSQL instances.

Requirements

To connect to PostgreSQL, you need:

  • PostgreSQL 10.x or higher.
  • Network access from the Trino coordinator and workers to PostgreSQL. Port 5432 is the default port.

Configuration

The connector can query a database on a PostgreSQL server. Create a catalog properties file that specifies the PostgreSQL connector by setting the connector.name to postgresql.

For example, to access a database as the postgresql catalog, create the file etc/catalog/postgresql.properties. Replace the connection properties as appropriate for your setup:

connector.name=postgresql
connection-url=jdbc:postgresql://example.net:5432/database
connection-user=root
connection-password=secret

The connection-url defines the connection information and parameters to pass to the PostgreSQL JDBC driver. The parameters for the URL are available in the PostgreSQL JDBC driver documentation. Some parameters can have adverse effects on the connector behavior or not work with the connector.

The connection-user and connection-password are typically required and determine the user credentials for the connection, often a service user. You can use secrets to avoid actual values in the catalog properties files.

Connection security

If you have TLS configured with a globally-trusted certificate installed on your data source, you can enable TLS between your cluster and the data source by appending a parameter to the JDBC connection string set in the connection-url catalog configuration property.

For example, with version 42 of the PostgreSQL JDBC driver, enable TLS by appending the ssl=true parameter to the connection-url configuration property:

connection-url=jdbc:postgresql://example.net:5432/database?ssl=true

For more information on TLS configuration options, see the PostgreSQL JDBC driver documentation.

Multiple PostgreSQL databases or servers

The PostgreSQL connector can only access a single database within a PostgreSQL server. Thus, if you have multiple PostgreSQL databases, or want to connect to multiple PostgreSQL servers, you must configure multiple instances of the PostgreSQL connector.

To add another catalog, simply add another properties file to etc/catalog with a different name, making sure it ends in .properties. For example, if you name the property file sales.properties, Trino creates a catalog named sales using the configured connector.

General configuration properties

The following table describes general catalog configuration properties for the connector:

Property nameDescriptionDefault value
case-insensitive-name-matchingSupport case insensitive schema and table names.false
case-insensitive-name-matching.cache-ttl1m
case-insensitive-name-matching.config-filePath to a name mapping configuration file in JSON format that allows Trino to disambiguate between schemas and tables with similar names in different cases.null
case-insensitive-name-matching.refresh-periodFrequency with which Trino checks the name matching configuration file for changes.0 (refresh disabled)
metadata.cache-ttlDuration for which metadata, including table and column statistics, is cached.0 (caching disabled)
metadata.cache-missingCache the fact that metadata, including table and column statistics, is not availablefalse
metadata.cache-maximum-sizeMaximum number of objects stored in the metadata cache10000
write.batch-sizeMaximum number of statements in a batched execution. Do not change this setting from the default. Non-default values may negatively impact performance.1000

Procedures

  • system.flush_metadata_cache()

    Flush JDBC metadata caches. For example, the following system call flushes the metadata caches for all schemas in the example catalog

    USE example.myschema;
    CALL system.flush_metadata_cache();

Case insensitive matching

When case-insensitive-name-matching is set to true, Trino is able to query non-lowercase schemas and tables by maintaining a mapping of the lowercase name to the actual name in the remote system. However, if two schemas and/or tables have names that differ only in case (such as "customers" and "Customers") then Trino fails to query them due to ambiguity.

In these cases, use the case-insensitive-name-matching.config-file catalog configuration property to specify a configuration file that maps these remote schemas/tables to their respective Trino schemas/tables:

{
"schemas": [
{
"remoteSchema": "CaseSensitiveName",
"mapping": "case_insensitive_1"
},
{
"remoteSchema": "cASEsENSITIVEnAME",
"mapping": "case_insensitive_2"
}],
"tables": [
{
"remoteSchema": "CaseSensitiveName",
"remoteTable": "tablex",
"mapping": "table_1"
},
{
"remoteSchema": "CaseSensitiveName",
"remoteTable": "TABLEX",
"mapping": "table_2"
}]
}

Queries against one of the tables or schemes defined in the mapping attributes are run against the corresponding remote entity. For example, a query against tables in the case_insensitive_1 schema is forwarded to the CaseSensitiveName schema and a query against case_insensitive_2 is forwarded to the cASEsENSITIVEnAME schema.

At the table mapping level, a query on case_insensitive_1.table_1 as configured above is forwarded to CaseSensitiveName.tablex, and a query on case_insensitive_1.table_2 is forwarded to CaseSensitiveName.TABLEX.

By default, when a change is made to the mapping configuration file, Trino must be restarted to load the changes. Optionally, you can set the case-insensitive-name-mapping.refresh-period to have Trino refresh the properties without requiring a restart:

case-insensitive-name-mapping.refresh-period=30s

Non-transactional INSERT

The connector supports adding rows using INSERT statements </sql/insert>. By default, data insertion is performed by writing data to a temporary table. You can skip this step to improve performance and write directly to the target table. Set the insert.non-transactional-insert.enabled catalog property or the corresponding non_transactional_insert catalog session property to true.

Note that with this property enabled, data can be corrupted in rare cases where exceptions occur during the insert operation. With transactions disabled, no rollback can be performed.

Type mapping

The data type mappings are as follows:

PostgreSQLTrinoNotes
BITBOOLEAN
BOOLEANBOOLEAN
SMALLINTSMALLINT
INTEGERINTEGER
BIGINTBIGINT
REALDATE
DOUBLEDOUBLE

NUMERIC(p, s)

DECIMAL(p, s)

DECIMAL(p, s) is an alias ofNUMERIC(p, s). See PostgreSQL for more information.

CHAR(n)CHAR(n)
VARCHAR(n)VARCHAR(n)
ENUMVARCHAR
BINARYVARBINARY
DATEDATE
TIME(n)TIME(n)
TIMESTAMP(n)TIMESTAMP(n)
TIMESTAMPTZ(n)TIMESTAMP(n) WITH TIME ZONE
MONEYVARCHAR
UUIDUUID
JSONJSON
JSONBJSON
HSTOREMAP(VARCHAR, VARCHAR)
ARRAYDisabled, ARRAY or JSONSee PostgreSQL for more information.

Decimal type handling

DECIMAL types with precision larger than 38 can be mapped to a Trino DECIMAL by setting the decimal-mapping configuration property or the decimal_mapping session property to allow_overflow. The scale of the resulting type is controlled via the decimal-default-scale configuration property or the decimal-rounding-mode session property. The precision is always 38.

By default, values that require rounding or truncation to fit will cause a failure at runtime. This behavior is controlled via the decimal-rounding-mode configuration property or the decimal_rounding_mode session property, which can be set to UNNECESSARY (the default), UP, DOWN, CEILING, FLOOR, HALF_UP, HALF_DOWN, or HALF_EVEN (see RoundingMode).

Array type handling

The PostgreSQL array implementation does not support fixed dimensions whereas Trino support only arrays with fixed dimensions. You can configure how the PostgreSQL connector handles arrays with the postgresql.array-mapping configuration property in your catalog file or the array_mapping session property. The following values are accepted for this property:

  • DISABLED (default): array columns are skipped.
  • AS_ARRAY: array columns are interpreted as Trino ARRAY type, for array columns with fixed dimensions.
  • AS_JSON: array columns are interpreted as Trino JSON type, with no constraint on dimensions.

Type mapping configuration properties

The following properties can be used to configure how data types from the connected data source are mapped to Trino data types and how the metadata is cached in Trino.

Property nameDescriptionDefault value

unsupported-type-handling

Configure how unsupported column data types are handled:

  • IGNORE, column is not accessible.
  • CONVERT_TO_VARCHAR, column is converted to unboundedVARCHAR.

The respective catalog session property isunsupported_type_handling.

IGNORE

jdbc-types-mapped-to-varcharAllow forced mapping of comma separated lists of data types to convert to unbounded VARCHAR

Querying PostgreSQL

The PostgreSQL connector provides a schema for every PostgreSQL schema. You can see the available PostgreSQL schemas by running SHOW SCHEMAS:

SHOW SCHEMAS FROM postgresql;

If you have a PostgreSQL schema named web, you can view the tables in this schema by running SHOW TABLES:

SHOW TABLES FROM postgresql.web;

You can see a list of the columns in the clicks table in the web database using either of the following:

DESCRIBE postgresql.web.clicks;
SHOW COLUMNS FROM postgresql.web.clicks;

Finally, you can access the clicks table in the web schema:

SELECT * FROM postgresql.web.clicks;

If you used a different name for your catalog properties file, use that catalog name instead of postgresql in the above examples.

SQL support

The connector provides read access and write access to data and metadata in PostgreSQL. In addition to the globally available and read operation statements, the connector supports the following features:

  • INSERT
  • DELETE
  • TRUNCATE
  • sql-schema-table-management

SQL DELETE

If a WHERE clause is specified, the DELETE operation only works if the predicate in the clause can be fully pushed down to the data source.

ALTER TABLE

The connector does not support renaming tables across multiple schemas. For example, the following statement is supported:

ALTER TABLE catalog.schema_one.table_one RENAME TO catalog.schema_one.table_two

The following statement attempts to rename a table across schemas, and therefore is not supported:

ALTER TABLE catalog.schema_one.table_one RENAME TO catalog.schema_two.table_two

ALTER SCHEMA

The connector supports renaming a schema with the ALTER SCHEMA RENAME statement. ALTER SCHEMA SET AUTHORIZATION is not supported.

Performance

The connector includes a number of performance improvements, detailed in the following sections.

Table statistics

The PostgreSQL connector can use table and column statistics for cost based optimizations, to improve query processing performance based on the actual data in the data source.

The statistics are collected by PostgreSQL and retrieved by the connector.

To collect statistics for a table, execute the following statement in PostgreSQL.

ANALYZE table_schema.table_name;

Refer to PostgreSQL documentation for additional ANALYZE options.

Pushdown

The connector supports pushdown for a number of operations:

  • Pushdown
  • Pushdown
  • Pushdown

Aggregate pushdown for the following functions:

  • avg
  • count
  • max
  • min
  • sum
  • stddev
  • stddev_pop
  • stddev_samp
  • variance
  • var_pop
  • var_samp
  • covar_pop
  • covar_samp
  • corr
  • regr_intercept
  • regr_slope

Cost-based join pushdown

The connector supports cost-based join-pushdown to make intelligent decisions about whether to push down a join operation to the data source.

When cost-based join pushdown is enabled, the connector only pushes down join operations if the available /optimizer/statistics suggest that doing so improves performance. Note that if no table statistics are available, join operation pushdown does not occur to avoid a potential decrease in query performance.

The following table describes catalog configuration properties for join pushdown:

Property nameDescriptionDefault value
join-pushdown.enabledEnable join pushdown <join-pushdown>. Equivalent catalog session property <session-properties-definition> isjoin_pushdown_enabled.true
join-pushdown.strategyStrategy used to evaluate whether join operations are pushed down. Set to AUTOMATIC to enable cost-based join pushdown, orEAGER to push down joins whenever possible. Note thatEAGER can push down joins even when table statistics are unavailable, which may result in degraded query performance. Because of this, EAGER is only recommended for testing and troubleshooting purposes.AUTOMATIC

Predicate pushdown support

The connector does not support pushdown of range predicates, such as >, <, or BETWEEN, on columns with character string types like CHAR or VARCHAR. Equality predicates, such as IN or =, and inequality predicates, such as != on columns with textual types are pushed down. This ensures correctness of results since the remote data source may sort strings differently than Trino.

In the following example, the predicate of the first query is not pushed down since name is a column of type VARCHAR and > is a range predicate. The other queries are pushed down.

-- Not pushed down
SELECT * FROM nation WHERE name > 'CANADA';
-- Pushed down
SELECT * FROM nation WHERE name != 'CANADA';
SELECT * FROM nation WHERE name = 'CANADA';

There is experimental support to enable pushdown of range predicates on columns with character string types which can be enabled by setting the postgresql.experimental.enable-string-pushdown-with-collate catalog configuration property or the corresponding enable_string_pushdown_with_collate session property to true. Enabling this configuration will make the predicate of all the queries in the above example get pushed down.