Server System Variables The server attempts to obtain the number of file descriptors using the maximum of those values, capped to the maximum unsigned integer value. If that many descriptors cannot be obtained, the server attempts to obtain as many as the system permits. The effective value is 0 on systems where MySQL cannot change the number of open files. On Unix, the value cannot be set greater than the value displayed by the  ulimit -n  command. Controls the heuristics applied during query optimization to prune less-promising partial plans from the optimizer search space. A value of 0 disables heuristics so that the optimizer performs an exhaustive search. A value of 1 causes the optimizer to prune plans based on the number of rows retrieved by intermediate plans. The maximum depth of search performed by the query optimizer. Values larger than the number of relations in a query result in better query plans, but take longer to generate an execution plan for a query. Values smaller than the number of relations in a query return an execution plan quicker, but the resulting plan may be far from being optimal. If set to 0, the system automatically picks a reasonable value. 848 •Prior to MySQL 8.0.19: •The operating system limit if that limit is positive but not Infinity. •If the operating system limit is Infinity:  open_files_limit  value if specified at startup, 5000 if not. • optimizer_prune_level • optimizer_search_depth Command-Line Format --optimizer-prune-level=# System Variable optimizer_prune_level Scope Global, Session Dynamic Yes SET_VAR  Hint Applies Yes Type Integer Default Value 1 Minimum Value 0 Maximum Value 1 Command-Line Format --optimizer-search-depth=# System Variable optimizer_search_depth Scope Global, Session Dynamic Yes SET_VAR  Hint Applies Yes Type Integer Default Value 62 Minimum Value 0 Maximum Value 62