UPDATE子句中VALUES（col_name）函数的性能

Question

The question is about SQL legacy code for MySQL database.

It is known, that when doing INSERT ... ON DUPLICATE KEY UPDATE statement VALUES(col_name) function can be used to refer to column values from the INSERT portion instead of passing there exact values:

INSERT INTO table (a,b,c) VALUES (1,2,3)
  ON DUPLICATE KEY UPDATE b=VALUES(b), c=VALUES(c)

My legacy code contains a lot of huge inserts in parametrized style (they are used in batch-inserts):

INSERT INTO table (a,b,c, <...dozens of params...>) VALUES (?,?,?,<...dozens of values...>)
  ON DUPLICATE KEY UPDATE b=?, c=?, <...dozens of params...>

The question is: would it increase performance of batch-inserts if I will change all these queries to use VALUES(col_name) function (in UPDATE portion)?

My queries are executed from java code using jdbc driver. So, what I guess, is that for long text values it should significantly reduce size of queries. What about MySQL it self? Would it really in general give me increasing of speed?

Answer 1

Batched inserts can may run 10 times as fast and one row at a time. The reason for this is all the network, etc, overhead.

Another technique is to change from a single batched IODKU into two statements -- one to insert the new rows, one to do the updates. (I don't know if that will run any faster.) Here is a discussion of the two steps, in the context of "normalization".

Another thing to note: If there is an AUTO_INCREMENT involved (not as one of the columns mentioned), then IODKU may "burn" ids for the cases where it does an 'update'. That is, the IODKU (and INSERT IGNORE and a few others) get all the auto_incs that it might need, then proceeds to use the ones it does need and waste the others.

You get into "diminishing returns" if you try to insert more than a few hundred rows in a batch. And you stress the rollback log.