The SET statement can be used to override some cluster-wide configuration parameters for a specific request:
SET <IDENTIFIER> <STRING_LITERAL>
As parameter identifiers are qualified names (containing a '.') they have to be escaped using backticks (``). Note that changing query parameters will not affect query correctness but only impact performance characteristics, such as response time and throughput.
The system can execute each request using multiple cores on multiple machines (a.k.a., partitioned parallelism) in a cluster. A user can manually specify the maximum execution parallelism for a request to scale it up and down using the following parameter:
compiler.parallelism: the maximum number of CPU cores can be used to process a query. There are three cases of the value p for compiler.parallelism:
p < 0 or p > the total number of cores in a cluster: the system will use all available cores in the cluster;
p = 0 (the default): the system will use the storage parallelism (the number of partitions of stored datasets) as the maximum parallelism for query processing;
all other cases: the system will use the user-specified number as the maximum number of CPU cores to use for executing the query.
SET `compiler.parallelism` "16"; SELECT u.name AS uname, m.message AS message FROM GleambookUsers u JOIN GleambookMessages m ON m.authorId = u.id;
In the system, each blocking runtime operator such as join, group-by and order-by works within a fixed memory budget, and can gracefully spill to disks if the memory budget is smaller than the amount of data they have to hold. A user can manually configure the memory budget of those operators within a query. The supported configurable memory parameters are:
compiler.groupmemory: the memory budget that each parallel group-by operator instance can use; 32MB is the default budget.
compiler.sortmemory: the memory budget that each parallel sort operator instance can use; 32MB is the default budget.
compiler.joinmemory: the memory budget that each parallel hash join operator instance can use; 32MB is the default budget.
For each memory budget value, you can use a 64-bit integer value with a 1024-based binary unit suffix (for example, B, KB, MB, GB). If there is no user-provided suffix, "B" is the default suffix. See the following examples.
SET `compiler.groupmemory` "64MB"; SELECT msg.authorId, COUNT(*) FROM GleambookMessages msg GROUP BY msg.authorId;
SET `compiler.sortmemory` "67108864"; SELECT VALUE user FROM GleambookUsers AS user ORDER BY ARRAY_LENGTH(user.friendIds) DESC;
SET `compiler.joinmemory` "132000KB"; SELECT u.name AS uname, m.message AS message FROM GleambookUsers u JOIN GleambookMessages m ON m.authorId = u.id;