[MDEV-19045] Change in behavior upon creation of unlimited non-unique indexes - Jira

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Before ~~MDEV-371~~, an attempt to add an unlimited (non-unique) index on a blob column caused an error, regardless the strictness of SQL_MODE:

create table t1 (f text);

alter table t1 add index ind1 (f);

10.3 dcdeb394
mysqltest: At line 2: query 'alter table t1 add index ind1 (f)' failed: 1170: BLOB/TEXT column 'f' used in key specification without a key length

Now instead it causes a warning, also regardless the strictness of SQL_MODE, and an index is created with automatically specified length:

10.4 c0ba036b
alter table t1 add index ind1 (f);
Warnings:
Note 1071 Specified key was too long; max key length is 1000 bytes
show create table t1;
Table Create Table
t1 CREATE TABLE `t1` (
`f` text DEFAULT NULL,
KEY `ind1` (`f`(1000))
) ENGINE=MyISAM DEFAULT CHARSET=latin1

If the change is intentional, it needs to be documented somewhere.

is caused by

MDEV-371 Unique indexes for blobs