Looks like a variation of bug #1322, although it seems unlikely that pr_id alone
would cause it.

As far as I remember, adding an "auto_increment" field automatically
creates an index on it, so for a test I altered the script
./maintenance/archives/patch-page_restrictions_sortkey.sql
which contains:

---cut---cut---cut---

• Add a sort-key to page_restrictions table.
• First immediate use of this is as a sort-key for coming

modifications

• of Special:Protectedpages.
• Andrew Garrett, February 2007

ALTER TABLE /*$wgDBprefix*/page_restrictions

ADD COLUMN pr_id int unsigned not null auto_increment,
ADD UNIQUE KEY pr_id (pr_id(10));

---cut---cut---cut---

by omitting the "ADD UNIQUE KEY" clause, and rerun ./maintenance/
update.php
Indeed, the result is exactly the same as before.

An inspection of the page_restrictions table reveals an oddity:
there is a PRIMARY INDEX on (pr_page, pr_page) while at the same
time there is an INDEX on (pr_page) imho does not make sense.

Since there was no data in the page_restrictions table, I wanted to
drop the PRIMARY INDEX, rerun the script, and later recreate it with a
reduced sice:

ALTER TABLE page_restrictions DROP PRIMARY KEY;
#1071 - Specified key was too long. Max key length is 500

pr_page: pr_page(8)
pr_type: varchar(255)

This leads me to the assumption that, just before, the size of
pr_type might have been increased.

ALTER TABLE page_restrictions CHANGE pr_type pr_type VARCHAR( 48
) NOT NULL;

Next ran
./maintenance/update.php
withe the original
./maintenance/archives/patch-page_restrictions_sortkey.sql
with success.

So a fix should possible obtain the maximum key length from the
server, and if need be, reduce either the field size, or the part of
the field going into the index, accordingly.

Can you please explain how you managed to get a maximum key length of 500? I've only seen maximums of 1000 and 1024 before, so the schema was adjusted to work for 1000, with 3 bytes per character.

The maximum key length was apparently 500 on MyISAM prior to 4.1.2.

http://dev.mysql.com/doc/refman/4.1/en/myisam-storage-engine.html

Note that pr_type should be varchar(64). This was changed and backported to 1.10 as well. Some installations have it as 255 (by local test site has it too).

• This bug has been marked as a duplicate of bug 4445 ***