PostgreSQL - Temporary Table

In PostgreSQL, a temporary table (or temp table) is a short-lived table that exists temporarily and is visible only within the current session. Once the session ends, the temporary table is automatically dropped.

Here are some key points and uses of temporary tables:

Creation:

You can create a temporary table using the CREATE TEMPORARY TABLE or CREATE TEMP TABLE command:

CREATE TEMPORARY TABLE temp_table_name (column_definitions);

For example:

CREATE TEMP TABLE temp_users AS SELECT * FROM users WHERE active = false;

Visibility:
- Temporary tables are only visible within the session in which they are created.
- Other sessions will not see the temporary table.
- If there's already a permanent table with the same name, the temporary table takes precedence in name resolution.
Lifespan:
- By default, a temporary table will last only for the duration of the session in which it was created.
- You can also specify the ON COMMIT option to drop the table at the end of a transaction. For instance:
```
CREATE TEMP TABLE temp_table_name (column_definitions) ON COMMIT DROP;
```
Usage:
Temporary tables can be useful in various scenarios:
- Intermediate Processing: For complex data manipulations where intermediate results are needed.
- Testing: If you need to test some queries without affecting the main tables.
- Backup: As a quick backup before performing certain operations.
- Performance: Sometimes, working with a subset of data in a temporary table can be faster than continuously querying a larger, permanent table, especially if the operations involve sorting or joining.
Location:
Temporary tables are generally stored in the pg_temp schema. However, you don't need to specify the schema when referencing them.
Indexes:
You can create indexes on temporary tables, which can improve the performance of queries involving the temp table. Remember, like the table itself, these indexes are temporary and will be dropped at the end of the session.
Considerations:
- While temporary tables can be beneficial, excessive or inappropriate use might lead to performance issues. Every creation of a temp table involves I/O operations and, if overused, could strain the system.
- Consider the scope and purpose of a temporary table. If data needs to persist beyond the current session, a regular table or other mechanisms like materialized views might be more appropriate.

Always think about the specific needs of your operations and the potential impacts on the database system when deciding to use temporary tables.

PostgreSQL create temporary table example: To create a temporary table in PostgreSQL, use the CREATE TEMPORARY TABLE statement:
```
CREATE TEMPORARY TABLE temp_table (
   id SERIAL PRIMARY KEY,
   name VARCHAR(255)
);
```
Create temporary table with data in PostgreSQL: You can create a temporary table and insert data in a single query:
```
CREATE TEMPORARY TABLE temp_table AS
SELECT id, name FROM source_table WHERE condition;
```
Insert into temporary table in PostgreSQL: Insert data into a temporary table:
```
INSERT INTO temp_table (name) VALUES ('John'), ('Jane'), ('Doe');
```
Drop temporary table in PostgreSQL: Drop a temporary table at the end of the session or when it's no longer needed:
```
DROP TEMPORARY TABLE IF EXISTS temp_table;
```