1. RUM Conjecture of Database Access Method

2. Database Access Method
Defines how to store and access data in a database system
Heap file
Sorted file
B+ Tree index
Hash Index …

3. Requirements of A Good Access Method
Read optimized
how much time it cost to read the data
e.g., range search, equality search
Update optimized
how much time it cost to update the data
e.g., insert, delete, update
Memory (Space) optimized
how much space it uses to store the data
e.g., auxiliary structures, duplicates

4. However… RUM Conjecture
RUM Conjecture: An access method that can minimize two out of the read, update, and memory overheads, cannot optimize the third overhead!
In other words, there might not be the “best” access method that is optimal for all the three requirements!

5. Example: Append-Only Log
Read unoptimized
A search may need to scan the entire log
Space unoptimized
When delete/update an entry, old entry is not deleted in-place
Update optimized
Each update simply append the entry to the log

6. Example: B+Tree Read optimized: Update unoptimized: Space unoptimized:
Each search only visits logB(N) pages
Update unoptimized:
An update requires visiting at least logB(N) pages
Space unoptimized:
Pages often are not full

7. Example: Hash Index Read optimized: Update unoptimized:
A query simply checks O(1) bucket
Update unoptimized:
An update may need to split overflow buckets
Space unoptimized:
Buckets often are not full

8. Other Access Methods