16.3. Cost based optimizations
- Presto supports the following cost based optimizations:
- Cost Based Join Enumeration
- Cost Based Join Distribution Selection
Cost Based Join Enumeration
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 passed over the network. If a join that produces a lot of data is performed early in the execution, then subsequent stages will need to process large amounts of data for longer than necessary, increasing the time and resources needed for the query.
With cost based join enumeration, Presto uses Table Statistics provided by the connectors to estimate the costs for different join orders and automatically pick the join order with the lowest computed costs.
The join enumeration strategy is governed by
optimizer.join-reordering-strategy set in
config.properties. This serves as
the default for
join_reordering_strategy session property.
- The valid values are:
AUTOMATIC(default) - full automatic join enumeration enabled
ELIMINATE_CROSS_JOINS- eliminate unnecessary cross joins
NONE- purely syntactic join order
AUTOMATIC and statistics are not available or if for any other reason a cost
could not be computed, the
ELIMINATE_CROSS_JOINS strategy is used
Cost Based Join Distribution Selection
Presto uses a hash based join algorithm. That implies that for each join operator a hash table must be created from one join input (called build side). The other input (probe side) is then iterated and for each row the hash table is queried to find matching rows.
- There are two types of join distributions:
- PARTITIONED - each node participating in query builds a hash table from only fraction of data
- BROADCAST - each node participating in query builds a hash table from all of data (data is replicated)
Each type have their trade offs. Partitioned joins require redistributing both tables using a hash of the join key. This can be slower (sometimes substantially) than broadcast joins, but allows much larger joins. In particular, broadcast joins will be 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, Presto automatically chooses the type of distributed join to use for each join. During cost based join enumeration, Presto also considers switching the probe side with build side.
The join distribution strategy is governed by
join-distribution-type set in
config.properties. This serves as a default
join_distribution_type session property.
- 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
In order for the Presto optimizer to use the cost based strategies, the connector implementation must provide statistics. Currently the only connector that supports statistics is the Hive Connector.