SingleStore connector#

The SingleStore (formerly known as MemSQL) connector lets you query and create tables in an external SingleStore database.

SEP includes additional enterprise features that are built on top of the existing Trino connector functionality. For more information on connector key feature differences between Trino and SEP, see the connectors feature matrix.

Requirements#

To connect to SingleStore, you need:

  • SingleStore version 7.1.4 or higher.

  • Network access from the SEP coordinator and workers to SingleStore. Port 3306 is the default port.

  • A valid Starburst Enterprise license.

Configuration#

To configure the Singlestore connector, create a catalog properties file that specifies the Singlestore connector by setting the connector.name to Singlestore.

For example, to access a database as the example catalog, create the file etc/catalog/example.properties. Replace 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 exposing 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.

To enable TLS append useSsl=true 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. If you have additional SingleStore servers, configure another catalog.

To add another catalog, add a new properties file to etc/catalog. For example, if you name the property file sales.properties, SEP creates a catalog named sales.

General configuration properties#

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

Property name

Description

case-insensitive-name-matching

Support case insensitive schema and table names. Defaults to false.

case-insensitive-name-matching.cache-ttl

Duration for which case insensitive schema and table names are cached. Defaults to 1m.

case-insensitive-name-matching.config-file

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 null.

case-insensitive-name-matching.config-file.refresh-period

Frequency with which Trino checks the name matching configuration file for changes. The duration value defaults to 0s (refresh disabled).

metadata.cache-ttl

Duration for which metadata, including table and column statistics, is cached. Defaults to 0s (caching disabled).

metadata.cache-missing

Cache the fact that metadata, including table and column statistics, is not available. Defaults to false.

metadata.schemas.cache-ttl

Duration for which schema metadata is cached. Defaults to the value of metadata.cache-ttl.

metadata.tables.cache-ttl

Duration for which table metadata is cached. Defaults to the value of metadata.cache-ttl.

metadata.statistics.cache-ttl

Duration for which tables statistics are cached. Defaults to the value of metadata.cache-ttl.

metadata.cache-maximum-size

Maximum number of objects stored in the metadata cache. Defaults to 10000.

write.batch-size

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 1000.

dynamic-filtering.enabled

Push down dynamic filters into JDBC queries. Defaults to true.

dynamic-filtering.wait-timeout

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 20s.

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 example trino-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 catalog

    USE 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.

Run SHOW SCHEMAS to see the available SingleStore databases:

SHOW SCHEMAS FROM example;

Examples#

If you used a different name for your catalog properties file, use that catalog name instead of example.

If you have a SingleStore database named web, run SHOW TABLES to see the tables it contains:

SHOW TABLES FROM example.web;

To see a list of the columns in the clicks table in the web database, run either of the following:

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

To access the clicks table in the web database, run the following:

SELECT * FROM example.web.clicks;

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 to Trino type mapping#

Singlestore type

Trino type

Notes

BIT

BOOLEAN

BOOLEAN

BOOLEAN

TINYINT

TINYINT

TINYINT UNSIGNED

SMALLINT

SMALLINT

SMALLINT

SMALLINT UNSIGNED

INTEGER

INTEGER

INTEGER

INTEGER UNSIGNED

BIGINT

BIGINT

BIGINT

BIGINT UNSIGNED

DECIMAL(20, 0)

DOUBLE

DOUBLE

REAL

DOUBLE

DECIMAL(p, s)

DECIMAL(p, s)

See Singlestore DECIMAL type handling

CHAR(n)

CHAR(n)

TINYTEXT

VARCHAR(255)

TEXT

VARCHAR(65535)

MEDIUMTEXT

VARCHAR(16777215)

LONGTEXT

VARCHAR

VARCHAR(n)

VARCHAR(n)

LONGBLOB

VARBINARY

DATE

DATE

TIME

TIME(0)

TIME(6)

TIME(6)

DATETIME

TIMESTAMP(0)

DATETIME(6)

TIMESTAMP(6)

JSON

JSON

No other types are supported.

Trino to Singlestore type mapping#

The connector maps Trino types to the corresponding Singlestore types following this table:

Trino to Singlestore type mapping#

Trino type

Singlestore type

Notes

BOOLEAN

BOOLEAN

TINYINT

TINYINT

SMALLINT

SMALLINT

INTEGER

INTEGER

BIGINT

BIGINT

DOUBLE

DOUBLE

REAL

FLOAT

DECIMAL(p, s)

DECIMAL(p, s)

See Singlestore DECIMAL type handling

CHAR(n)

CHAR(n)

VARCHAR(65535)

TEXT

VARCHAR(16777215)

MEDIUMTEXT

VARCHAR

LONGTEXT

VARCHAR(n)

VARCHAR(n)

VARBINARY

LONGBLOB

DATE

DATE

TIME(0)

TIME

TIME(6)

TIME(6)

TIMESTAMP(0)

DATETIME

TIMESTAMP(6)

DATETIME(6)

JSON

JSON

No other types are supported.

Decimal type handling#

DECIMAL types with unspecified precision or scale are ignored unless the decimal-mapping configuration property or the decimal_mapping session property is set to allow_overflow. Then such types are mapped to a Trino DECIMAL with a default precision of 38 and default scale of 0. To change the scale of the resulting type, use the decimal-default-scale configuration property or the decimal_default_scale 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

unsupported-type-handling

Configure how unsupported column data types are handled:

  • IGNORE, column is not accessible.

  • CONVERT_TO_VARCHAR, column is converted to unbounded VARCHAR.

The respective catalog session property is unsupported_type_handling.

IGNORE

jdbc-types-mapped-to-varchar

Allow forced mapping of comma separated lists of data types to convert to unbounded VARCHAR

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 EXECUTE#

This connector supports the following commands for use with ALTER TABLE EXECUTE:

collect_statistics#

The collect_statistics command is used with Managed statistics to collect statistics for a table and its columns.

The following statement collects statistics for the example_table table and all of its columns:

ALTER TABLE example_table EXECUTE collect_statistics;

Collecting statistics for all columns in a table may be unnecessarily performance-intensive, especially for wide tables. To only collect statistics for a subset of columns, you can include the columns parameter with an array of column names. For example:

ALTER TABLE example_table
    EXECUTE collect_statistics(columns => ARRAY['customer','line_item']);

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

Fault-tolerant execution support#

The connector supports Fault-tolerant execution of query processing. Read and write operations are both supported with any retry policy.

Performance#

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

Parallelism#

The connector is able to read data from SingleStore using multiple parallel connections for tables partitioned as described in the SingleStore documentation.

Parallelism is disabled by default. Set the following catalog configuration property to enable parallelism:

singlestore.parallelism-type=RESULT_TABLE_PARALLELISM

When this feature is enabled, SEP reads each partition of a SingleStore table in parallel and uses materialized result tables.

Table statistics#

The SingleStore 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 SingleStore and retrieved by the connector.

Table-level statistics are based on SingleStore’s INFORMATION_SCHEMA.TABLE_STATISTICS table.

SingleStore can automatically update its table and column statistics. In some cases, you may want to force a statistics update. For example, after creating new columns or after changing data in the table.

To force an update, run the following statement in the SingleStore database:

ANALYZE TABLE table_name;

Note

SingleStore statistics are estimates, and SEP and SingleStore may use statistics information in different ways. For this reason, the accuracy of table and column statistics returned by the SingleStore connector might be lower than that of others connectors.

Improving statistics accuracy

You can improve statistics accuracy and access column-level statistics with histogram statistics. Column-level statistics are based on SingleStore’s column statistics INFORMATION_SCHEMA.ADVANCED_HISTOGRAMS table. If that table is not available, the information is based on the INFORMATION_SCHEMA.OPTIMIZER_STATISTICS table.

The ADVANCED_HISTOGRAMS table includes additional stats such as the MIN and MAX values for a column. These statistics are not available in the LEGACY_HISTOGRAMS table. SingleStore’s ADVANCED_HISTOGRAMS feature requires a cardinality_estimation_level greater than or equal to 6.5.

To check and determine whether ADVANCED_HISTOGRAMS are available to you, execute the following statement in SingleStore:

SELECT
   COLUMN_NAME,
   IF(RANGE_STATS=1, true, false) as histograms_available,
   IF(ADVANCED_HISTOGRAMS=1, 'Advanced', 'Legacy') as histogram_type
FROM INFORMATION_SCHEMA.OPTIMIZER_STATISTICS
WHERE DATABASE_NAME = 'db' AND TABLE_NAME = 'table';

If available in your SingleStore version, use the following statement to populate the ADVANCED_HISTOGRAMS table:

ANALYZE TABLE table_name COLUMNS ALL ENABLE;

Refer to the SingleStore documentation for information about options, limitations, and additional considerations.

Managed statistics#

The connector supports Managed statistics which lets SEP collect and store table and column statistics that can then be used for performance optimizations in query planning.

Statistics must be collected manually using the built-in collect_statistics command, see ALTER TABLE EXECUTE for details and examples.

Pushdown#

The connector supports pushdown for a number of operations:

In addition, the connector supports pushdown for the following aggregate functions:

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.

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 Table 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 name

Description

Default value

join-pushdown.enabled

Enable join pushdown. Equivalent catalog session property is join_pushdown_enabled.

true

join-pushdown.strategy

Strategy used to evaluate whether join operations are pushed down. Set to AUTOMATIC to enable cost-based join pushdown, or EAGER to push down joins whenever possible. Note that EAGER 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 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';

Dynamic filtering#

Dynamic filtering is enabled by default. It causes the connector to wait for dynamic filtering to complete before starting a JDBC query.

You can disable dynamic filtering by setting the dynamic-filtering.enabled property in your catalog configuration file to false.

Wait timeout#

By default, table scans on the connector are delayed up to 20 seconds until dynamic filters are collected from the build side of joins. Using a large timeout can potentially result in more detailed dynamic filters. However, it can also increase latency for some queries.

You can configure the dynamic-filtering.wait-timeout property in your catalog properties file:

dynamic-filtering.wait-timeout=1m

You can use the dynamic_filtering_wait_timeout catalog session property in a specific session:

SET SESSION example.dynamic_filtering_wait_timeout = 1s;

Compaction#

The maximum size of dynamic filter predicate, that is pushed down to the connector during table scan for a column, is configured using the domain-compaction-threshold property in the catalog properties file:

domain-compaction-threshold=100

You can use the domain_compaction_threshold catalog session property:

SET SESSION domain_compaction_threshold = 10;

By default, domain-compaction-threshold is set to 32. When the dynamic predicate for a column exceeds this threshold, it is compacted into a single range predicate.

For example, if the dynamic filter collected for a date column dt on the fact table selects more than 32 days, the filtering condition is simplified from dt IN ('2020-01-10', '2020-01-12',..., '2020-05-30') to dt BETWEEN '2020-01-10' AND '2020-05-30'. Using a large threshold can result in increased table scan overhead due to a large IN list getting pushed down to the data source.

Metrics#

Metrics about dynamic filtering are reported in a JMX table for each catalog:

jmx.current."io.trino.plugin.jdbc:name=example,type=dynamicfilteringstats"

Metrics include information about the total number of dynamic filters, the number of completed dynamic filters, the number of available dynamic filters and the time spent waiting for dynamic filters.

Starburst Cached Views#

The connectors supports table scan redirection to improve performance and reduce load on the data source.

Security#

The connector includes a number of security-related features, detailed in the following sections.

Kerberos authentication#

The connector supports Kerberos authentication. To configure Kerberos authentication, add the following catalog configuration properties to the catalog properties file:

singlestore.authentication.type=KERBEROS
kerberos.client.principal=example@example.com
kerberos.client.keytab=etc/kerberos/example.keytab
kerberos.config=etc/kerberos/krb5.conf

In this configuration, the user example@example.com connects to the database. The related Kerberos service ticket is located in the etc/kerberos/example.keytab file.

The SingleStore connector authenticates to Kerberos using the Java Authentication and Authorization Service (JAAS). The file is set with the java.security.auth.login.config JVM system property.

If this system property is not set, the connector automatically generates a file. The file contents include values from the catalog configuration, and sets the system property to the path of the generated file.

Krb5ConnectorContext {
  com.sun.security.auth.module.Krb5LoginModule required
  useKeyTab=true
  storeKey=true
  doNotPrompt=true
  isInitiator=true
  principal="${kerberos.client.principal}"
  keyTab="${kerberos.client.keytab}";
 };

A single JAAS configuration file is shared for the entire JVM. This means multiple SingleStore catalogs must use the same principal and keytab. Users can create their own JAAS configuration and set the system property in the jvm.config configuration. There must be a Krb5ConnectorContext for the connector to work correctly.