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The server checks the value of secure_file_priv  at startup and writes a warning to the error log if the value is insecure. A non- NULL  value is considered insecure if it is empty, or the value is the data directory or a subdirectory of it, or a directory that is accessible by all users. If secure_file_priv  is set to a nonexistent path, the server writes an error message to the error log and exits. 

• select_into_buffer_size 

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When using SELECT INTO OUTFILE  or SELECT INTO DUMPFILE  to dump data into one or more files for backup creation, data migration, or other purposes, writes can often be buffered and then trigger a large burst of write I/O activity to the disk or other storage device and stall other queries that are more sensitive to latency. You can use this variable to control the size of the buffer used to write data to the storage device to determine when buffer synchronization should occur, and thus to prevent write stalls of the kind just described from occurring. 

select_into_buffer_size  overrides any value set for read_buffer_size . ( select_into_buffer_size  and read_buffer_size  have the same default, maximum, and minimum values.) You can also use select_into_disk_sync_delay  to set a timeout to be observed afterwards, each time synchronization takes place. 

As of MySQL 8.0.26, setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 5.1.9.1, “System Variable Privileges” . 

• select_into_disk_sync 

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