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PostgreSQL temporary tables

I need to perform a query 2.5 million times. This query generates some rows which I need to AVG(column) and then use this AVG to filter the table from all values below average. I then need to INSERT these filtered results into a table.

The only way to do such a thing with reasonable efficiency, seems to be by creating a TEMPORARY TABLE for each query-postmaster python-thread. I am just hoping these TEMPORARY TABLEs will not be persisted to hard drive (at all) and will remain in memory (RAM), unless they are out of working memory, of course.

I would like to know if a TEMPORARY TABLE will incur disk writes (which would interfere with the INSERTS, i.e. slow to whole process down)

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Nicholas Leonard Avatar asked Jan 28 '09 01:01

Nicholas Leonard


People also ask

What are temporary tables in PostgreSQL?

A temporary table, as the name implies, is a short-lived table that exists for the duration of a database session. PostgreSQL automatically drops the temporary tables at the end of a session or a transaction.

Where are temporary tables stored in Postgres?

Temporary tables get put into a schema called "pg_temp_NNN", where "NNN" indicates which server backend you're connected to. This is implicitly added to your search path in the session that creates them.

Does Postgres have temporal tables?

Requirements. Temporal Tables Extension requires PostgreSQL 9.2 or higher.

How do I drop a temp table in PostgreSQL?

In PostgreSQL, one can drop a temporary table by the use of the DROP TABLE statement. Syntax: DROP TABLE temp_table_name; Unlike the CREATE TABLE statement, the DROP TABLE statement does not have the TEMP or TEMPORARY keyword created specifically for temporary tables.


2 Answers

Please note that, in Postgres, the default behaviour for temporary tables is that they are not automatically dropped, and data is persisted on commit. See ON COMMIT.

Temporary table are, however, dropped at the end of a database session:

Temporary tables are automatically dropped at the end of a session, or optionally at the end of the current transaction.

There are multiple considerations you have to take into account:

  • If you do want to explicitly DROP a temporary table at the end of a transaction, create it with the CREATE TEMPORARY TABLE ... ON COMMIT DROP syntax.
  • In the presence of connection pooling, a database session may span multiple client sessions; to avoid clashes in CREATE, you should drop your temporary tables -- either prior to returning a connection to the pool (e.g. by doing everything inside a transaction and using the ON COMMIT DROP creation syntax), or on an as-needed basis (by preceding any CREATE TEMPORARY TABLE statement with a corresponding DROP TABLE IF EXISTS, which has the advantage of also working outside transactions e.g. if the connection is used in auto-commit mode.)
  • While the temporary table is in use, how much of it will fit in memory before overflowing on to disk? See the temp_buffers option in postgresql.conf
  • Anything else I should worry about when working often with temp tables? A vacuum is recommended after you have DROPped temporary tables, to clean up any dead tuples from the catalog. Postgres will automatically vacuum every 3 minutes or so for you when using the default settings (auto_vacuum).

Also, unrelated to your question (but possibly related to your project): keep in mind that, if you have to run queries against a temp table after you have populated it, then it is a good idea to create appropriate indices and issue an ANALYZE on the temp table in question after you're done inserting into it. By default, the cost based optimizer will assume that a newly created the temp table has ~1000 rows and this may result in poor performance should the temp table actually contain millions of rows.

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vladr Avatar answered Sep 27 '22 20:09

vladr


Temporary tables provide only one guarantee - they are dropped at the end of the session. For a small table you'll probably have most of your data in the backing store. For a large table I guarantee that data will be flushed to disk periodically as the database engine needs more working space for other requests.

EDIT: If you're absolutely in need of RAM-only temporary tables you can create a table space for your database on a RAM disk (/dev/shm works). This reduces the amount of disk IO, but beware that it is currently not possible to do this without a physical disk write; the DB engine will flush the table list to stable storage when you create the temporary table.

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Adam Hawes Avatar answered Sep 27 '22 18:09

Adam Hawes