SingleStore connector#
The SingleStore (formerly known as MemSQL) connector allows querying and creating tables in an external SingleStore database.
Requirements#
To connect to SingleStore, you need:
SingleStore version 7.1.4 or higher.
Network access from the Trino coordinator and workers to SingleStore. Port 3306 is the default port.
Configuration#
To configure the SingleStore connector, create a catalog properties file in
etc/catalog
named, for example, example.properties
, to mount the
SingleStore connector as the example
catalog. Create the file with the
following contents, replacing the connection properties as appropriate for your
setup:
connector.name=singlestore
connection-url=jdbc:singlestore://example.net:3306
connection-user=root
connection-password=secret
The connection-url
defines the connection information and parameters to pass
to the SingleStore JDBC driver. The supported parameters for the URL are
available in the SingleStore JDBC driver documentation.
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.
Enable TLS between your cluster and SingleStore by appending the useSsl=true
parameter to the connection-url
configuration property:
connection-url=jdbc:singlestore://example.net:3306/?useSsl=true
For more information on TLS configuration options, see the JDBC driver documentation.
Multiple SingleStore servers#
You can have as many catalogs as you need, so if you have additional
SingleStore servers, 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
will create a catalog named sales
using the configured connector.
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 |
Appending query metadata#
The optional parameter query.comment-format
allows you to configure a SQL
comment that is sent to the datasource with each query. The format of this
comment can contain any characters and the following metadata:
$QUERY_ID
: The identifier of the query.$USER
: The name of the user who submits the query to Trino.$SOURCE
: The identifier of the client tool used to submit the query, for exampletrino-cli
.$TRACE_TOKEN
: The trace token configured with the client tool.
The comment can provide more context about the query. This additional
information is available in the logs of the datasource. To include environment
variables from the Trino cluster with the comment , use the
${ENV:VARIABLE-NAME}
syntax.
The following example sets a simple comment that identifies each query sent by Trino:
query.comment-format=Query sent by Trino.
With this configuration, a query such as SELECT * FROM example_table;
is
sent to the datasource with the comment appended:
SELECT * FROM example_table; /*Query sent by Trino.*/
The following example improves on the preceding example by using metadata:
query.comment-format=Query $QUERY_ID sent by user $USER from Trino.
If Jane
sent the query with the query identifier
20230622_180528_00000_bkizg
, the following comment string is sent to the
datasource:
SELECT * FROM example_table; /*Query 20230622_180528_00000_bkizg sent by user Jane from Trino.*/
Note
Certain JDBC driver settings and logging configurations might cause the comment to be removed.
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
32
for this threshold.
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
catalogUSE example.example_schema; 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.
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.
Querying SingleStore#
The SingleStore connector provides a schema for every SingleStore database.
You can see the available SingleStore databases by running SHOW SCHEMAS
:
SHOW SCHEMAS FROM example;
If you have a SingleStore database named web
, you can view the tables
in this database by running SHOW TABLES
:
SHOW TABLES FROM example.web;
You can see a list of the columns in the clicks
table in the web
database using either of the following:
DESCRIBE example.web.clicks;
SHOW COLUMNS FROM example.web.clicks;
Finally, you can access the clicks
table in the web
database:
SELECT * FROM example.web.clicks;
If you used a different name for your catalog properties file, use
that catalog name instead of example
in the above examples.
Type mapping#
Because Trino and Singlestore each support types that the other does not, this connector modifies some types when reading or writing 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.
Singlestore to Trino type mapping#
The connector maps Singlestore types to the corresponding Trino types following this table:
Singlestore type |
Trino type |
Notes |
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No other types are supported.
Trino to Singlestore type mapping#
The connector maps Trino types to the corresponding Singlestore types following this table:
Trino type |
Singlestore type |
Notes |
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No other types are supported.
Decimal type handling#
DECIMAL
types with unspecified precision or scale are mapped to a Trino
DECIMAL
with a default precision of 38 and default scale of 0. The scale can
be changed 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).
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 read access and write access to data and metadata in a SingleStore database. In addition to the globally available and read operation statements, the connector supports the following features:
UPDATE#
Only UPDATE
statements with constant assignments and predicates are
supported. For example, the following statement is supported because the values
assigned are constants:
UPDATE table SET col1 = 1 WHERE col3 = 1
Arithmetic expressions, function calls, and other non-constant UPDATE
statements are not supported. For example, the following statement is not
supported because arithmetic expressions cannot be used with the SET
command:
UPDATE table SET col1 = col2 + 2 WHERE col3 = 1
The =
, !=
, >
, <
, >=
, <=
, IN
, NOT IN
operators are supported in
predicates. The following statement is not supported because the AND
operator
cannot be used in predicates:
UPDATE table SET col1 = 1 WHERE col3 = 1 AND col2 = 3
All column values of a table row cannot be updated simultaneously. For a three column table, the following statement is not supported:
UPDATE table SET col1 = 1, col2 = 2, col3 = 3 WHERE col3 = 1
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 RENAME TO#
The connector does not support renaming tables across multiple schemas. For example, the following statement is supported:
ALTER TABLE example.schema_one.table_one RENAME TO example.schema_one.table_two
The following statement attempts to rename a table across schemas, and therefore is not supported:
ALTER TABLE example.schema_one.table_one RENAME TO example.schema_two.table_two
Performance#
The connector includes a number of performance improvements, detailed in the following sections.
Pushdown#
The connector supports pushdown for a number of operations:
Note
The connector performs pushdown where performance may be improved, but in order to preserve correctness an operation may not be pushed down. When pushdown of an operation may result in better performance but risks correctness, the connector prioritizes correctness.
Join pushdown#
The join-pushdown.enabled
catalog configuration property or
join_pushdown_enabled
catalog session property control whether the connector pushes
down join operations. The property defaults to false
, and enabling join
pushdowns may negatively impact performance for some queries.
Predicate pushdown support#
The connector does not support pushdown of any predicates on columns with
textual types like CHAR
or VARCHAR
.
This ensures correctness of results since the data source may compare strings
case-insensitively.
In the following example, the predicate is not pushed down for either query
since name
is a column of type VARCHAR
:
SELECT * FROM nation WHERE name > 'CANADA';
SELECT * FROM nation WHERE name = 'CANADA';