Unsticking VACUUM

Every PostgreSQL release adds new features, but sometimes the key to a release has less to do with what you add than with what you take away.  PostgreSQL 8.4, for example, removed the settings max_fsm_pages and max_fsm_relations, and replaced them with a per-relation free space map that no longer requires manual sizing.  Those parameters are now gone, and more importantly, something that you previously needed to understand and manage was replaced with something that just works.   People who are still running PostgreSQL 8.3, or older versions, want to understand exactly how the free space map works; people who are running PostgreSQL 8.4, or newer, don't care.  It's enough to know that it does work.

Now, about eight months ago, I wrote a blog entry on troubleshooting stuck vacuums.  I would not say that this is an everyday problem, but in ten years of working with PostgreSQL, I've seen it a few times, and it's very unpleasant.  It's easy to miss the fact that you have a problem at all, because in most cases, nothing immediately breaks.  Instead, system performance just slowly degrades, gradually enough that you may not realize what the problem is until things have gotten pretty bad and you need to CLUSTER or VACUUM FULL to recover.

Hint Bits

Heikki Linnakangas was doing some benchmarking last week and discovered something surprising: in some circumstances, unlogged tables were actually slower than permanent tables.  Upon examination, he discovered that the problem was caused by CLOG contention, due to hint bits not being set soon enough.  This leads to a few questions:

1. What is CLOG?
2. What are hint bits?
3. How does setting hint bits prevent CLOG contention?
4. Why weren't hint bits being set sooner?

Let's take those in order.

Fast Counting

Since I wrote my previous blog entry on index-only scans, quite a bit of additional work has been done.  Tom Lane cleaned up the code and improved the costing model, but possibly the most interesting thing he did was to allow index-only scans to be used for queries that don't involve an indexable condition at all.  The classic example is SELECT COUNT(*) FROM table.  In previous versions of PostgreSQL, there's just one way to implement this: sequential scan the table and count 'em up.  In PostgreSQL 9.2, that method will still, of course, be available, but now there will be another choice: pick any index you like and do a full index scan, checking whether each tuple is all-visible either using the visibility map or via a heap fetch.  So, how well does it work?

PostgreSQL Crash Debugging

As I mentioned in a previous blog post, I spend some of my time working in and with EnterpriseDB's support department.  And what that means is that every customer I talk to has a problem, typically a fairly serious problem, and they want  me to help them fix it.  Of course, to fix it, you first have to be able to identify the problem, and sometimes that's not so simple.  Database crashes can be among the more difficult cases to debug.

Deadlocks

Last week, someone pinged me on instant messenger to ask about the following message, which their PostgreSQL instance had just produced:

DETAIL: Process 22986 waits for ShareLock on transaction 939; blocked by process 22959.
Process 22959 waits for ShareLock on transaction 940; blocked by process 22986.

This message is a complaining about a deadlock.  But unless you've seen and debugged these a few times before, it might not be entirely obvious to you what's actually going on here.  What, exactly, did the offending processes do that caused the problem?

Index-Only Scans: We've Got 'Em

Tom Lane committed a patch for index-only scans by myself and Ibrar Ahmed, which also incorporated some previous work by Heikki Linnakangas, after hacking on it some more himself. Woohoo!

There is, of course, more work to be done here - performance fine-tuning, cost estimation, extensions to the core functionality - but the core of the feature is now in.  If you get a chance, please test it out and let us know how it works for you.

For those that may not have been following along at home, what we're essentially doing here is allowing any index to act as a "covering index".  If all of the columns the query needs are available from the index tuple, we'll skip fetching the corresponding heap (table) page if every tuple on that page is visible to all running transactions.

Although I know we're not even really done with this feature yet, I can't help wondering what's next.  Index-only scans have so often be cited as "the big performance feature that PostgreSQL is missing" that it's become something of a cliché.  Now that we have them, what will take their place as the next big thing?

CommitFest In Progress

I've seen a lot of articles lately about the great new features (and removed limitations) in PostgreSQL 9.1.  Unless you're a regular reader of pgsql-hackers, you could almost forget about the fact that PostgreSQL 9.2 development is in full swing.  In fact, there's a CommitFest going on right now and we could use a few more reviewers.

Many of the features that were submitted to this CommitFest are small improvements - minor fine-tuning of existing features, like generating better column names for subquery expressions, or fixing things so that LIKE can more reliably make use of indexes when non-English characters are involved.  But some of the big features that will hopefully become part of PostgreSQL 9.2 are also beginning to materialize.