Trino supports several cost based optimizations, described below.
The order in which joins are executed in a query can have a significant impact on the query’s performance. The aspect of join ordering that has the largest impact on performance is the size of the data being processed and transferred over the network. If a join which produces a lot of data is performed early in the query’s execution, then subsequent stages need to process large amounts of data for longer than necessary, increasing the time and resources needed for processing the query.
With cost-based join enumeration, Trino uses Table statistics provided by connectors to estimate the costs for different join orders and automatically picks the join order with the lowest computed costs.
The join enumeration strategy is governed by the
session property, with the
optimizer.join-reordering-strategy configuration property providing the
The possible values are:
AUTOMATIC(default) - enable full automatic join enumeration
ELIMINATE_CROSS_JOINS- eliminate unnecessary cross joins
NONE- purely syntactic join order
If you are using
AUTOMATIC join enumeration and statistics are not
available or a cost can not be computed for any other reason, the
ELIMINATE_CROSS_JOINS strategy is used instead.
Join distribution selection#
Trino uses a hash-based join algorithm. For each join operator, a hash table must be created from one join input, referred to as the build side. The other input, called the probe side, is then iterated on. For each row, the hash table is queried to find matching rows.
There are two types of join distributions:
Partitioned: each node participating in the query builds a hash table from only a fraction of the data
Broadcast: each node participating in the query builds a hash table from all of the data. The data is replicated to each node.
Each type has advantages and disadvantages. Partitioned joins require redistributing both tables using a hash of the join key. These joins can be much slower than broadcast joins, but they allow much larger joins overall. Broadcast joins are faster if the build side is much smaller than the probe side. However, broadcast joins require that the tables on the build side of the join after filtering fit in memory on each node, whereas distributed joins only need to fit in distributed memory across all nodes.
With cost-based join distribution selection, Trino automatically chooses whether to use a partitioned or broadcast join. With cost-based join enumeration, Trino automatically chooses which sides are probe and build.
The join distribution strategy is governed by the
session property, with the
join-distribution-type configuration property
providing the default value.
The valid values are:
AUTOMATIC(default) - join distribution type is determined automatically for each join
BROADCAST- broadcast join distribution is used for all joins
PARTITIONED- partitioned join distribution is used for all join
Capping replicated table size#
The join distribution type is automatically chosen when the join reordering
strategy is set to
AUTOMATIC or when the join distribution type is set to
AUTOMATIC. In both cases, it is possible to cap the maximum size of the
replicated table with the
property or with the
join_max_broadcast_table_size session property. This
allows you to improve cluster concurrency and prevent bad plans when the
cost-based optimizer misestimates the size of the joined tables.
By default, the replicated table size is capped to 100MB.
Syntactic join order#
If not using cost-based optimization, Trino defaults to syntactic join ordering. While there is no formal way to optimize queries for this case, it is possible to take advantage of how Trino implements joins to make them more performant.
Trino uses in-memory hash joins. When processing a join statement, Trino loads the right-most table of the join into memory as the build side, then streams the next right-most table as the probe side to execute the join. If a query has multiple joins, the result of this first join stays in memory as the build side, and the third right-most table is then used as the probe side, and so on for additional joins. In the case where join order is made more complex, such as when using parentheses to specify specific parents for joins, Trino may execute multiple lower-level joins at once, but each step of that process follows the same logic, and the same applies when the results are ultimately joined together.
Because of this behavior, it is optimal to syntactically order joins in your SQL queries from the largest tables to the smallest, as this minimizes memory usage.
As an example, if you have a small, medium, and large table and are using left joins:
SELECT * FROM large_table l LEFT JOIN medium_table m ON l.user_id = m.user_id LEFT JOIN small_table s ON s.user_id = l.user_id
This means of optimization is not a feature of Trino. It is an artifact of how joins are implemented, and therefore this behavior may change without notice.
In order for the Trino optimizer to use the cost based strategies, the connector implementation must provide Table statistics.