Exasol connector#
The Exasol connector allows querying an Exasol database.
Requirements#
To connect to Exasol, you need:
Exasol database version 7.1 or higher.
Network access from the Trino coordinator and workers to Exasol. Port 8563 is the default port.
Configuration#
To configure the Exasol connector as the example catalog, create a file
named example.properties in etc/catalog. Include the following
connection properties in the file:
connector.name=exasol
connection-url=jdbc:exa:exasol.example.com:8563
connection-user=user
connection-password=secret
The connection-url defines the connection information and parameters to pass
to the JDBC driver. See the
Exasol JDBC driver documentation
for more information.
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 using actual values in catalog
properties files.
Note
If your Exasol database uses a self-signed TLS certificate you must
specify the certificate’s fingerprint in the JDBC URL using parameter
fingerprint, e.g.: jdbc:exa:exasol.example.com:8563;fingerprint=ABC123.
Data source authentication#
The connector can provide credentials for the data source connection in multiple ways:
inline, in the connector configuration file
in a separate properties file
in a key store file
as extra credentials set when connecting to Trino
You can use secrets to avoid storing sensitive values in the catalog properties files.
The following table describes configuration properties for connection credentials:
Property name |
Description |
|---|---|
|
Type of the credential provider. Must be one of |
|
Connection user name. |
|
Connection password. |
|
Name of the extra credentials property, whose value to use as the user
name. See |
|
Name of the extra credentials property, whose value to use as the password. |
|
Location of the properties file where credentials are present. It must
contain the |
|
The location of the Java Keystore file, from which to read credentials. |
|
File format of the keystore file, for example |
|
Password for the key store. |
|
Name of the key store entity to use as the user name. |
|
Password for the user name key store entity. |
|
Name of the key store entity to use as the password. |
|
Password for the password key store entity. |
General configuration properties#
The following table describes general catalog configuration properties for the connector:
Property name |
Description |
|---|---|
|
Support case insensitive schema and table names. Defaults to |
|
Duration for which case insensitive schema and table
names are cached. Defaults to |
|
Path to a name mapping configuration file in JSON format that allows
Trino to disambiguate between schemas and tables with similar names in
different cases. Defaults to |
|
Frequency with which Trino checks the name matching configuration file
for changes. The duration value defaults to |
|
Duration for which metadata, including table and
column statistics, is cached. Defaults to |
|
Cache the fact that metadata, including table and column statistics, is
not available. Defaults to |
|
Duration for which schema metadata is cached.
Defaults to the value of |
|
Duration for which table metadata is cached.
Defaults to the value of |
|
Duration for which tables statistics are cached.
Defaults to the value of |
|
Maximum number of objects stored in the metadata cache. Defaults to |
|
Maximum number of statements in a batched execution. Do not change
this setting from the default. Non-default values may negatively
impact performance. Defaults to |
|
Push down dynamic filters into JDBC queries. Defaults to |
|
Maximum duration for which Trino waits for dynamic
filters to be collected from the build side of joins before starting a
JDBC query. Using a large timeout can potentially result in more detailed
dynamic filters. However, it can also increase latency for some queries.
Defaults to |
Domain compaction threshold#
Pushing down a large list of predicates to the data source can compromise
performance. Trino compacts large predicates into a simpler range predicate
by default to ensure a balance between performance and predicate pushdown.
If necessary, the threshold for this compaction can be increased to improve
performance when the data source is capable of taking advantage of large
predicates. Increasing this threshold may improve pushdown of large
dynamic filters.
The domain-compaction-threshold catalog configuration property or the
domain_compaction_threshold catalog session property can be used to adjust the default value of
256 for this threshold.
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
Type mapping#
Because Trino and Exasol each support types that the other does not, this connector modifies some types when reading data. Data types may not map the same way in both directions between Trino and the data source. Refer to the following sections for type mapping in each direction.
Exasol to Trino type mapping#
Trino supports selecting Exasol database types. This table shows the Exasol to Trino data type mapping:
Exasol database type |
Trino type |
Notes |
|---|---|---|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
No other types are supported.
Mapping numeric types#
An Exasol DECIMAL(p, s) maps to Trino’s DECIMAL(p, s) and vice versa
except in these conditions:
No precision is specified for the column (example:
DECIMALorDECIMAL(*)).Scale (
s) is greater than precision.Precision (
p) is greater than 36.Scale is negative.
Mapping character types#
Trino’s VARCHAR(n) maps to VARCHAR(n) and vice versa if n is no greater
than 2000000. Exasol does not support longer values.
If no length is specified, the connector uses 2000000.
Trino’s CHAR(n) maps to CHAR(n) and vice versa if n is no greater than 2000.
Exasol does not support longer values.
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 name |
Description |
Default value |
|---|---|---|
|
Configure how unsupported column data types are handled:
The respective catalog session property is |
|
|
Allow forced mapping of comma separated lists of data types to convert to
unbounded |
SQL support#
The connector provides globally available and read operation statements to access data and metadata in the Exasol database.
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.example_schema;
CALL system.flush_metadata_cache();
system.execute('query')#
The execute procedure allows you to execute a query in the underlying data
source directly. The query must use supported syntax of the connected data
source. Use the procedure to access features which are not available in Trino
or to execute queries that return no result set and therefore can not be used
with the query or raw_query pass-through table function. Typical use cases
are statements that create or alter objects, and require native feature such
as constraints, default values, automatic identifier creation, or indexes.
Queries can also invoke statements that insert, update, or delete data, and do
not return any data as a result.
The query text is not parsed by Trino, only passed through, and therefore only subject to any security or access control of the underlying data source.
The following example sets the current database to the example_schema of the
example catalog. Then it calls the procedure in that schema to drop the
default value from your_column on your_table table using the standard SQL
syntax in the parameter value assigned for query:
USE example.example_schema;
CALL system.execute(query => 'ALTER TABLE your_table ALTER COLUMN your_column DROP DEFAULT');
Verify that the specific database supports this syntax, and adapt as necessary based on the documentation for the specific connected database and database version.
Table functions#
The connector provides specific table functions to access Exasol.
query(varchar) -> table#
The query function allows you to query the underlying database directly. It
requires syntax native to Exasol, because the full query is pushed down and
processed in Exasol. This can be useful for accessing native features which are
not available in Trino or for improving query performance in situations where
running a query natively may be faster.
The native query passed to the underlying data source is required to return a table as a result set. Only the data source performs validation or security checks for these queries using its own configuration. Trino does not perform these tasks. Only use passthrough queries to read data.
As a simple example, query the example catalog and select an entire table::
SELECT
*
FROM
TABLE(
example.system.query(
query => 'SELECT
*
FROM
tpch.nation'
)
);
As a practical example, you can use the WINDOW clause from Exasol:
SELECT
*
FROM
TABLE(
example.system.query(
query => 'SELECT
id, department, hire_date, starting_salary,
AVG(starting_salary) OVER w2 AVG,
MIN(starting_salary) OVER w2 MIN_STARTING_SALARY,
MAX(starting_salary) OVER (w1 ORDER BY hire_date)
FROM employee_table
WINDOW w1 as (PARTITION BY department), w2 as (w1 ORDER BY hire_date)
ORDER BY department, hire_date'
)
);
Note
The query engine does not preserve the order of the results of this
function. If the passed query contains an ORDER BY clause, the
function result may not be ordered as expected.