String splitting and operations on only some results - postgresql

Question

I have strings that look like this:
schedulestart | event_labels
2018-04-04 | 9=TTR&11=DNV&14=SWW&26=DNV&2=QQQ&43=FTW
When I look at it in the database. I have code that relies in this string in this format to display a schedule with events with those labels on those days.
Now I find myself needing to break down the string in postgres for reporting/analysis, and I can't really pull out the string and parse it in another language, so I have to stick to postgres.
I've figured out a way to unpack the string so my results look like this:
User ID | Schedule Start | Unpacked String
2 | 2018-04-04 | TTR
2 | 2018-04-04 | 9
2 | 2018-04-04 | DNV
2 | 2018-04-04 | 11
2 | 2018-04-04 | SWW
2 | 2018-04-04 | 14
2 | 2018-04-04 | DNV
2 | 2018-04-04 | 26
select schedulestart, unnest(string_to_array(unnest(string_to_array(event_labels, '&')), '=')) from table;
Now what I need is a way to actually perform an interval calculation (so 2018-04-04+11 days::interval), and I can if I only get a numbers list, but I need to also bind that result to each string. So the goal is an output like this:
eventdate | event_label
2018-04-12 | TTR
2018-04-20 | DNV
Where eventdate is the schedule start + which day of the schedule the event is on. I'm not sure how to take the unpacked string I created and use it to perform date calculations, and tie it to the string.
I've considered doing only one unnest, so that it's 11=TTR and 14=DNV, but I'm not sure how to take that to my desired result either. Is there a way to read a string until you reach a certain character, and then use that in calculations, and then read every character past a certain character in a string into a new column?
I'm aware completely rewriting how this is handled would be ideal, but I did not initially write it, and I don't have the time or means to rewrite the ~20 locations this is used.

Answer 1

Here is your table (I added userid column):
CREATE TABLE test(userid INTEGER, schedulestart DATE, event_labels VARCHAR);
And input data:
INSERT INTO test(userid,schedulestart , event_labels) VALUES
(2,DATE '2018-04-04', '9=TTR&11=DNV&14=SWW&26=DNV&2=QQQ&43=FTW');
And finally the solution:
SELECT
userid,
(schedulestart + (SPLIT_PART(kv,'=',1)||' days')::INTERVAL)::DATE AS eventdate,
SPLIT_PART(kv,'=',2) AS event_label
FROM (
SELECT
userid,schedulestart,
REGEXP_SPLIT_TO_TABLE(event_labels, '&') AS kv
FROM test
WHERE userid = 2
) a