My analytics table has 190M rows, about 150GB ins size.
Storage engine is MyISAM with

key-buffer-size=16G

myisam_sort_buffer_size=2G

myisam-max-sort-file-size=200G.

Machine has 32GB memory.
While creating 10 simple indexes (no covering, no prefix, just column names using BTREE) I noticed:

• first index - 40 minutes


• second index - 50 minutes


• ...


• fifth index - 2 hours


• eleventh index - 3 1/2 hours

Each index adds between 1GB and 3GB to the .myi file.
Has anybody experienced a similar increase in run-time? Is there any way to get the performance of the first index creation for the other indexes?

You must be doing something odd with your DDL or you have a very, very old version of MySQL.

MyISAM Index creation can be very lethargic. I wrote about this almost ten years ago.

• 
  Oct 10, 2006 : Why does mysql drop index very very slow in a large table? (from the MySQL General Discussion Forum)
  


• 
  May 12, 2011 : Does MySQL still handle indexes in this way? (where I mention this again)

Without any additional details from you, I can only guess. Here it goes:

I suspect your must be just doing the following:

ALTER TABLE mytable ADD INDEX (column01);

ALTER TABLE mytable ADD INDEX (column02);

ALTER TABLE mytable ADD INDEX (column03);

...

ALTER TABLE mytable ADD INDEX (column10);

ALTER TABLE mytable ADD INDEX (column11);

I would like to suggest the following

SUGGESTION #1 : Build all 11 indexes at the same time

You should create all 11 indexes in a single DDL statement

ALTER TABLE myisamtable

    ADD INDEX (column01),

    ADD INDEX (column02),

    ADD INDEX (column03),

    ...

    ADD INDEX (column10),

    ADD INDEX (column11)

;

SUGGESTION #2 : Delay indexes building

You can disable the building of non-unique indexes so that sorting and be done.

Doing so can make a smaller BTREE for each index

ALTER TABLE mytable DISABLE KEYS;

... 

ALTER TABLE mytable ENABLE KEYS;

SUGGESTION #3 : Use a temp table

If you wish to test this manually, you can make a temp table, add all the indexes to the empty temp table, disable indexes, insert the data, enable indexes, and switch table names.

Here is a sample of what I just described to you

CREATE TABLE mytable_new LIKE mytable;

ALTER TABLE mytable_new

    ADD INDEX (column01),

    ADD INDEX (column02),

    ADD INDEX (column03),

    ...

    ADD INDEX (column10),

    ADD INDEX (column11)

;

ALTER TABLE mytable_new DISABLE KEYS;

INSERT INTO mytable_new SELECT * FROM mytable;

ALTER TABLE mytable_new ENABLE KEYS;

ALTER TABLE mytable RENAME mytable_old;

ALTER TABLE mytable_new RENAME mytable;

DROP TABLE mytable_old;

GIVE IT A TRY !!!

Rolando,
Thanks a lot for your explanation. I followed disc size and file access during this ordeal (46 hours, completed this morning) and yes .. even in MySql 5.6 this is what happens.
My statements were:

CREATE INDEX idx1 ....;

CREATE INDEX idx2 ....;

CREATE INDEX idx3 ....;

...

and I saw in the "show processlist" how all the data was first copied to a temp table and then indexed. Will use

ALTER TABLE ADD INDEX, ADD INDEX, ADD INDEX ...

in the future.

Thanks! Problem solved .. question answered!