Temporal Database

A temporal database is a collection of transactions ordered by their timestamps. A sample temporal database generated from the set of items, I={a,b,c,d,e,f}, is shown in below table:

TID

Timestamp

Transactions

1

1

a, b, c

2

2

d, e

3

4

a, e, f

4

7

d, f, g

Types of temporal databases:

  • Regular temporal database: Uniform time gap exists between any two transactions.

  • Irregular temporal database: Non-uniform time gap exists between any two transactions.

    • Type-1 irregular temporal database: Every transaction will have a distinct timestamp.

    • Type-2 irregular temporal database: Multiple transactions can have a common timestamp.

Rules to create a temporal database:

  • Since TID of a transaction implicitly represents the row number, this information can be ignored to save space.

  • The first column in the database must represent a timestamp.

  • The timestamp of the first transaction must always start from 1. The timestamps of remaining transactions follow thereafter. In other words, the timestamps in a temporal database must be relative to each other, rather than being absolute timestamps.

  • Irregular time gaps can exist between the transactions.

  • Multiple transactions can have a same timestamp. In other words, multiple transactions can occur at a particular timestamp. (Please note that some pattern mining algorithms, especially variants of ECLAT, may not work properly if multiple transactions share a common timestamp.)

  • All items in a transaction must be seperated with a separator.

  • The items in a temporal database can be integers or strings.

  • ‘ Tab space ’ is the default seperator. However, temporal databases can be constructed using other seperators, such as comma and space.

Format of a temporal database:

>>>  timestamp<sep>item1<sep>item2<sep>...<sep>itemN

Examples:

  • Regular temporal database: Uniform time gap exists between the transactions.

    1 a b c

    2 d e

    4 a e f

    7 d f g

  • Irregular temporal database (Type-1): Non-uniform time gap exists between the transactions. More important, every transaction contains a unique timestamp.

    1 a b c

    2 d e

    4 a e f

    7 d f g

  • Irregular temporal database (Type-2): Non-uniform time gap exists between the transactions. More important, multiple transactions can have same timestamps.

    1 a b c

    1 d e

    4 a e f

    8 d f g