Database Indexing

Sequential Index §

• Can be dense or sparse
  • Primary Index: Key indices are often sparse, because the rows are ordered using the key index
  • Secondary Index: Index on other attributes are often dense b/c/ v.v.
• You can manually create indexes in SQL: {postgresql}CREATE INDEX indexname ON tablename(columnname)
• Multi-column index: Index $(A,B)$ on $R(A,B,C)$ has keys that have $A,B$ concatenated and sorted together.
  • See: sql server - what does a B-tree index on more than 1 column look like? - Stack Overflow
  • Quote: “With most implementations, the key is simply a longer key that includes all of the key values, with a separator. No magic there;-). In your example the key values could look something like:"123499|John Doe|Conway, NH","32144|Bill Gates| Seattle, WA"

Index Sequential Access Method (ISAM) §

• Lookup:
• Insert / Delete:

B+ Trees §

• Architectural Improvement on ISAM
  • Each block can fan-out a specific amount $n$
  • Each block containes $n-1$ entries to indicate ranges
  • Example:
• Leaves are sequential → Range queries are possible:
• Guaranteed to be well-balanced (=all nodes are at least half-full)
  • Guaranteed by recursive node splitting (on insert) and recursive node coalescing or entry stealing (on delete)
  • Example:
• Performance: access time is $O(\text{height})=O({\log }_{\text{fanout}}\text{\#Tuples})$
  • → You can fit this in memory (4-level B+ Trees are often good enough)
  • …recall you also need to node-split of node-coalesce sometimes too