Temporal Data in PostgreSQLTemporal Data in PostgreSQL

Scott BaileyThe Evergreen State CollegePostgreSQL Conference West 2009http://scottrbailey.wordpress.com

AgendaAgenda

✗ Provided data types✗ Extended data types✗ Temporal Databases✗ What is in store for PostgreSQL✗ Tips, tricks and questions

Thinking About TimeThinking About Time

✗ Temporal Quiz✗ How many minutes in a day?✗ How many seconds in a day?✗ How long does it take the Earth to complete one

rotation?✗ Actually, what is a second?✗ What number should be at the top of the clock?

✗ Extremely complicated!

Basic Data TypesBasic Data Types

✗ Timestamp &Timestamp with time zone✗ Granularity 1 microsecond✗ Can only subtract two timestamps and

add/subtract with intervals.✗ Timestamp with out time zone is default for

compliance with SQL specification.✗ Which should you use?

✗ Date ✗ Granularity 1 day✗ Can add/subtract dates & integers

Basic Types Cont.Basic Types Cont.

✗ Interval – a duration of time✗ Granularity 1 microsecond✗ Add/Subtract/Multiply/Divide

✗ Time / Time with time zone✗ Granularity 1 microsecond✗ Acts like both time and a (small) interval✗ Timezone info is useless without date

✗ Depricated types✗ abstime (Unix timestamp), reltime (limited

interval), timespan (interval), tinterval (period)

Period Data TypePeriod Data Type

✗ Period – defines an anchored interval or timespan✗ Often what we are modeling is not an instantaneous

event but a period over which some state is true. ✗ Typically implemented as a start and end timestamp.

Could be implemented with an anchor time and an interval.

✗ Theory has been around for a while. ✗ Current implementations – PgTemporal, Chronos✗ Write down a period representing today.✗ Can be open or closed intervals but are typically half-

open. ✗ [ ] indicates endpoints are contained () not contained.

Period Data TypePeriod Data Type

✗ Why a half open interval?✗ Period functions

✗ Positional✗ before(), after(), adjacent(), overlaps(), overleft(), over-

right(), starts(), ends()✗ Containment

✗ contains(), contained_by()✗ Manipulation

✗ shift(), grow(), shrink()✗ Set functions – periods are contiguous sets

✗ period_union(), period_intersect(), period_minus(), period_exclude()

Period IssuesPeriod Issues✗ When one end point is unknown

✗ Since – period(timestamp, NULL) ✗ Until – period(NULL, timestamp) not as common✗ Hard to do much with infinity.

✗ Referential integrity✗ Primary keys – exclusion, can't be two places at

once.✗ Foreign keys – containment, lifetime of child item

must be contained by parent's lifetime.

Non-contiguous Time SetsNon-contiguous Time Sets✗ Why would we need non-contiguous sets?✗ Set functions for periods can be applied to arrays of periods!

✗ Now we can solve (pretty easily) in SQL what would have taken LOTS of procedural code.

✗ Period Arrays vs Non-Contiguous SetsPeriod Array

NCS

Bitemporal DataBitemporal Data✗ “Temporal Database” vs temporal data

✗ Both Valid Time (VT) and Transaction Time (TT).

✗ Valid Time – A period for which a fact is true.✗ Employee X was in position Y from A to B.✗ Store not just the current fact but a historical record.

✗ Transaction Time – Records the time a row was inserted and superseded

✗ Provides time travel or temporal rollback.✗ Very much like the MVCC approach in PostgreSQL. ✗ Logging transaction ids with timestamp would let us

map xmin & xmax to times.

Current ImplementationsCurrent Implementations✗ Lots of theory out there, very few actual implementations.

✗ Oracle – Currently the best implementation in a general purpose database.

✗ Workspace Manager – version enable tables✗ Automatically renames table, adds versioning

metadata (VT), creates view w/ original table name, and defines INSTEAD OF triggers for DML.

✗ Handles temporal constraints.✗ Provides a period data type and a subset of the

functions provided in pgTemporal, Chronos.✗ Great implementation but solves only a single problem

domain and does not allow you to reuse them.

The Future for PostgreSQLThe Future for PostgreSQL✗ Version 8.5

✗ Temporal keys and constraints✗ Changes to the type system✗ Period data type either added to core or as a

contrib module!

✗ Version 8.6?✗ Full support for periods, NCS, table versioning and

time travel!✗ Hands down the most complete temporal

implementation of ANY general purpose database.

Additional ResourcesAdditional Resources✗ TSQL2 Specification -

http://www.cs.arizona.edu/~rts/tsql2.html

✗ Developing Time-Oriented Database Applications – Snodgrasshttp://www.cs.arizona.edu/~rts/tdbbook.pdf

✗ Temporal Data and the Relational Model – Date, Darwen, Lorentzoshttp://books.google.com/books?isbn=1558608559

✗ TimeDB – For Oracle and IBM Cloudscapehttp://www.timeconsult.com/Software/Software.html

✗ Oracle Workspace Manager - http://www.oracle.com/technology/products/database/workspace_manager/index.html