Server System Variables SET transaction_isolation =  value ; For each of those syntaxes, these semantics apply: •Sets the isolation level for all subsequent transactions performed within the session. •Permitted within transactions, but does not affect the current ongoing transaction. •If executed between transactions, overrides any preceding statement that sets the next-transaction isolation level. •Corresponds to  SET SESSION TRANSACTION ISOLATION LEVEL  (with the  SESSION  keyword). •To set the next-transaction isolation level, use this syntax: SET @@transaction_isolation =  value ; •Sets the isolation level only for the next single transaction performed within the session. •Subsequent transactions revert to the session isolation level. •Not permitted within transactions. •Corresponds to  SET TRANSACTION ISOLATION LEVEL  (without the  SESSION  keyword). For that syntax, these semantics apply: For more information about  SET TRANSACTION  and its relationship to the  transaction_isolation system variable, see Section 13.3.7, “SET TRANSACTION Statement” . • transaction_prealloc_size There is a per-transaction memory pool from which various transaction-related allocations take memory. The initial size of the pool in bytes is  transaction_prealloc_size . For every allocation that cannot be satisfied from the pool because it has insufficient memory available, the pool is increased by  transaction_alloc_block_size  bytes. When the transaction ends, the pool is truncated to transaction_prealloc_size  bytes. By making  transaction_prealloc_size  sufficiently large to contain all statements within a single transaction, you can avoid many  malloc()  calls. Command-Line Format --transaction-prealloc-size=# System Variable transaction_prealloc_size Scope Global, Session Dynamic Yes SET_VAR  Hint Applies No Type Integer Default Value 4096 Minimum Value 1024 Maximum Value 131072 Block Size 1024 911