Enrich Running Tasks with ROOT_PROC_INST_ID_

Given is a processdefinition which contains a multi-instance-callactivity.
The called processdefinition contains UserTasks.

The collection from which the mutli-instance is consuming is 1001 elements long, which leads to 1001 sub-processinstances wich leads to 1001 tasks.

We want to list all tasks which are (directly) created and currently running from our root-processinstance.

If you have history enabled, than you could query this list:

processEngine.getHistoryService()
                     .createHistoricTaskInstanceQuery()
                     .rootProcessInstanceId("1234")
                     .unfinished()

Asking the historic-tables is not recommended because it could contain a huge amount of data.

If you don't have history enabled, you have to query all processInstanceIds which have "1234" as rootProcessInstance and execute page-requests aftewards, because you will get 1001 IDs, which are not allowed in IN-clauses.

Describe the solution you'd like

Enrich the TaskEntity with the rootProcessInstanceId and enrich the query:

  processEngine.getTaskService()
                     .createTaskQuery()
                     .rootProcessInstanceId("1234")
                     .list()

Additional context

Since we are adding a column, we have to fill existing rows the data via SQL.
We could take the ACT_RU_EXECUTION.ROOT_PROC_INST_ID_ since the EXECUTION_ID_ is already in the table ACT_RU_TASK.EXECUTION_ID_

I'm not quite sure if this should already add an index on the new column. If not, this should be noted in die JavaDoc of this new method.

Start with TaskEntity and the TaskService createTaskQuery() entry point, then trace ACT_RU_TASK.EXECUTION_ID_ to ACT_RU_EXECUTION.ROOT_PROC_INST_ID_. Done means the rootProcessInstanceId("1234") query returns currently running tasks without history, existing rows are populated via SQL, and the index decision is documented.