The Impala SQL dialect supports query hints, for fine-tuning the inner workings of queries. Specify hints as a temporary workaround for expensive queries, where missing statistics or other factors cause inefficient performance. The hints are represented as keywords surrounded by [] square brackets; include the brackets in the text of the SQL statement.

The [BROADCAST] and [SHUFFLE] hints control the execution strategy for join queries. Specify one of the following constructs immediately after the JOIN keyword in a query:

• [SHUFFLE] - Makes that join operation use the "partitioned" technique, which divides up corresponding rows from both tables using a hashing algorithm, sending subsets of the rows to other nodes for processing. (The keyword SHUFFLE is used to indicate a "partitioned join", because that type of join is not related to "partitioned tables".) Since the alternative "broadcast" join mechanism is the default when table and index statistics are unavailable, you might use this hint for queries where broadcast joins are unsuitable; typically, partitioned joins are more efficient for joins between large tables of similar size.
• [BROADCAST] - Makes that join operation use the "broadcast" technique that sends the entire contents of the right-hand table to all nodes involved in processing the join. This is the default mode of operation when table and index statistics are unavailable, so you would typically only need it if stale metadata caused Impala to mistakenly choose a partitioned join operation. Typically, broadcast joins are more efficient in cases where one table is much smaller than the other. (Put the smaller table on the right side of the JOIN operator.)

To see which join strategy is used for a particular query, examine the EXPLAIN output for that query.

  Note:

Because hints can prevent queries from taking advantage of new metadata or improvements in query planning, use them only when required to work around performance issues, and be prepared to remove them when they are no longer required, such as after a new Impala release or bug fix.

In particular, the [BROADCAST] and [SHUFFLE] hints are expected to be needed much less frequently in Impala 1.2.2 and higher, because the join order optimization feature in combination with the COMPUTE STATS statement now automatically choose join order and join mechanism without the need to rewrite the query and add hints. See Performance Considerations for Join Queries for details.

For example, this query joins a large customer table with a small lookup table of less than 100 rows. The right-hand table can be broadcast efficiently to all nodes involved in the join. Thus, you would use the [broadcast] hint to force a broadcast join strategy:

select customer.address, state_lookup.state_name
  from customer join [broadcast] state_lookup
  on customer.state_id = state_lookup.state_id;

This query joins two large tables of unpredictable size. You might benchmark the query with both kinds of hints and find that it is more efficient to transmit portions of each table to other nodes for processing. Thus, you would use the [shuffle] hint to force a partitioned join strategy:

select weather.wind_velocity, geospatial.altitude
  from weather join [shuffle] geospatial
  on weather.lat = geospatial.lat and weather.long = geospatial.long;

For joins involving three or more tables, the hint applies to the tables on either side of that specific JOIN keyword. The joins are processed from left to right. For example, this query joins t1 and t2 using a partitioned join, then joins that result set to t3 using a broadcast join:

select t1.name, t2.id, t3.price
  from t1 join [shuffle] t2 join [broadcast] t3
  on t1.id = t2.id and t2.id = t3.id;

For more background information and performance considerations for join queries, see Joins.