Here are the key differences between them:
USE_CONCAT:
This hint instructs the optimizer to transform OR conditions into a series of UNION ALL operations. This can improve performance by allowing the optimizer to handle each part of the OR condition separately.
It is particularly useful when the OR conditions involve different columns or when the selectivity of the conditions varies significantly.
OR_EXPAND:
Similar to USE_CONCAT, this hint also transforms OR conditions into UNION ALL operations. However, it is more aggressive in its approach.
OR_EXPAND is typically used when the optimizer might not automatically choose to expand the OR conditions, but you want to force it to do so for performance reasons.
In summary, both hints aim to optimize queries with OR conditions by converting them into UNION ALL operations, but they differ in their aggressiveness and specific use cases.