Values larger than 1/3 of a buffer page cannot be indexed.

hi,

when trying to insert a long-long value, I get the following error:

 index row size 3120 exceeds maximum 2712 for index "ir_translation_ltns"
HINT:  Values larger than 1/3 of a buffer page cannot be indexed.
Consider a function index of an MD5 hash of the value, or use full text indexing.

is there a way to generate this recommended function index of an md5 hash on an already existing database and tables (filled with data)?

thanks,
Viktor