Postgres OnLine Journal   

We have covered this briefly before, but its an important enough concept to cover again in more detail.

Problem: You are running out of disk space on the drive you keep PostgreSQL data on

Create a new tablespace on a separate drive and move existing tables to it, or create a new tablespace and use for future tables.

What is a tablespace and how to create a tablespace

A tablespace in PostgreSQL is similar to a tablespace in Oracle and a filegroup in SQL Server. It segments a piece of physical disk space for use by the PostgreSQL process for holding data. Below are steps to creating a new tablespace. Tablespaces have existed since PostgreSQL 8.0.

More about tablespaces in PostgreSQL is outlined in the manual PostgreSQL 8.3 tablespaces

While it is possible to create a table index on a different tablespace from the table, we won't be covering that.

This has been bugging me for a long time and I finally complained about it and Tom Lane kindly gave a reason for the problem and that its by design and not a bug.

So I thought I would post the situation here without getting into too many embarassing specifics in case others have suffered from a similar fate and can learn from this.

The situation:

• You create a function lets call it myniftyfunc() in the public schema.
• Then you create another function that depends on myniftyfunc(), lets call it mysuperniftyfunc() also in public schema.
• Then because your function is such a super nifty function, you decide to create a functional index with that super function on your table that sits in mysuperdata schema - mysuperdata.mysupertable

Your super nifty function is doing its thing; your table is happy; the planner is spitting out your queries lightning fast using the super nifty index on your super table; The world is good.

One day you decide to restore your nifty database backup and to your chagrin, your nifty index is not there. The planner is no longer happily spitting out your queries lighting fast and everything has come to a painful crawl. Your super nifty index is gone. What happened to super nifty functional index?

I have to admit that I'm the type of person that assumes the public schema is always there and always in search_path and that my assumption is a flawed one. After all the public schema is there by default on new databases for convenience, but one can change it not to be in the search_path and in fact pg_dump does just that. So if everything you have is kept in public schema -- you don't run into this particular misfortune. If however you have your functions in public and your tables in different schemas, during restore -- the search path is changed to the schema being restored and your super functional indexes based on super functions that depend on other super functions fail because public is no longer in the search_path.

You'll often hear the term planner statistics thrown around by database geeks. Did you update your statistics. This lingo isn't even limited to PostgreSQL, but is part and parcel to how most decent databases work. For example in PostgreSQL you do a vacuum analyze to update your planner statistics in addition to cleaning up dead space. In SQL Server you do an UPDATE STATISTICS. In MySQL you do an ANALYZE TABLE or a more invasive OPTIMIZE TABLE.

Normally all this "update your stats so your planner can be happy" is usually unnecessary unless you just did a bulk load or a bulk delete or you are noticing your queries are suddenly slowing down. These stat things are generally updated behind the scenes by most databases on an as needed basis.

What makes SQL really interesting and a bit different from procedural languages is that it is declarative (like functional and logical programming languages) and relies on the database planner to come up with strategies for navigating the data. Its strategy is not fixed as it is in procedural languages. A big part of this strategy is decided on by the query planner which looks at distributions of data. Given different WHERE conditions for similar queries, it could come up with vastly different strategies if one value has a significantly higher distribution in a table than another. This is also the mystery of why it sometimes refuses to use an index on a field because it has decided a table scan is more efficient and also why some people consider HINTS evil because they pollute the imperative nature of the language.