Re: simple function index question

Interestingly a version of this:

CREATE INDEX TX_QSET_DIR_MAP_PRI_URI_TYP_UK
ON tx_question_set_dir_map
USING btree
(question_set2tx_question_set ,uri_type,
(CASE WHEN uri_type = 201900 THEN null::varchar(100)
ELSE question_set_dir_map_key END);

Worked for me. Try it if you are still interested.
Just do proper casting of null (it was varchar(100) in my case).

​This is a multi-column index - with the third column being an expression - as opposed to a single-column index of a composite.

David J.
​
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You are right.
But the question is whether OP needs a single_column_index_of_a_composite or multi_column_index will do.

Regards,
Igor Neyman