Most of queries on Tickets has id == EffectiveId condition
that is required to skip merged tickets. Usually percentage
of merged tickets is very low, so this condition is true.
However, DBs like Pg and Oracle estimate number of returned
records very wrong. Each column is very distinct and statisticly
speaking chance that one record has the same value in id and
EffectiveId is low, but this is only true when correlation
is not accounted and it's not accounted in mentioned DBs.

We add IsMerged column with "boolean" type that reflects
id != EffectiveId. It fixes estimations of DB optimizers.
Query execution plans get better.

Instead of adding the column we could use NULLs in EffectiveId
for records where now id = EffectiveId, but this means ORed
conditions in JOIN clause for rare queries. Tests show that
such solution result in terrible perfomance degradation on
mysql in these rare situations.