Starburst SingleStore (MemSQL) connector#

The Starburst SingleStore (formerly known as MemSQL) connector is an extended version of the SingleStore connector. It can be used to connect and query SingleStore databases. Configuration and usage is identical.

Requirements#

SQL support#

The connector supports all of the SQL statements listed in the SingleStore connector documentation.

Performance#

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

Table statistics#

This feature is available for free, and does not require a valid license.

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. You can do that by executing 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 available. If that table is not available, the information is based on the INFORMATION_SCHEMA.OPTIMIZER_STATISTICS table instead.

The ADVANCED_HISTOGRAMS table includes additional stats such as the MIN and MAX values for a column, which are not available in the LEGACY_HISTOGRAMS table. SingleStore’s ADVANCED_HISTOGRAMS feature requires version 6.5 or later, and a cardinality_estimation_level >= 6.5.

To check for these conditions 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 they are 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.

Pushdown#

The connector supports pushdown of all operations listed in the SingleStore connector documentation.

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

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

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 catalogname.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."com.starburstdata.presto.plugin.jdbc.dynamicfiltering:name=catalogname,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 SingleStore connector includes a number of security-related features, detailed in the following sections.

Kerberos authentication#

The connector supports Kerberos authentication. Use the following properties in the catalog properties file to configure it.

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

With this configuration the user example@example.com, defined in the principal property, is used to connect to the database, and the related Kerberos service ticket is located in the example.keytab file.

The SingleStore connector authenticates to Kerberos using the Java Authentication and Authorization Service (JAAS). The file is set via the java.security.auth.login.config JVM system property. If this system property is not set, the connector automatically generates a file with the following contents using 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, however there must be a Krb5ConnectorContext for the connector to work correctly.