SingleStore DB

Other Schema Concepts
Reference Tables

Reference tables are relatively small tables that do not need to be distributed and are present on every node in the cluster. Reference tables are implemented via primary-secondary replication to every node in the cluster from the master aggregator. Replication enables reference tables to be dynamic: updates that you perform to a reference table on the master aggregator are quickly reflected on every machine in the cluster. Since reference tables are replicated to every node in the cluster, it eliminates the need to transfer the table’s data across the network during query execution. Reference tables should mostly be used for tables that change rarely, because the write operations on reference tables consume a lot more resources.

Data Definition Language (DDL) commands for reference tables can be run from the child, or master aggregator. DDL commands run on a child aggregator are forwarded to the master and propagated from there. Data Manipulation Language (DML) for reference tables can not be run from child aggregators because forwarding is not supported for these commands. See Node Requirements for SingleStore DB Commands for more information.

SingleStore DB aggregators can take advantage of reference tables’ ubiquity by pushing joins between reference tables and a distributed table onto the leaves. Imagine you have a distributed clicks table storing billions of records and a smaller customers table with just a few million records. Since the customers table is small, it can be replicated on every node in the cluster. If you run a join between the clicks table and the customers table, then the bulk of the work for the join will occur on the leaves.

Important

Reference tables are a convenient way to implement dimension tables.

Temporary Tables

Temporary tables exist, storing data in memory, for the duration of a client session. This means they are scoped to the connection that opened them, cannot be queried by other users, and are dropped once the connection has ended. They can also be dropped manually without removing the connection.

SingleStore DB does not write logs or take snapshots of temporary tables. Temporary tables are designed for temporary, intermediate computations and can only be created in rowstores. Since temporary tables are neither persisted nor replicated in SingleStore DB, they have high availability disabled. This means that if a node in a cluster goes down and a failover occurs, all the temporary tables on the cluster lose data. Whenever a query references a temporary table after a node failover, it returns an error. For example,

"Temporary table <table> is missing on leaf <host> due to failover. The table will need to be dropped and recreated."

Important

To prevent loss of data on node failover, use SingleStore DB tables that have high availability enabled.

Views cannot reference temporary tables because temporary tables only exist for the duration of a client session. Although SingleStore DB does not materialize views, views are available as a service for all clients, and so cannot depend on client session-specific temporary tables.

CREATE TEMPORARY TABLE can be run on any aggregator, not just the master aggregator. Temporary tables are sharded tables, and can be modified and queried like any permanent table, including distributed joins.

Global Temporary Tables

Another type of temporary table is the global temporary table. Like temporary tables, these can only be created in rowstores, and are not persisted.

Unlike temporary tables, global temporary tables are shared across sessions, i.e. they exist beyond the duration of a session. They are never automatically dropped and must always be dropped manually. They can also be queried by other users since they are not session dependent.

Global temporary tables can be used as a temporary space for processing data that can be accessed from multiple connections, for example, performing ETL (Extract, Transform, and Load) operations on modern dynamic databases.

If failover occurs, global temporary tables lose data and enter an errored state; they need to be dropped and recreated. However, dropping a global or non-global temporary table does not drop its plancache from the disk and retains the cache if the table is recreated with the same schema.

DDL such as CREATE GLOBAL TEMPORARY TABLE and DML for global temporary tables can be run on a master, or a child aggregator node.

Note: Both global temporary tables and non-global temporary tables cannot be altered.

Types of Indexes

SingleStore DB, like all standard SQL databases, allows the creation of indexes on tables, which speed up certain access patterns. As with MySQL, SingleStore DB supports two index type keywords. The BTREE keyword is for compatibility with MySQL syntax and will create the default SingleStore DB skip list index instead. The CLUSTERED COLUMNSTORE index type is for columnstore tables.

Skip List Indexes

The default index type in SingleStore DB is a skip list. Skip lists in SingleStore DB are meant to replace the B-Tree indexes used by most other databases, including MySQL. Skip lists are optimized to run in memory as they can be implemented lock free and offer extremely fast insert performance. Like B-Trees, they offer an expected O(log(n)) lookup performance and can be traversed in sorted order.

Unlike B-Trees in MySQL, skip lists in SingleStore DB are uni-directional (singly linked). Each column in a compound skip list index can be specified as ascending (ASC) or descending (DESC). The default is ASC. Which one you pick will not impact lookup performance, but it does impact scan performance depending on the direction the index is scanned. Scanning a skip list in reverse order is approximately twice as costly as scanning in forward order. So, if you have an ASC index and you run a query that would traverse the index in descending order (ORDER BY DESC for example), then the query will require a more expensive iteration than if the index were DESC.

SingleStore DB supports skip lists only on rowstore tables. For more information on what a Skip List is and why it is used in SingleStore DB see: The Story Behind SingleStore’s Skiplist Indexes

Columnstore Indexes

Columnstore indexes leverage columnstore technology to efficiently store and retrieve large numbers of values from disk (using flash or SSD is recommended). Because columnstore indexes typically provide significant data compression, are backed by disk, and thus don’t have the requirement that all data must fit in memory, which other types of indexes in SingleStore DB do, they are typically very useful for analytical workloads. SingleStore DB currently supports clustered columnstore indexes which, when added to a table, will make the entire table structure backed by the columnstore. Currently columnstore indexes cannot be combined with in-memory row store indexes on the same table. For more information about using columnstore indexes in SingleStore DB, see How the Columnstore Works.

Hash Table Indexes

Warning

A HASH index will only be utilized if the query employs an equality filter for every column in the index.

Due to the restrictive case detailed above, HASH indexes should only be used when there is a demonstrated need and measurable benefit on your particular dataset and workload. In these specific cases, HASH indexes provide fast exact-match access to unique values. This is because the hash index is stored in a sparse array of buckets indexed by a hash function on the relevant columns eg: hash(column_a, column_b). Queries can quickly find exact match data by examining only the bucket identified by the hash function. However they cannot easily scan over a subset of the index. For multi-column indexes, query filters must match all of the index columns to be able to take advantage of the index. SingleStore DB supports HASH indexes on both columnstore and rowstore tables. For more information, see USING HASH behavior.

Consider an example table:

CREATE TABLE t(a int, b int, INDEX(a, b) USING HASH);

Suppose we are running queries like:

SELECT * FROM t WHERE a < 3;

EXPLAIN shows us that since we are performing a range scan and not filtering on all the columns in our hash index, a full Table Scan is performed.

EXPLAIN SELECT * FROM t WHERE a < 3;
****
+------------------------------------------------+
| EXPLAIN                                        |
+------------------------------------------------+
| Gather partitions:all                          |
| Project [t.a, t.b]                             |
| Filter [t.a < 3]                               |
| TableScan db.t                                 |
+------------------------------------------------+

The hash index is only utilized if the query uses only equality predicates, and filters on all columns in the hash index.

EXPLAIN SELECT * FROM t WHERE a=3 AND b=7;
****
+--------------------------------------------------------------------------------------------------------------------------+
| EXPLAIN                                                                                                                  |
+--------------------------------------------------------------------------------------------------------------------------+
| Gather partitions:all                                                                                                    |
| Project [t.a, t.b]                                                                                                       |
| IndexRangeScan db.t, KEY a (a, b) USING HASH storage:lf_hashtable scan:[a = 3 AND b = 7]                                 |
+--------------------------------------------------------------------------------------------------------------------------+

Important

A columnstore table cannot have a multi-column HASH index.

Runtime Plan Choosing

SingleStore DB can dynamically select which index to use for a query at runtime. Instead of collecting statistics and building histograms, SingleStore DB can compute these statistics for a given query on-demand by inspecting its indexes. If a query can match more than one index, SingleStore DB compiles an execution plan for each choice, along with the necessary expression logic to cheaply analyze and evaluate which plan to choose at runtime. This process eliminates the need to manually recompute statistics on indexes.

Index Hints

SingleStore DB supports the following index hint syntax:

tbl_name [index_hint]

index_hint:
    USE {INDEX | KEY} (index_list)
  | IGNORE {INDEX | KEY} (index_list)
  | FORCE {INDEX | KEY} (index_list)

index_list:
    index_name [, index_name] ...
  • USE and FORCE hints force the use of one of the specified indexes to run the query. In SingleStore DB, there is no difference between a USE and FORCE hint.

  • IGNORE hints disallow the specified indexes from being used to run the query.

The EXPLAIN <query> statement can be used to show which indexes the query considers and which one it will actually use.

Index Commands