This topic describes the restrictions on parallel query and how to use parallel query.

Restrictions on parallel query

Parallel query cannot improve performance in the following situations:

  • The number of table entries is less than 20,000.
  • System tables or temporary tables
  • SELECT... FOR UPDATE and SELECT... FOR SHARE statements
  • Queries on a full-text indexed table
  • Stored procedures
  • User defined functions (UDFs)
  • Recursive CTEs
  • Window functions
  • GIS
  • XML functions
  • GROUP BY WITH ROLLUP
  • Locking functions
  • Non-B-tree indexes
  • INDEX MERGE
  • Queries for serializable transactions
  • The number of parallel threads is equal to or more than four times the number of CPU cores.