Link cost table to payer_plan_period table

[gowthamrao]

Proposal to link cost table to payer_plan_period_id.
Add the pk of payer_plan_period to cost table

Cost

Add new field - optional

Field	Required	Type	Description
payer_plan_period_id	No	integer	A foreign key to the payer_plan_period_id in the payer_plan_period table that was responsible for the payment related to the cost

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Use cases -
http://forums.ohdsi.org/t/how-to-represent-visit-payer-information-in-the-model/369
http://forums.ohdsi.org/t/add-drug-benefit-flag-to-payer-plan-table/2093 (can be addressed by plan)
Meets use cases of both payers and providers. Providers may not have denominator file that contains timespans when a person had coverage but did not receive care - in those cases, the payer_plan_period_start_date and payer_plan_period_end_date will be the period the person received care (i.e. same as visit_start_date and visit_end_date). Payers who have beneficiary denominator file will use payer_plan_period to represents spans of time with coverage irrespective of visits.

Backward compatible - yes

Related to:
#107
#120

[gowthamrao]

looks like cost table is already linked to payer-plan-period. so this proposal is not required and may be deleted!