refactor: Optimize getUserIdMappingWithEitherSuperTokensUserIdOrExternalUserId query

[mavwolverine]

Summary of change

Optimize getUserIdMappingWithEitherSuperTokensUserIdOrExternalUserId query to use union all instead of or

Related issues

#1094

explain analyze 
SELECT * FROM subscriberdb.st_userid_mapping WHERE app_id = 'public' AND (supertokens_user_id = '123' OR external_user_id = '123');

-> Filter: ((subscriberdb.st_userid_mapping.supertokens_user_id = '123') or (subscriberdb.st_userid_mapping.external_user_id = '123'))  (cost=732 rows=6566) (actual time=38.3..120 rows=1 loops=1)
    -> Index lookup on st_userid_mapping using PRIMARY (app_id='public')  (cost=732 rows=34559) (actual time=0.0114..102 rows=72308 loops=1)

Also, for our environment, this query is taking around 0.100 ms.

With union all;

explain analyze 
SELECT *FROM subscriberdb.st_userid_mapping WHERE app_id = 'public' AND supertokens_user_id = '123'
union all
SELECT * FROM subscriberdb.st_userid_mapping WHERE app_id = 'public' AND external_user_id = '123';

-> Append  (cost=0..0 rows=1) (actual time=0.00295..0.00323 rows=1 loops=1)
    -> Stream results  (cost=0..0 rows=0) (actual time=678e-6..678e-6 rows=0 loops=1)
        -> Zero rows (no matching row in const table)  (cost=0..0 rows=0) (actual time=73e-6..73e-6 rows=0 loops=1)
    -> Stream results  (cost=0..0 rows=1) (actual time=0.00165..0.00182 rows=1 loops=1)
        -> Rows fetched before execution  (cost=0..0 rows=1) (actual time=72e-6..133e-6 rows=1 loops=1)

This query runs in less than 0.20ms.

As a single query, 0.100ms and 0.20ms doesn't seem to be a big difference, but with thousands of users suddenly jumping on the site at 9AM, it seems to be causing latency issues and eventually downtime for us.

[mavwolverine]

@sattvikc Can you please look into this? Our production is down for second time :(