Time in Databases CSCI 6442 With thanks to Richard Snodgrass, 1985 ACM /85/005/0236

Agenda Taxonomy of time in databases Point-in-time databases 2

The Problem Suppose you are away from your company for 6 months assigned to work at an outpost in Australia, in the opal mines in Coober Pedy, underground, out of communication. You are eligible for a raise in the midst of this. When you return they notice this. Now it is approved and entered into the database within a few days, but you get the extra money from 3 months ago Which times get recorded in the database? 3

Types of Time Transaction time—time when the information was stored in the database Valid time—the time that the stored information models reality User-defined time—a time that is important to and is defined by the application 4

Static Databases A conventional database models the current state of a dynamically changing world When an update is made, the past values are forgotten completely This sort of database is called a static database It can’t tell you what Jones’s salary was two years ago or how many raises Jones has had in the past four years 5

Static Rollback Databases We can provide the capability to roll back a database to a previous state, such as by using a log The effect of recent transactions is removed, until the desired state is reached Then the intended query can be posed Past values cannot be changed; only operations on current values are permitted, because this preserves the series of past operations Transaction time must be recorded for every change, to support rollback Current operations must be suspended while rollback operations are under way 6

Historical Databases Historical databases record the history of the values of data as it is best known As errors are discovered, they are corrected by changing the database Previous states are not retained; at any time, the database reflects the best-known history of the data Time represented in historical database is valid time, the time that the stored information models 7

Temporal Databases A temporal database allows one to view tuples valid at one moment as seen from another moment Such a database completely captures the history of current and retroactive changes 8

Snodgrass Wrote “A Taxonomy of Time in Databases” (1985) Regarded as the authoritative work Defines three types of time 9

Types of Time There are three times: Transaction time—when the change is made to the database Valid time—when the change was approved User-defined time—when the raise is effective (application- specific) 10

Static Database Database that reflects changing values with a snapshot at any particular time Does not necessarily reflect the state of the real world We discard past states of the database Can’t answer questions about what past values were Can’t show trends in change over time 11

Static Rollback Database Adds some time sequence Ability to back out transactions is provided Can back out one at a time to previous states of the database Such a database shows the history of transactions rather than the history of the actual data 12

Snodgrass’s Taxonomy Is a useful way to talk about time in databases His levels of databases are theoretical, a basis for research What if we want to model current and past states of a single table in a SQL database system? 13

Point in Time Databases We can model current and former values of data in a single relational table We need to add time to the data model so that changes are captured It is possible to use a single table to capture current and former values of data in a single relation As performance limitations of databases disappear because of increasing machine power, this becomes practical In fact, this approach is used in some advanced applications However, it is not widely known 14

Point In Time Databases A single database models the present and all past states of the data as it matters to the organization Enough information about past states is stored so that the past state can be obtained using a SQL query Instead of deleting rows they are marked as no longer valid When values are updated, the old values are kept as well This was impractical in 1970 because of cost and efficiency issues Is it practical today? 15

What We Store With each change to a row, we insert a new row, instead of changing the previous row Consider EMP(EMPNO, ENAME, SAL) We add the attributes START and END EMP(EMPNO, ENAME, SAL, START, END) New attributes are start and end times for validity When we change a row, that time is the END time for validity of the previous row, START time of validity for the new row These times are usually what Snodgrass calls “user-defined” time because that’s the time that matters to the organization His “valid time” is just a date of transaction approval Transaction time is when entered into the database Let’s call this time “effective time” because it’s the time when the row’s values are effective 16

Point In Time Operations Insert: New row is added Start time is set Stop time is left null Delete: Stop time is set Update: New row added with new values, start time Old row is kept and stop time is set Read: Current values: STOP ISNULL As of values: ASOF >= START AND (ASOF < STOP OR STOP ISNULL) 17

Example: Retroactive Pay Increase Chen is given a salary increase The salary increase is approved at date t 1 The increase is effective at date t 2. However, the increase does not get entered into the database until some other time, t 3 >t 2 (called a retroactive salary increase). In this case, t 1 is an attribute of the approval process, t 2 is the start of effective time and t 3 is the transaction time. Most important to the organization is the time when Chen is getting the higher pay 18

An Example: EMP Schema Definition: EMP(EMPNO,ENAME,JOB,SAL,DEPTNO) Now we add START and STOP for validity dates: EMP(EMPNO,ENAME,JOB,SAL,DEPTNO,START,STOP) START gives the first time of validity for the values in the row STOP gives the last time of validity for the values in the row A row with no value for STOP is the current value 19

Example To find the status of EMP on 1/1/2008 SELECT * FROM EMP WHERE START = to_date( '01-JAN-2008','dd-mmm-yyyy') OR STOP ISNULL); DELETE statements will now be UPDATEs instead 20

Complexities Not every column value needs to be tracked Can simplify by not tracking some column values This is a semantic issue Change in primary key If primary key changes, hard to relate changed row to original row Potential case for use of generated key Cascading changes Other tables may change because of one change Use trigger instead of automatic foreign key update Cascading deletes Other tables may have rows deleted because of one change Use trigger instead of automatic foreign key delete 21

Capabilities These techniques allow you to implement a database that can show its state as of any time in the past This can be done while the database is actively being used and even updated by multiple processes One process can be looking at the database as of two years ago while another is doing current processing You can record retroactive changes You can implement applications to process retroactive changes 22

Bottom Line This is a practical approach that potentially simplifies data models and processing One table replaces duplication of data and completely different functions One somewhat more complex function replaces two (or more) separate functions Use can be presented a unifying interface that simplifies use of the application It’s a tradeoff—sometimes use a separate history table 23

Homework You have the opportunity to explore this You get to build a point-in-time EMP table Then you use it to give a retroactive raise And then compute how much we owe to the person with the retroactive raise 24