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Using join indexes

Join operations are often the most resource-intensive aspect of queries, slowing down queries and consuming engine resources. Firebolt join indexes can help accelerate queries that include joins. Firebolt can read the join index cached in engine RAM rather than creating the join at query runtime. Join indexes are created & held in RAM automatically for eligible queries. This reduces disk I/O and compute resources at query runtime. Queries run faster and you can use engine resources more efficiently to reduce cost. With this optimization, we have seen real-world, production queries run 200x faster.

How to use a join index

To see how this works, let us look at an example. Say we have the following query pattern which is run hundreds of times per second with different values for l.player_id and

SELECT, SUM(l.score) 
FROM   game_plays as l
JOIN   player_info as r 
ON     l.player_id = r.player_id
WHERE  l.player_id = XXX AND = YYY

On the first run of this query, the relevant data from the right-hand side table player_info is read and stored in a specialized data structure, which is cached in RAM. This can take tens of seconds if the player_info table is large (e.g., contains millions of rows).

However, on subsequent runs of the query pattern, the cached data structure can be reused - so all subsequent queries will only take a few milliseconds (if the left-hand side with potential field restrictions is small, as here).

Requirements on join queries

Not all join queries create (and use) join indexes. Here is a set of requirements that must be met:

  • The right side of the join in the query must be directly a table. Subselects or views are not supported.
  • Restrictions on fields from the right side of the join need to be applied in an OUTER SELECT, wrapping the query.
  • Since the join index data structure is cached in RAM, the right side table may not be too large (by default the size of the cache is limited to 20% of the RAM).
  • All types of joins (INNER, LEFT, RIGHT, …) are supported.
  • The right table in the join can be a FACT or DIMENSION table.


See Join indexes in the Firebolt indexes in action blog post.

Legacy index commands

Prior to release of DB version 3.19.0, join indexes needed to be created and manually refreshed. Below are legacy instructions.

When to use a join index (legacy)

Join indexes are most effective when you define them for a single, large dimension table that you join with many fact tables. A join index is also most effective when the query with a JOIN returns only a small subset of many available columns in the dimension table. You can create as many join indexes per dimension table as you want.

How join indexes work (legacy)

When you create a join index, you specify a dimension table, the unique join key column in the dimension table, and columns in the dimension table that are used in the SELECT query with the JOIN. When you run the query, the Firebolt optimizer looks for a join index to use. If the query uses the join key column and dimension columns that you defined in the join index, Firebolt reads the join index from engine RAM.

How to define a join index (legacy)

To create a join index, use the CREATE JOIN INDEX statement. You can create as many join indexes for a dimension table as you need.

The join index definition establishes the name of the join index and specifies the dimension table it applies to. The definition then specifies the join key column. The join key column must appear in the first position. This column must have been created using the UNIQUE attribute and have no duplicate values. For more information, see CREATE FACT|DIMENSION TABLE.

CREATE JOIN INDEX [IF NOT EXISTS] <unique_join_index_name> ON <dim_table_name>