This is the documentation for Impala 2.1.x, included as part of CDH 5.3.x.
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NUM_SCANNER_THREADS Query Option

Maximum number of scanner threads (on each node) used for each query. By default, Impala uses as many cores as are available (one thread per core). You might lower this value if queries are using excessive resources on a busy cluster. Impala imposes a maximum value automatically, so a high value has no practical effect.

Type: numeric

Default: 0

  Note: Currently, a known issue (IMPALA-488) could cause excessive memory usage during a COMPUTE STATS operation on a Parquet table. As a workaround, issue the command SET NUM_SCANNER_THREADS=2 in impala-shell before issuing the COMPUTE STATS statement. Then issue UNSET NUM_SCANNER_THREADS before continuing with queries.
Page generated September 15, 2016.