Is there an equivalent of EXPLAIN that will work in front of an ALTER TABLE query?

Question

It looks like the MySQL EXPLAIN prefix only works in front of certain queries. Is there an equivalent of EXPLAIN that will work in front of an ALTER TABLE query?

I would love to be able to find out how long my planned ALTER TABLE statement is likely to take.

Background: I have a table from someone else that contains 300 columns of data. I know that I'm only going to need to use a few of those columns, and in order to figure out which columns I need, I'm planning to do a full-text search for a few key words. But in order to do that, I need to add a full-text index. And since I'm new to this size of data set, I'm not entirely sure that this is a realistic plan. I'm hoping something like EXPLAIN (or, more likely, a substitute tool from this thread) might help determine that.

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EDIT: In answer to a couple questions below, I should mention that this table has about 4 million rows and is on a local testing machine. So I can just run this thing blindly if needed. I just don't prefer to if possible. Thanks for all the good information so far.

Answer 1

Most "Alter table" will trigger the copy to tmp table operation, which it will create temp table with new schema, then lock table, copy data from old table to new table, then rename, drop old table.

So most time consumed is copy to temp table, it's depend on how big of that table if the server have enough memory. Use show table status to check how big of the table (data_length+ index_length), sample on other table to know the transfer speed on your mysql server, then you can estimate how long it will take.

Another way mentioned on mysql doc about explain on DML, but I didn't got result, maybe not finished yet :

http://dev.mysql.com/doc/refman/5.6/en/explain.html

As of MySQL 5.6.3, permitted explainable statements for EXPLAIN are SELECT, DELETE, INSERT, REPLACE, and UPDATE. Before MySQL 5.6.3, SELECT is the only explainable statement.