Improving a Process That Updates Data :: SQL Server 2012

The same company that hired you to consult about its inefficient nightly process from the first scenario hires you again. They ask for your advice regarding the following update processes:
  1. The database has a table holding about 100 million rows. About a third of the existing rows need to be updated. Can you provide recommendations as to how to handle the update in order not to cause unnecessary performance problems in the system?

  2. There’s an UPDATE statement that modifies rows in one table based on information from related rows in another table. The UPDATE statement currently uses a separate subquery for each column that needs to be modified, obtaining the value of the respective column from the related row in the source table. The statement also uses a subquery to filter only rows that have matches in the source table. The process is very slow. Can you suggest ways to improve it?
  1. The customer should consider developing a process that handles the large update in chunks. If done in one big transaction, the process will very likely result in a significant increase in the transaction log size. The process will also likely result in lock escalation leading to blocking problems.

  2. The customer should consider using an UPDATE statement based on a join instead of the existing use of subqueries. The amount of code will be significantly reduced, and the performance will likely improve. Each subquery requires a separate visit to the related row. So using multiple subqueries to obtain values from multiple columns will result in multiple visits to the data. With a join, through one visit to the matching row, you can obtain any number of column values that you need.