What are practical differences between `REPLACE` and `INSERT ... ON DUPLICATE KEY UPDATE` in MySQL?

Question

What I need is to set the values of all the fields of a record with a particular key (the key is composite actually), inserting the record if there is no record with such a key yet.

REPLACE seems as meant to do the job, but at the same time its manual page suggests INSERT ... ON DUPLICATE KEY UPDATE.

What of them should I better choose and why?

The only "side effect" of REPLACE that comes into my mind is that it would increment autoincrement values (fortunately I don't use any) while INSERT ... ON DUPLICATE KEY UPDATE probably wouldn't. What are the other practical differences to take in mind? In what particular cases can REPLACE be preferred over INSERT ... ON DUPLICATE KEY UPDATE and vice versa?

Solution 1

REPLACE internally performs a delete and then an insert. This can cause problems if you have a foreign key constraint pointing at that row. In this situation the REPLACE could fail or worse: if your foreign key is set to cascade delete, the REPLACE will cause rows from other tables to be deleted. This can happen even though the constraint was satisfied both before and after the REPLACE operation.

Using INSERT ... ON DUPLICATE KEY UPDATE avoids this problem and is therefore prefered.

Solution 2

To answer the question in terms of performance, I did a test using both the methods

Replace Into involves:
1.Try insert on the table
2. If 1 fails, delete row and insert new row

Insert on Duplicate Key Update involves:
1.Try insert on table
2.If 1 fails, update row

If all the steps involved are inserts, there should be no difference in performance. The speed has to depend on the number of updates involved. Worst case is when all the statements are updates

I have tried both the statements on my InnoDB table involving 62,510 entries (only updates). On camparing speeds:
Replace Into: 77.411 seconds
Insert on Duplicate Key Update: 2.446 seconds

Insert on Duplicate Key update is almost 32 times faster.

Table Size: 1,249,250 rows with 12 columns on an Amazon m3.medium

Solution 3

When using REPLACE instead of INSERT ... ON DUPLICATE KEY UPDATE, I sometimes observe key locking or deadlock problems when multiple queries arrive quickly for a given key. The atomicity of the latter (in addition to not causing cascade deletes) is all the more reason to use it.

Solution 4

In what particular cases can REPLACE be preferred over INSERT ... ON DUPLICATE KEY UPDATE and vice versa?

I've just found out the hard way that in the case of tables with a FEDERATED storage engine INSERT...ON DUPLICATE KEY UPDATE statements are accepted, but fail (with an Error 1022: Can't write; duplicate key in table...) if a duplicate-key violation occurs - see corresponding bullet point on this page of the MySQL Reference Manual.

Fortunately, I was able to use REPLACE instead of INSERT...ON DUPLICATE KEY UPDATE within my after insert trigger to achieve the desired outcome of replicating changes to a FEDERATED table.

Solution 5

If you don't list all the columns, I think REPLACE will reset any unmentioned columns with their default values in the replaced rows. ON DUPLICATE KEY UPDATE will leave unmentioned columns unchanged.

View more solutions

Comments

[Ivan]

Good answer, but in the actual case of mine this problem is not going to be met. The chance of collision can though be considered 50/50. What should I choose then? And as INSERT ... ON DUPLICATE KEY UPDATE looks considerably "better" then in what particular cases can "REPLACE" be a better choice?

[Nathan Stretch]

I've done a fair bit of research and as far as I can tell, there is no common reason to use REPLACE instead of INSERT ... ON DUPLICATE KEY UPDATE. It's essentially a legacy feature. Unless there's some particular reason why your code relies on rows being deleted and re-added, with the associated effects on indexes and auto-increment values, there doesn't appear to be any reason to use it.

[radtek]

On REPLACE will update your PK auto-increment value if it does a DELETE and INSERT. Which is exactly what I want. I do not want the consumer to find the record under the same PK, so they get no rows. When I want them to find it (actual update), I use UPDATE

[radtek]

Cool stats, did you try Insert on Duplicate Key Replace? Was it slower?

[LemonPi]

So the other half of the question: when would you prefer REPLACE over INSERT ... ON DUPLICATE KEY UPDATE? Why would an INSERT + DELETE ever be preferred over an UPDATE?

[izogfif]

Beware: INSERT IGNORE query will finish successfully (and will issue a warning) if a foreign constraint fails! If you want to catch an error like this, better use ON DUPLICATE KEY UPDATE without IGNORE.

[izogfif]

@radtek you can only write ON DUPLICATE KEY UPDATE, you can't write ON DUPLICATE KEY REPLACE. If you want to update all values of existing row upon duplicate key, you have to write ON DUPLICATE KEY UPDATE col1=VALUES(col1), col2=VALUES(col2), ... - you have to list all columns manually.

[radtek]

I know I was just asking what was faster and looks like update is.

[Payel Senapati]

what is the purpose of REPLACE command then if it is so useless and only causes problems? Shouldn't this feature be depreciated then?