Partitioned Tables Partitions / partitioning / partitioned tables For very large tables Improve querying Easier admin Backup and recovery easier Optimiser knows when partitioning used Can use in SQL also

Creating a PT Create table JOHN ( IDnumber namevarchar2(25) agenumber constraint john_pk primary key (ID) ) partition by range (age) (partition PART1 values less than (21) partition PART2 values less than (40) partition PART3 values less than (maxvalue))

Warning Specification of partition is exclusive e.g. partition by range (name) (partition part1 values less than (‘F’) implies that f is excluded Maxvalue is a general term to pick up anything that failed so far Works for text as well as number

Hash Partition Only in Oracle 8i and above Uses a numerical algorithm based on partition key to determine where to place data Range partition = consecutive values together Hash = consecutive values may be in different partitions –Also gives more partitions = reduces the risk of contention

What is Hash? Imagine 8GB table – split in 8 / 1 GB No intuitive way to split data Or obvious way is totally imbalanced –1 partition 7BG MB –Huge variations in performance Randomise breakdown of data so objects of similar size –Select one column –Select number of chunks –Oracle does the rest!

Mechanics of Hashing Each record is allocated into a bucket based on key value – e.g. Name = John Applying the hashing function to the value John uniquely returns the bucket number where the record is located: e.g. using prime number –divide KEY by a prime number –If text, translation into numeric value using ASCII code –use remainder of the division = address on the disk –if record already at same address - pointer to overflow area.

Hashing Process A hash table, or a hash map, is a data structure that associates keys with values. The primary operation it supports efficiently is a lookup: given a key (e.g. a person's name), find the corresponding value (e.g. that person's telephone number). It works by transforming the key using a hash function into a hash, a number that is used as an index in an array to locate the desired location ("bucket") where the values should be.

Hash Example

Questions Why do we use hashing? How does it work? (exercise) se.des/cis350/hashing/WEB/HashApplet.h tm

Hash Partition - SQL Create table JOHN ( Namevarchar2(25) primary key, Agenumber, Years abroadnumber ) Partition by hash (age) Partitions 2 Store in (Part1_john, Part2_john);

Sub-Partitions Create table FRED ( Namevarchar2(25) primary key, Agenumber, Years abroadnumber ) Partition by range (years abroad) Subpartition by hash (name) Subpartitions 5 (partition Part1 values less than (1) partition Part2 values less than (3) partition Part3 values less than (6) partition Part4 values less than (MAXVALUE));

Indexing Partitions Performance requirements may mean Partitioned tables should be indexed Create index JOHN_NAME on JOHN (name) Local Partitions (Part1, Part2, Part3, Part4) Local means create separate index for each partition of the table Alternative is to create a global index with values from different partitions Global indexes cannot be created for Hash partitions