Keith's Ramblings…

WARNING: If accidentally read, induce vomiting

Partitioning With Custom Time Intervals

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This has been something I’ve had in the works since I started on this extension, but it took a while to settle on the best method to use. The pre-determined time ranges for PG Partition Manager were done because it was easy and, as it turns out, provide a very noticeable performance improvement when used with the static partitioning method. Note that custom time partitioning, while the most flexible, is the least performant of the 3 methods now supported with this extension.

The biggest roadblock to implementing this was trying to find an answer to the following question:

If an interval can be any value, how do you determine the partition that the currently inserted timestamp goes into?

Seems a simple enough question, but when you try and implement it, you realize the impact solving it can have on the potential performance of every insert to the table. My solution in the end (which I was trying to avoid) was to create a lookup table. Thankfully the new range type added in PostgreSQL 9.2 made this a whole lot easier than it could’ve been, but it does mean this feature is only supported on the latest versions. If anyone has an efficient solution as an algorithm that can be implemented solely in the partitioning function, I’d be extremely grateful for a contribution.

Until then, pg_partman 1.6.0 hopefully now supports any interval value from 1 second up to the minimum and maximum values for the timestamp type that PostgreSQL supports. Less than that is potentially possible, but even a partitioning interval of 1 second seemed ridiculous enough to me, so I doubt I’m going to pursue it. And I say hopefully because it’s impossible for me to make unit tests for every possible interval there is. I test for intervals less than 1 minute (due to the new child table naming pattern) and greater than 100 years (due to the seemingly non-standard century partition boundaries (2001, 2101, 2201, etc)) so hopefully that will be enough for now. Would greatly appreciate feedback.

This version also bring the ability to manually designate your starting partition at creation. Be aware that if you’re using static partitioning, the trigger function on the parent may not cover all the partitions this could create. The partitioning functions (partition_data_time() & partition_data_id()) will create these tables automatically when you migrate data out of the parent. But if you’re starting from scratch, I thought this might make things easier so if you’re loading data externally, you can have the tables already set to have data loaded directly into them. Below is an example of using both the new “time-custom” type and setting a custom start partition (by default it would’ve only created 4 partitions behind CURRENT_DATE)

I’ve gotten a lot of positive responses (and bug fixes!) from the community so far and am glad that so many people are finding this a useful tool. It’s nice to see that the effort has been worth it.

Written by Keith

February 13th, 2014 at 11:34 am

Posted in PostgreSQL

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Version Specific PostgreSQL Extension Features

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When I say version specific here, I’m speaking of the PostgreSQL version (9.1, 9,2, etc), not the extension version. An upcoming update to PG Partition Manager takes advantage of the range data type, which is a 9.2 feature. But I also wanted to keep backward compatibility with 9.1 by excluding this new feature.

The more important thing is to get the extension installed in the first place, so the Makefile was where I started.

Keep in mind, I know very little about Makefiles in the first place, so my solution is using just what I already knew how to do in the original Makefile. There may be a better way, and if there is, I’d love feedback. I just expanded on the already existing lines that check for a minimal version of 9.1 to even allow the extension to install. And I organized my code folders to have one specifically for 9.2+ features (sql/92/tables/). After that it’s just another check to see if the PG92 variable is true, and if so, grab the code from that additional folder.

Now the next challenge is checking the version when installing an update to an already existing installation. First, we need a means of checking if the current version is new enough. I made up the below function for inclusion in my extension since I’ll actually be doing version checking elsewhere as well.

Now we can just take advantage of the DO block syntax to allow script code in a plain SQL file.

The full implementation of this will be available in the 1.6.0 update for pg_partman that I’ll be releasing in the near future. It contains a feature I’ve wanted to get done for a while now: custom time intervals!

Written by Keith

January 22nd, 2014 at 10:44 am

Posted in PostgreSQL

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Managing Constraint Exclusion in Table Partitioning

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One of the biggest advantages of table partitioning in databases is taking advantage of a feature called constraint exclusion. The PostgreSQL docs explain this best:

With constraint exclusion enabled, the planner will examine the constraints of each partition and try to prove that the partition need not be scanned because it could not contain any rows meeting the query’s WHERE clause. When the planner can prove this, it excludes the partition from the query plan.

Using the following table partitioned on col1, some examples of how this works will follow

Without constraint exclusion, doing a simple SELECT * for a smaller subset of the partition set still does a scan across all tables (constraint exclusion is turned on by default with the setting “partition”).

Even though this is doing INDEX scans, doing this against a partition set with much larger amounts of data would be extremely expensive and cause significant delays. With constraint exclusion on, you can see the plan is greatly simplified and only the tables that need to be queried actually are

However, when you create a partitioned set, usually only the column that has a constraint on it is the column that is controlling the partitioning. As soon as you do a query with a WHERE condition on one of the other columns, you lose all the advantages you thought you gained. In this case its even worse because it causes full sequential scans across the whole set. Indexes could help, but again, not if your partition set is very large.

You could try and add constraints before you insert the data, but often that is either hard or impossible. Another alternative is to go back and create constraints on old partitions that are no longer being edited based on the data they contain. That is the solution I built into pg_partman with the 1.5.0 release.

The second constraint is the new one that was created based on the data currently in the child table. The configuration column constraint_cols is an ARRAY that can contain as many columns for the given table that you’d like to have constraints set for. The above is how you can go back and add additional constraints to an already existing partition set. The python script will apply the constraint to all relevant child partitions. It determines which children to apply the constraints to by the premake pg_partman configuration value for that partition set (default is 4, so it will not apply constraints to child tables newer than the current and the previous 4. See the pg_partman docs for more details on what the premake value is used for).

So, while it won’t apply constraints to all the old children, it can at least allow constraint exclusion to potentially exclude a huge majority of them. In this case it was able to exclude partitions _p30, _p40 & _p50. As more partitions are added, more constraints would get added to older partitions and at most the tables that would need to be scanned would be the current one, the last 4, the empty future ones and whichever partitions contained the relevant data. On a partition set with thousands of partitions and/or partitions with millions of rows each, that would have significant performance benefits.

The big caveat with this is that it would prevent edits on any of these older tables if the value would change the constraint boundaries. If there comes a time when you must edit this data, some included functions can help. The drop_constraints() and apply_constraints() functions can drop then reapply all constraints managed by pg_partman on a given child table (see docs for their parameters). They’ve been designed to cleanly handle if constraints created by pg_partman already exist or if the columns contain only NULL values. These functions only work on a single child table at a time. If you need to drop/apply to the entire partition set, the python script used  above can make that much easier.

You can set this up during partition creation with a parameter to the create_parent() function. Or it can be added at any time later like I did above. Once the additional constraint columns are configured, the normal partition maintenance procedures that pg_partman runs will take care of creating the constraints for you.

This is a pretty tricky situation to handle and its one of the things that makes partitioning a big challenge to set up correctly. And it’s something most people don’t realize until they’ve already gotten their partitioning set up and only encounter the issue when they start noticing the performance impacts of scanning their now very large partition set. Hopefully this feature will allow more people to take advantage of partitioning when long term storage of data is required within PostgreSQL. If anyone has any feedback or suggestions for making this feature better, I’d greatly appreciate it.

Written by Keith

January 8th, 2014 at 11:21 am

Monitoring Streaming Slave Lag Effectively

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One of the easiest ways to monitor slave lag when using streaming replication is to turn hot standby on your slave and use pg_last_xact_replay_timestamp() and/or the other recovery information functions. Here’s an example query to run on the slave systems to get the number of seconds behind it is:

The issue with this query is that while your slave(s) may be 100% caught up, the time interval being returned is always increasing until new write activity occurs on the master that the slave can replay. This can cause your monitoring to give false positives that your slave is falling behind if you have things set up to ensure your slaves are no more than a few minutes behind. A side affect of this monitoring query can also give you an indication that writes to your master have stopped for some reason.

One of our clients has a smaller sized database that doesn’t get quite as much write traffic as our typical clients do. But it still has failover slaves and still needs to be monitored just like our other larger clients to ensure it doesn’t fall too far behind. So, my coworker introduced me to the pg_stat_replication view that was added in PostgreSQL 9.1. Querying this from the master returns information about streaming replication slaves connected to it.

He also provided a handy query to get back a simple, easy to understand numeric value to indicate slave lag. The issue I ran into using the query is that this view uses pg_stat_activity as one of its sources. If you’re not a superuser, you’re not going to get any statistics on sessions that aren’t your own (and hopefully you’re not using a superuser role as the role for your monitoring solution). So, instead I made a function with SECURITY DEFINER set, made a superuser role the owner, and gave my monitoring role EXECUTE privileges on the function.

Running this query gives back a few handy columns that should be good enough for most monitoring tools. You can easily add more columns from pg_stat_replication or any other tables you need to join against for more info.

This can also be useful to monitor slave lag when you don’t have hot standby turned on for your slaves to allow read-only queries.

Combining both of the replication monitoring solutions mentioned in this post should give you a much better overall picture of the status of your master/slave systems.

Written by Keith

October 30th, 2013 at 10:51 am

Posted in PostgreSQL

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PostgreSQL Extension Code Organization

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I was going to title this “Why is my extension failing to install in 9.3?”, but I figured a more general post about how I’ve been organizing my extension code would be more helpful. But I will also be answering that question along the way since my organization method provided a solution for me.

For extension installation, the code that installs objects within the database must be in a file in the format extension–version.sql. If your extension is primarily C code with just a few functions to provide calling methods, the SQL is usually pretty easy to maintain in a single file. You can organize the pre-compiled source code as needed and the Makefile can then designate the files that get compiled and installed during installation. But if your entire extension is PL/pgSQL (or any of the other scripting languages), it can really start to get hard to manage all that code in a single file, which is what I see most people doing.

Extensions were my first venture into even looking at the contents of a Makefile. I used the template from David Wheeler’s PGXN HowTo to get started. Learning what the bit of code below does gave me a clue to a way to manage things a little easier.

Essentially that tells make to copy the contents of whatever is after the colon into the filename given before the colon.  So my solution was to split all my functions, tables, types, etc out into their own files organized into folders and then have make pull it all together into a single file.

The order of the files listed after the colon is important since make pulls things together in the order given. In the single sql file for my pg_partman extension, types must be created before tables and tables must be created before functions. Your extension may need things done in a different order, so the important thing is to just organize your files in a manner that you can give make a specific order to merge things.

The main extension file (extension–version.sql) doesn’t necessarily have to contain all the installation code for the extension. You can include commands to read in your code stored in other files instead of having make merge it all together. But then whenever you have a new file, you have to remember to include it in the master file and possibly add it to the Makefile as well to ensure it gets copied over. This also puts more files in the PGSHARED folder. More of a pet peeve, but I like keeping that folder less cluttered.

The other thing I do to organize my extension code is to put the updates into a separate folder. For a while, I hadn’t realized that the following line in the PGXN Makefile was for copying all update files (filename format extension–oldversion–newversion.sql) along with the core extension file to the PGSHARED folder

So my extensions had just included instructions to manually copy updates to PGSHARED. But, when the 9.3 beta came out and I started testing things, I (along with many other extension developers) started getting this error during make install

This pgsql-hackers mailing list thread contains a discussion of what is causing this error & possible patches for PGXS to fix it back to the way it used to be. From what I’m understanding, it’s because the wildcard in the DATA value ($(wildcard sql/*–*.sql)) matches the second file’s naming pattern (sql/$(EXTENSION)–$(EXTVERSION).sql) and it’s trying to copy the same file over. A change in 9.3 PGXS is now causing an error to be thrown instead of either just ignoring it or copying it anyway (not sure what it originally did before 9.3).

Because I have my updates in a separate folder than where the core extension file is created, I can have a different DATA value and avoid this problem all together

Unfortunately since many people used the PGXN Makefile template, a lot of third-party extensions are running into this error after they upgrade to 9.3. Hopefully clarifying what is causing the error can help you fix your Makefile so your extensions can be installed on 9.3 without any problems.

Some of my extensions also contain scripts or alternate versions of functions for different purposes. For those I just make an “extras” folder and keep them there. And then for the scripts, I’ve added something like the following line to the Makefile to have them installed along with the extension code.

For more Makefile options see the PostgreSQL documentation on the Extension Building Infrastructure.

I’m not sure this code organization method for my extensions is the best way to do things. But the extension system is still fairly new, so the standard methods for doing things are still in flux. The only real issue I run into now is having to make sure the code in the update files matches the code in the individual object files. I use Meld to compare my update file to the object files to make sure things match up. Not the most elegant way, but it’s worked for quite a few updates of my extensions over the last year or so.

If anyone has any suggestions or sees any issues with this organization method, I’d appreciate feedback.

 

Written by Keith

October 25th, 2013 at 10:43 am

Posted in PostgreSQL

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