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# 5. ADVANCED POSTGRES SQL FEATURES

Having covered the basics of using POSTGRES SQL to access your data, we will now discuss those features of POSTGRES that distinguish it from conventional data managers. These features include inheritance, time travel and non-atomic data values (array- and set-valued attributes). Examples in this section can also be found in `advance.sql` in the tutorial directory. (Refer to the introduction of the previous chapter for how to use it.)

## 5.1. Inheritance

Let's create two classes. The capitals class contains state capitals which are also cities. Naturally, the capitals class should inherit from cities.

             CREATE TABLE cities (
                     name            text,
                     population      float,
                     altitude        int            -- (in ft)
             );

             CREATE TABLE capitals (
                     state           char2
             ) INHERITS (cities);

In this case, an instance of capitals inherits all attributes (name, population, and altitude) from its parent, cities. The type of the attribute name is text, a built-in POSTGRES type for variable length ASCII strings. The type of the attribute population is float4, a built-in POSTGRES type for double precision floating point numbres. State capitals have an extra attribute, state, that shows their state. In POSTGRES, a class can inherit from zero or more other classes,[4] and a query can reference either all instances of a class or all instances of a class plus all of its descendants. For example, the following query finds all the cities that are situated at an attitude of 500 'ft or higher:

             SELECT name, altitude
             FROM cities
             WHERE altitude > 500;

             +----------+----------+
             |name      | altitude |
             +----------+----------+
             |Las Vegas | 2174     |
             +----------+----------+
             |Mariposa  | 1953     |
             +----------+----------+

On the other hand, to find the names of all cities, including state capitals, that are located at an altitude over 500 'ft, the query is:

             SELECT c.name, c.altitude
             FROM cities* c
             WHERE c.altitude > 500;

which returns:

             +----------+----------+
             |name      | altitude |
             +----------+----------+
             |Las Vegas | 2174     |
             +----------+----------+
             |Mariposa  | 1953     |
             +----------+----------+
             |Madison   | 845      |
             +----------+----------+

Here the * after cities indicates that the query should be run over cities and all classes below cities in the inheritance hierarchy. Many of the commands that we have already discussed -- select, update and delete -- support this * notation, as do others, like alter command.

## 5.2. Time Travel

POSTGRES supports the notion of time travel. This feature allows a user to run historical queries. For example, to find the current population of Mariposa city, one would query:

             SELECT * FROM cities WHERE name = 'Mariposa';

             +---------+------------+----------+
             |name     | population | altitude |
             +---------+------------+----------+
             |Mariposa | 1320       | 1953     |
             +---------+------------+----------+

POSTGRES will automatically find the version of Mariposa's record valid at the current time. One can also give a time range. For example to see the past and present populations of Mariposa, one would query:

             SELECT name, population
             FROM cities['epoch', 'now']
             WHERE name = 'Mariposa';

where "epoch" indicates the beginning of the system clock.[5] If you have executed all of the examples so far, then the above query returns:

             +---------+------------+
             |name     | population |
             +---------+------------+
             |Mariposa | 1200       |
             +---------+------------+
             |Mariposa | 1320       |
             +---------+------------+

The default beginning of a time range is the earliest time representable by the system and the default end is the current time; thus, the above time range can be abbreviated as ``[,].''

## 5.3. Non-Atomic Values

One of the tenets of the relational model is that the attributes of a relation are atomic. POSTGRES does not have this restriction; attributes can themselves contain sub-values that can be accessed from the query language. For example, you can create attributes that are arrays of base types.

### 5.3.1. Arrays

POSTGRES allows attributes of an instance to be defined as fixed-length or variable-length multi-dimensional arrays. Arrays of any base type or user-defined type can be created. To illustrate their use, we first create a class with arrays of base types.

             * CREATE TABLE SAL_EMP (
                     name            text,
                     pay_by_quarter  int4[],
                     schedule        char16[][]
             );

The above query will create a class named SAL_EMP with a text string (name), a one-dimensional array of int4 (pay_by_quarter), which represents the employee's salary by quarter and a two-dimensional array of char16 (schedule), which represents the employee's weekly schedule. Now we do some INSERTSs; note that when appending to an array, we enclose the values within braces and separate them by commas. If you know C, this is not unlike the syntax for initializing structures.

             INSERT INTO SAL_EMP
                  VALUES ('Bill',
                          '{10000, 10000, 10000, 10000}',
                          '{{"meeting", "lunch"}, {}}');

             INSERT INTO SAL_EMP
                  VALUES ('Carol',
                          '{20000, 25000, 25000, 25000}',
                          '{{"talk", "consult"}, {"meeting"}}');

By default, POSTGRES uses the "one-based" numbering convention for arrays -- that is, an array of n elements starts with array[1] and ends with array[n]. Now, we can run some queries on SAL_EMP. First, we show how to access a single element of an array at a time. This query retrieves the names of the employees whose pay changed in the second quarter:

             * SELECT name
               FROM SAL_EMP
               WHERE SAL_EMP.pay_by_quarter[1] <>
                       SAL_EMP.pay_by_quarter[2];

             +------+
             |name  |
             +------+
             |Carol |
             +------+

This query retrieves the third quarter pay of all employees:

             * SELECT SAL_EMP.pay_by_quarter[3] FROM SAL_EMP;

             +---------------+
             |pay_by_quarter |
             +---------------+
             |10000          |
             +---------------+
             |25000          |
             +---------------+

We can also access arbitrary slices of an array, or subarrays. This query retrieves the first item on Bill's schedule for the first two days of the week.

             * SELECT SAL_EMP.schedule[1:2][1:1]
               FROM SAL_EMP
               WHERE SAL_EMP.name = 'Bill';

             +-------------------+
             |schedule           |
             +-------------------+
             |{{"meeting"},{""}} |
             +-------------------+

4. i.e., the inheritance hierarchy is a directed acyclic graph.
5. On UNIX systems, this is always midnight, January 1, 1970 GMT.

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