Keith's Ramblings…

Per-Table Autovacuum Tuning

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A pattern that seems to drive my blog posts definitely seems to be the frequency of client questions. And that is definitely the case here again. Vacuum tuning to manage bloat and transaction id wraparound on production systems has been a hot topic and lately this has even been getting down to tuning autovacuum on the individual table basis. I’ve already discussed bloat pretty extensively in previous posts. While I’d like to get into the details of transaction ID wraparound, that really isn’t the focus of this post, so I’ll defer you to the documentation.

One setting I will discuss first though is autovacuum_freeze_max_age. Basically when any table’s max XID value reaches this, a more aggressive “emergency” autovacuum is kicked off. If many tables hit this at the same time (a common occurrence with data-warehouses that have many large, sparsely written tables), that can kick off some pretty high and long lasting IO caused by these autovacuums. I highly recommend increasing autovacuum_freeze_max_age from the default value of 200 million to 1 billion. However, I don’t recommend doing this unless you have some monitoring in place to watch for tables reaching both autovacuum_freeze_max_age and wraparound, the latter of which can shut your database down for some pretty extensive downtime.

So ideally, we want autovacuum running often enough on its own so you never encounter any of those bad situations above. The next thing I’d recommend tuning on any database before getting down to the table-level methods, no matter its size or traffic, are the default settings that control when autovacuum initially kicks in.

In my opinion, the defaults on these are not ideal for any situation. The scales are too high and the thresholds are too low. The scale factor settings are percentage values that say, “When this percentage of the table’s rows have changed (updates/deletes), run vacuum or analyze”. The threshold settings say that “When this many rows have changed, run vacuum or analyze”. These two settings are used together to determine the actual threshold that will kick in autovacuum. The formula taken from the documentation shows this:

So this means with the default settings, a 100,000 row table will have autovacuum kick in when 20050 rows have changed and autoanalyze will kick in when 10050 have changed. Not too bad, but my recommended default settings are are this:

This halves the scale factor settings so autovacuum is a bit more aggressive on how often it kicks in. My reason for increasing the threshold values is because when you have very small tables, you really don’t want autovacuum then being overly aggressive on them after decreasing the scale settings. Vacuuming too often may not necessarily be a bad thing, but it’s just wasted IO at that point.

So how about when you start getting into very large tables? Can you see why this could be a problem?

We would have to wait until nearly 160 million rows have changed before autovacuum would ever kick in. Even with my recommended defaults, 80 million rows is still quite a bit to wait for. For a high write rate table, this might work out ok, but for most cases this likely won’t be ideal. This affects bloat buildup since autovacuum may not run often enough to clear up available space for new/updated rows. And it could also cause the global XID value to increase quite high, more than likely reaching autovacuum_freeze_max_age before a normal autovacuum process would ever touch it. This is when we need to start tuning on a per-table basis and all four of the above settings can be set that way.

So the first question to ask is how often do we want autovacuum to run? I’ll start from a baseline of at least once per day for a starting point. This means we’ll need a way to see how many rows have changed per day.

The pg_stat_all_table view (or pg_stat_user_tables to exclude system tables) can help us get this. PostgreSQL doesn’t keep long term trending data on table writes, but it does keep counters. Specifically the n_tup_upd & n_tup_del columns can tell us how many tuples/rows where changed since the statistics for this database were last reset. So if we can collect this data once per day around the same time, we can get an idea of the write rate that would affect when autovacuum would kick in. Here’s a simple command you can add to a cronjob to generate some CSV data you could then pull into a spreadsheet and quickly do calculations.

Just add n_tup_upd+n_tup_del and compare that to the previous day’s total value and you have your rows changed per day. If you want it to run more/less often, then just adjust your collection interval as needed. So let’s say there were around 300,000 to 500,000 rows changed per day on our giant 800mil row table. We would want autovacuum to run when at least 400,000 rows changed. This means we want to ignore the scale factor setting completely and rely only on the threshold value. That’s the other problem with the scale factor and large tables: as they grow larger and larger, autovacuum runs less and less since the % value of that table grows larger. So to set the above settings on a per table basis, we can use the ALTER TABLE command to adjust the above settings for just that table.

We usually want analyze to run more often than a vacuum so queries can have accurate statistics. Analyzes are fairly lightweight, even on large tables, so it’s ok to be more aggressive about when those run.

Once we have those settings in place, we can then use the same stats view to see if autovacuum & autoanalyze are running when we want them too. last_autoanalyzelast_autovacuum are the timestamp of when they last completed.

You may need it to vacuum more often than this depending on the turn-over of updates, deletes & inserts to keep bloat under control. To determine that I recommend setting up bloat monitoring. If you see that it keeps growing, you can look at vacuuming more often or even adjusting the fillfactor settings for the table or indexes.

Another suggestion that I will make is for databases that have any tables that get few-to-no writes, especially large ones. First, upgrade to a recent version of PostgreSQL if you’re not on at least 9.6. Then, schedule a regular VACUUM of your database, or at least the tables that are no longer being touched often. 9.6 introduced a feature that if all of the rows on a page are frozen, then autovacuum is able to completely skip over that page and not have to evaluate each individual row. This can be a HUGE saving on both time and IO. And the larger the table the more the savings you’ll see in the long run. For anyone that is running their databases in the cloud where IO is money, this can save you quite a bit there. Typically tuning autovacuum will preclude needing to schedule a manual vacuum, but if it turns out it’s needed in some cases, running during off-peak hours won’t hurt anything.

The other issue that people often encounter is long running autovacuums or autovacuum incurring higher IO costs than anticipated. I’ll be working on another blog post in the future that covers these issues. Hopefully this gives you a good start on how to better tune autovacuum for your growing databases.

Written by Keith

October 1st, 2018 at 2:43 pm

Posted in PostgreSQL

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Removing A Lot of Old Data (But Keeping Some Recent)

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I’ve had this situation crop up a few times with clients and after a discussion on #postgresql on Freenode recently, decided a blog post may be in order. The pitfalls that lead me to this solution are useful to cover and it seems a useful set of steps to have documented and be able to share again later.

There comes a time for most people when you have a table that builds up quite a lot of rows and you then realize you didn’t actually need to keep all of it. But you can’t just run a TRUNCATE because you do want to keep some of the more recent data. If it’s just a few million small rows, it’s not a huge deal to just run a simple DELETE. But when that starts getting into the billions of rows, or your rows are very large (long text, bytea, etc), a simple DELETE may not be realistic.

The first, and arguably easiest way, to deal with this would be to run the DELETE in batches instead of one large transaction. This allows you to add a pause in between the batches to help control I/O. Another side affect of such large delete operations can be an excessively high amount of WAL generation. This not only contributes to I/O, this can also dramatically increase disk space usage. When you’re trying to delete data due to disk space constraints, this can end up making things even worse before they get better. And if you’re deleting quite a lot of data from over a long period of time that didn’t receive many deletes before, you likely won’t get much disk space back at the end without doing a VACUUM FULL or pg_repack (see my discussion on bloat here). And the biggest issue of all when disk space is critical is either one of those options requires that you have at least as much disk space available as a full copy of the unbloated table would take up. So if disk space is the crucial problem most people are trying to solve with removing old data, how can we easily do this?

The first option that’s most easily done on almost any RDBMS is to make a secondary table and have new data copied/redirected there. The I/O and WAL generation of moving the smaller, required amount of data is much less than removing all the old data. Then you can just drop the old table. And most people do this via a trigger-based method: have every write to the old table also write to the new one as well. Then once you’re sure things are working, you can take a brief lock on both the old and new tables and swap their names. PostgreSQL makes this last step much easier, having transactional DDL. I’ll cover the commands to do this later since my final solution is similar to this. The main problems surrounding this come when the table you’re trying to clean up is a very high-traffic table. Doing a trigger like this basically doubles all writes which could possibly cause some I/O issues. There’s also the fact of making sure you get that trigger function code right, otherwise all writes break. Can be a little nerve wracking on critical production systems. But there is a way to avoid both the additional I/O of double writes and the headache of writing triggers.

The actual solution we came up for this involves using the often overlooked feature of table inheritance in PostgreSQL. The steps basically go like this:

  1. Create a new table exactly like the first one
  2. Set ownership/privileges on the new table to match the old table
  3. Have the old table INHERIT the new table
  4. Swap old and new table names
  5. Move the data you want to keep from the old table to the new one
  6. Drop the old table

As soon as you do step 4, all new data is immediately going to the new table. Also, since the old table is a child of the new table, all your old data is still visible from the original table name. Moving the data from the old table to the new one is nearly transparent to any users of the table, the only issue being there may be some slightly slower queries during that transition period since the planner has to account for 2 tables. You can help mitigate this slightly by placing a constraint on the new table (before the swap when it’s empty) that only allows data in the new table’s window. This allows constraint exclusions to possibly ignore the old table while you get data moved. Placing a constraint on the old table probably wouldn’t help much since it would have to lock it for validation. Yes there will be additional I/O and disk usage from WAL while you move data from the old table to the new, but this can be controlled to a much greater degree since all new data isn’t also being written twice by a trigger. You can move the data in batches with any necessary pauses to control those I/O & WAL spikes.

The one thing this method does not account for is if there are updates to data in the old table that would technically place it in the new one while you’re in the process of moving the recent data you want to keep. That update will not be moved from the old table to do the new one. But in most cases where we’ve done this, those updates weren’t that critical since, if you’d done the original method of just deleting the old data, you would’ve been deleting that data that was getting updated anyway. But it’s something to be aware of if you go querying the old table and still see “new” data after you think you’re done migrating. It’s also something to be aware of if your application thinks it updated something when the old table was there and now it’s suddenly gone. You may need to stick with the trigger method above if that’s the case then.

An example of commands to do this is below. The LIKE clause to the CREATE TABLE statement is quite useful since this can automatically include all indexes, constraints, defaults, comments, and storage options. It does NOT include ownership or privileges however, so the one critical step in this piece is definitely #2 above. You can easily see a table’s privileges with the \dp option in psql. Also, explicitly obtaining the exclusive lock on both tables before doing the name switch ensures nothing weird happens during whatever brief moment could exist between the switch.

Once all these steps are done, you can then begin the process of moving your more recent data out of the old table and into the new via whichever method works best for you. One easy method to batch this is a CTE query that does the DELETE/INSERT with a SELECT in a single query to limit the rows moved.

And once that’s done, you can then DROP the old table, instantly recovering all that disk space with minimal WAL traffic and zero bloat aftermath!

Written by Keith

March 15th, 2017 at 11:32 am

Posted in PostgreSQL

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PostgreSQL 10 Built-in Partitioning

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Since I have a passing interest in partitioning in PostgreSQL, I figured I’d check out a recent commit to the development branch of PostgreSQL 10

Implement table partitioning –;a=commitdiff;h=f0e44751d7175fa3394da2c8f85e3ceb3cdbfe63

After many years of waiting, one of the major features missing from PostgreSQL is finally getting its first major step forward with the inclusion of a built in partitioning option. The syntax and usage is fairly straight forward so let’s jump straight into it with the examples from the documentation (slightly modified)

The basic syntax has two parts, one for initially creating a partitioned set

And another for adding a child table to that partition set

A ranged partition set is likely the most common use-case and is what pg_partman provides for time & id partitioning. Each child table is given a specific upper and lower bound of data. Once nice thing is that it can support multiple columns as well.

You can see that unlike all partitioning methods up until now, there is no user-visible trigger involved. All the data routing is handled internally which greatly simplifies setting things up and opens up greater opportunities for optimization. Also, constraints and some other properties are automatically inherited from the parent. But since indexes cannot be defined on the parent, they have to be defined per child. Above you can see you can define different defaults per child as well as other properties.

The other partitioning type is LIST and this is for explicitly defining values to go into specific children.

Here you can see the default of the sequence is inherited and an example of defining a constraint on the child. So what happens if you try to insert data and a child table is not defined for it?

Invalid data is currently rejected. It also seems to have incremented the sequence as well (I re-ran this again to be sure). Currently I’m not sure if there’s a way around the data rejection, and defining a trigger with a exception handler is not a good idea. As I found out the hard way in pg_partman, just the existence of an exception block in a trigger will also increase xid consumption since every row inserted will increment the global xid value. If this table has a high insert rate, you can quickly reach xid wraparound. I made a suggestion to the core devs to define a child table as the default and have any data that doesn’t match a child table go there instead.

Sub-partitioning is also possible and you can easily mix and match both range and list partitioning types. Here we redefine the cities_west table to be partitioned as well

Global indexes do not yet exist in PostgreSQL, so the issue of primary/unique keys being enforced across a partition set is still an issue. And while you can define foreign keys on individual children to other tables, defining them back to a partitioned set will not work. Also, foreign keys cannot be defined on the parent table with this new feature, so they’ll have to be set per child. Thankfully they just return an error if you try any of these things instead of allowing them and causing confusion the way most current partition setups do.

To get rid of a partitioned set, you’d use the CASCADE option just like with an inheritance set

Currently the TRUNCATE command run on the parent table will truncate the entire partition set, but I did see discussion on the mailing lists of a WHERE clause to only target specific children. That would be a great feature, but even better would be a WHERE clause to the DROP TABLE command to make retention maintenance to drop old tables much easier.

You may have noticed there’s no mention of ongoing creation of new partitions when needed. This is where I see pg_partman still being useful at this time. I’ve only just barely started playing with this feature to see how I can incorporate it. My main method of determining what characteristics were inherited to children (indexes, constraints, defaults, FKs etc) was done by defining them on the parent, but the current limitations on what can be defined on the parent with this new feature make this trickier to handle. Whether I’ll keep my old methods and just add this on as a new feature or do another major code revision will have to wait to be seen. It will all depend if I can find a way to implement all existing features I’ve added to pg_partman since I’d rather not take any steps backwards if I don’t have to. Reading the relevant threads on the mailing lists, it seems they have a lot more features for partitioning to be added in, so I may wait a bit before diving into incorporating this into pg_partman too tightly since I may over complicate things on my part due to a current lack of features. This email from Robert Haas gives me hope this this will be a well maintained core feature –

From the moment I started writing pg_partman, I knew built-in partitioning wasn’t too far around the corner. But we build the tools that we need at that time. I’m looking forward to seeing how this feature develops and I hope I can help out where possible to overcome any limitations encountered. Many thanks to Amit Langote and the review team!

Written by Keith

December 12th, 2016 at 11:48 am

Posted in PostgreSQL

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Cleaning Up PostgreSQL Bloat

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As a followup to my previous post on checking for bloat, I figured I’d share some methods for actually cleaning up bloat once you find it. I’ll also be providing some updates on the script I wrote due to issues I encountered and thanks to user feedback from people that have used it already.

First, as these examples will show, the most important thing you need to clean up bloat is extra disk space. This means it is critically important to monitor your disk space usage if bloat turns out to be an issue for you. And if your database is of any reasonably large size, and you regularly do updates & deletes, bloat will be an issue at some point. I’d say a goal is to always try and stay below 75% disk usage either by archiving and/or pruning old data that’s no longer needed. Or simply adding more disk space or migrating to new hardware all together. Having less 25% free can put you in a precarious situation where you may have a whole lot of disk space you can free up, but not enough room to actually do any cleanup at all or without possibly impacting performance in big ways (Ex. You have to drop & recreate a bloated index instead of rebuilding it concurrently, making previously fast queries extremely slow).


The above graph (y-axis terabytes) shows my recent adventures in bloat cleanup after using this new scan, and validates that what is reported by is actually bloat. In both this graph and the one below, there were no data purges going on and each of the significant line changes coincided exactly with a bloat cleanup session. You can see back on May 26-27th a huge drop in size. You can see an initial tiny drop followed by a fairly big increase then the huge drop. This is me first fixing one small, but very bloated index followed by running a pg_repack to take care of both table and a lot of index bloat. This is actually the group_members table I used as the example in my previous post. Over the next week or so I worked through roughly 80 bloated objects to recover about 270GB of disk space. Now, it may turn out that some of these objects will have their bloat return to their previous values quickly again and those could be candidates for exclusion from the regular report. But I figured I’d go through everything wasting more than a few hundred MB just so I can better assess what the actual normal bloat level of this database is. Here’s another example from another client that hadn’t really had any bloat monitoring in place at all before (that I was aware of anyway). It’s showing disk space available instead of total usage, hence the line going the opposite direction, and db12 is a slave of db11.


The easiest, but most intrusive, bloat removal method is to just run a VACUUM FULL on the given table. This will take an exclusive lock on the table (blocks all reads and writes) and completely rebuild the table to new underlying files on disk. This clears out 100% of the bloat in both the table and all indexes it contains at the expense of blocking all access for the duration. If you can afford the outage, it’s the easiest, most reliable method available. For very small tables this is likely your best option.

The next option is to use the REINDEX command. This can be run on several levels: INDEX, TABLE, DATABASE. 9.5 introduced the SCHEMA level as well. Running it on the TABLE level has the same consequence of likely locking the entire table for the duration, so if you’re going that route, you might as well just run a VACUUM FULL. Same for running at the DATABASE level, although if you’re running 9.5+, it did introduce parallel vacuuming to the vacuumdb console command, which would be much more efficient. When running on the INDEX level, things are a little more flexible. All writes are blocked to the table, but if a read-only query does not hit the index that you’re rebuilding, that is not blocked. If you can afford several shorter outages on a given table, or the index is rather small, this is the best route to take for bloat cleanup.

If you’ve got tables that can’t really afford long outages, then things start getting tricky. Before getting into pg_repack, I’d like to share some methods that can be used without third-party tools. Index bloat is the most common occurrence, so I’ll start with that.

If you’ve just got a plain old index (b-tree, gin or gist), there’s a combination of 3 commands that can clear up bloat with minimal downtime (depending on database activity). The CONCURRENTLY flag to the CREATE INDEX command allows an index to be built without blocking any reads or writes to the table. So say we had this bloated index

No dead tuples (so autovacuum is running efficiently) and 60% of the total index is free space that can be reclaimed. A handy command to get the definition of an index is pg_get_indexdef(regclass). In this case it’s a very easy index definition, but when you start getting into some really complicated functional or partial indexes, having a definition you can copy-n-paste is a lot safer.

Now we can write our set of commands to rebuild the index

I threw the ANALYZE calls in there just to ensure that the catalogs are up to date for any queries coming in during this rebuild. May not really be necessary, but I was doing this on a very busy table, so I’d rather be paranoid about it. Neither the CREATE nor the DROP command will block any other sessions that happen to come in while this is running. However, that final ALTER INDEX call can block other sessions coming in that try to use the given table. But the rename is optional and can be done at any time later. After the DROP command, your bloat has been cleaned up. While concurrent index creation does not block, there are some caveats with it, the major one being it can take much longer to rebuild the index. One of these for the second client above took 4.5 hours to complete. The documentation on building indexes concurrently goes into more detail on this, and how to deal with it possibly failing.

If you’re running this on a UNIQUE index, you may run into an issue if it was created as a UNIQUE CONSTRAINT vs a UNIQUE INDEX. Functionally, both are the same as far as PostgreSQL is concerned. And under the hood, creating a unique constraint will just create a unique index anyway. The big difference is you will not be able to drop a unique constraint concurrently. You will have to do an ALTER TABLE [..]. DROP CONSTRAINT […] call, which will require an exclusive lock, just like the RENAME above. Also, the index is more flexible since you can make a partial unique index as well. So it’s better to just make a unique index vs a constraint if possible.

PRIMARY KEYs are another special case. Functionally, they’re no different than a unique index with a NOT NULL constraint on the column. But they are marked specially in the catalog and some applications specifically look for them. You can do something very similar to the above, taking advantage of the USING clause to the ADD PRIMARY KEY command.

I gave full command examples here so you can see the runtimes involved. The concurrent index creation took quite a while (about 46 minutes), but everything besides the analyze commands was sub-second. Giving the command to create a primary key an already existing unique index to use allows it to skip the creation and validation usually done with that command.

As always, there are caveats to this. If the primary key, or any unique index for that matter, has any FOREIGN KEY references to it, you will not be able to drop that index without first dropping the foreign key(s). If there’s only 1 or 2 of those, you can likely do this in a transaction surrounding the drop & recreation of the primary key with commands that also drop and recreate the foreign keys. But if you start getting more in there, that’s just taking a longer and longer outage for the foreign key validation which will lock all tables involved. And also increasing the likelyhood of an error in the DDL you’re writing to manage recreating everything. In that case, it may just be better to take the outage to rebuild the primary key with the REINDEX command.

In all cases where I can use the above methods, I always try to use those first. They’re the native methods built into the database and, as long as you don’t typo the DDL commands, not likely to be prone to any issues cropping up later down the road. And since index bloat is primarily where I see the worst problems, it solves most cases (the second graph above was all index bloat). If you’re unable to use any of them, though, the pg_repack tool is very handy for removing table bloat or handling situations with very busy or complicated tables that cannot take extended outages. It’s gotten pretty stable over the last year or so, but just seeing some of the bugs that were encountered with it previously, I use it as a last resort for bloat removal. Also, if you’re running low on disk space, you may not have enough room for pg_repack since it requires rebuilding the entire table and all indexes in secondary tables before it can remove the original bloated table. As I said above, I did use it where you see that initial huge drop in disk space on the first graph, but before that there was a rather large spike to get there. In that case, the table had many, many foreign keys & triggers and was a very busy table, so it was easier to let pg_repack handle it.

For table bloat, Depesz wrote some blog posts a while ago that are still relevant with some interesting methods of moving data around on disk. This can also be handy when you are very low on disk space.

Bloat Removal Without Table Swapping

Bloat Removal By Tuples Moving

Since I initially wrote my blog post, I’ve had some great feedback from people using already. I’ve gotten several bugs fixed as well as adding some new features with version 2.1.0 being the latest available as of this blog post. json is now the preferred, structured output method if you need to see more details outside of querying the stats table in the database. I also added some additional options with –exclude_object_file  that allows for more fine grained filtering when you want to ignore certain objects in the regular report, but not forever in case they get out of hand. I updated the README with some examples of that since it’s a little more complex.

I also made note of the fact that this script isn’t something that’s made for real-time monitoring of bloat status. Since it’s doing full scans on both tables and indexes, this has the potential to force data out of shared buffers. So if you keep running it often, you may affect query performance of things that rely on data being readily available there. It’s best to run it maybe once a month or once a week at most during off-peak hours. If you have particularly troublesome tables you want to keep an eye on more regularly, the –tablename option allows you to scan just that specific table and nothing else. Once you’ve gotten the majority of your bloat issues cleaned up after your first few times running the script and see how bad things may be, bloat shouldn’t get out of hand that quickly that you need to run it that often. If it is, you may want to re-evaluate how you’re using PostgreSQL (Ex. MVCC makes it not great as a queuing system).

If anyone else has some handy tips for bloat cleanup, I’d definitely be interested in hearing them.

Written by Keith

June 8th, 2016 at 5:25 pm

Posted in PostgreSQL

Tagged with , , , ,

Checking for PostgreSQL Bloat

with 6 comments

My post almost 2 years ago about checking for PostgreSQL bloat is still one of the most popular ones on my blog (according to Google Analytics anyway). Since that’s the case, I’ve gone and changed the URL to my old post and reused that one for this post. I’d rather people be directed to correct and current information as quickly as possible instead of adding an update to my old post pointing to a new one. I’ve included my summary on just what exactly bloat is again below since that seemed to be the most popular part.

UPDATE: I wrote a followup post on methods for actually Cleaning Up PostgreSQL Bloat once you’ve identified it.

The intent of the original post was to discuss a python script I’d written for monitoring bloat status: Since that time, I’ve been noticing that the query used in v1.x of that script (obtained from the module) was not always accurate and was often not reporting on bloat that I knew for a fact was there (Ex: I just deleted over 300 million rows, vacuumed & analyzed the table and still no bloat? Sure it could happen, but highly unlikely). So I continued looking around and discovered the pgstattuple contrib module that comes with PostgreSQL. After discussing it with several of the core developers at recent PostgreSQL conferences (PGConfUS & PGCon) I believe this is a much, much better way to get an accurate assessment of the bloat situation. This encouraged me to do a near complete rewrite of my script and v2.0.0 is now available. It’s not a drop-in replacement for v1.x, so please check the –help for new options.

pgstattuple is a very simple, but powerful extension. It doesn’t require any additional libraries to be loaded and just adds a few functions you can call on database objects to get some statistics about them. The key function for bloat being the default one, pgstattuple(regclass), which returns information about live & dead tuples and free space contained in the given object. If you read the description below on what bloat actually is, you’ll see that those data points are exactly what we’re looking for. The difference between what this function is doing and what the query is doing is quite significant, though. The check_postgres query is doing its best to guess what is dead & free space based on the current statistics in the system catalogs. pgstattuple actually goes through and does a full scan on the given table or index to see what the actual situation is. This does mean this query can be very, very slow on large tables. The database I got the examples below from is 1.2TB and a full bloat check on it takes just under 1 hour. But with the inaccuracies I’ve seen being returned by the simpler query, this time can be well worth it. The script stores the statistics gathered in a table so they can be easily reviewed at any time and even used for monitoring purposes, just like check_postgres.

Before showing what the script can do, I just want to re-iterate some things from my old post because they’re important. Bloat percentage alone is a poor indicator of actual system health. Small tables may always have a higher than average bloat, or there may always be 1 or 2 pages considered waste, and in reality that has next to zero impact on database performance. Constantly “debloating” them is more a waste of time than the space used. So the script has some filters for object size, wasted space and wasted percentage. This allows the final output of the bloat report to provide a more accurate representation of where there may actually be problems that need to be looked into.

Another option is a filter for individual tables or indexes to be ignored. If you understand why bloat happens, you will come across cases where a table is stuck at a certain bloat point at all times, no matter how many times you VACUUM FULL it or run pg_repack on it (those two things do remove it, but it quickly comes back). This happens with tables that have a specific level of churn with the rows being inserted, updated & deleted. The number of rows being updated/deleted is balanced with the number of rows being inserted/updated as well as the autovacuum schedule to mark space for reuse. Removing the bloat from tables like this can actually cause decreased performance because instead of re-using the space that VACUUM marks as available, Postgres has to again allocate more pages to that object from disk first before the data can be added. So bloat is actually not always a bad thing and the nature of MVCC can lead to improved write performance on some tables. On to the new script!

So as an example of why this new, slower method can be worth it, here’s the bloat report for a table and its indexes from the old script using check_postgres

Here’s the results from the statistic table in the new version

Yes, all those indexes did exist before. The old query just didn’t think they had any bloat at all. There’s also a nearly 4x difference in wasted space in the table alone. It’s only 37% of the table in this case, but if you’re trying to clean up bloat due to low disk space, 12GB can be a lot. Another really nice thing pgstattuple provides is a distinction between dead tuples and reusable (free) space. You can see the dead tuple space is quite low in this example. That means autovacuum is running efficiently on this table and marking dead rows from updates & deletes as re-usable. If you see dead tuples is high, that could indicate autovacuum is not running properly and you may need to adjust some of the vacuum tuning parameters that are available. In this case, even a normal vacuum was not freeing the reusable space back to the operating system. See below for why this is. This means either a VACUUM FULL or pg_repack run is required to reclaim it. Here’s the result from making a new index on user_id:

You can see the new index group_members_user_id_idx1 is now down to only 9% wasted space and much smaller. Here’s the result after running pg_repack to clear both the table and all index bloat:

PostgreSQL 9.5 introduced the pgstattuple_approx(regclass) function which tries to take advantage of some visibility map statistics to increase the speed of gathering tuple statistics but possibly sacrificing some accuracy since it’s not hitting each individual tuple. It only works on tables, though. This option is available with the script using the –quick argument. There’s also the pgstatindex(regclass) that gives some more details on index pages and how the data in them is laid out, but I haven’t found a use for that in the script yet.

The same output options the old script had are still available: –simple to provide a text summary useful for emails & –dict which is a python dictionary that provides a structured output and also greater details on the raw statistics (basically just the data straight from the table). UPDATE: As of version 2.1.0 of the script, the –json & –jsonpretty options have been added and are the preferred structured output format unless you actually need a python dictionary. The table inside the database provides a new, easy method for reviewing the bloat information as well, but just be aware this is rebuilt from scratch every time the script runs. There’s also a new option which I used above (-t, –tablename) that you can use to get the bloat information on just a single table. See the –help for more information on all the options that are available.

Why Bloat Happens

For those of you newer to PostgreSQL administration, and this is the first time you may be hearing about bloat, I figured I’d take the time to explain why this scenario exists and why tools like this are necessary (until they’re hopefully built into the database itself someday). It’s something most don’t understand unless someone first explains it to them or you run into the headaches it causes when it’s not monitored and you learn about it the hard way.

MVCC (multi-version concurrency control) is how Postgres has chosen to deal with multiple transactions/sessions hitting the same rows at (nearly) the same time. The documentation, along with wikipedia provide excellent and extensive explanations of how it all works, so I refer you there for all the details. Bloat is a result of one particular part of MVCC, concentrated around the handling of updates and deletes.

Whenever you delete a row, it’s not actually deleted, it is only marked as unavailable to all future transactions taking place after the delete occurs. The same happens with an update: the old version of a row is kept active until all currently running transactions have finished, then it is marked as unavailable. I emphasize the word unavailable because the row still exists on disk, it’s just not visible any longer. The VACUUM process in Postgres then comes along and marks any unavailable rows as space that is now available for future inserts or updates. The auto-vacuum process is configured to run VACUUM automatically after so many writes to a table (follow the link for the configuration options), so it’s not something you typically have to worry about doing manually very often (at least with more modern versions of Postgres).

People often assume that VACUUM is the process that should return the disk space to the file system. It does do this but only in very specific cases. That used space is contained in page files that make up the tables and indexes (called objects from now on) in the Postgres database system. Page files all have the same size and differently sized objects just have as many page files as they need. If VACUUM happens to mark every row in a page file as unavailable AND that page also happens to be the final page for the entire object, THEN the disk space is returned to the file system. If there is a single available row, or the page file is any other but the last one, the disk space is never returned by a normal VACUUM. This is bloat. Hopefully this explanation of what bloat actually is shows you how it can sometimes be advantageous for certain usage patterns of tables as well, and why I’ve included the option to ignore objects in the report.

If you give the VACUUM command the special flag FULL, then all of that reusable space is returned to the file system. But VACUUM FULL does this by completely rewriting the entire table (and all its indexes) to new pages and takes an exclusive lock on the table the entire time it takes to run (CLUSTER does the same thing, but what that does is outside the scope of this post). For large tables in frequent use, this is problematic. pg_repack has been the most common tool we’ve used to get around that. It recreates the table in the background, tracking changes to it, and then takes a brief lock to swap the old bloated table with the new one.

Why bloat is actually a problem when it gets out of hand is not just the disk space it uses up. Every time a query is run against a table, the visibility flags on individual rows and index entries is checked to see if is actually available to that transaction. On large tables (or small tables with a lot of bloat) that time spent checking those flags builds up. This is especially noticeable with indexes where you expect an index scan to improve your query performance and it seems to be making no difference or is actually worse than a sequential scan of the whole table. And this is why index bloat is checked independently of table bloat since a table could have little to no bloat, but one or more of its indexes could be badly bloated. Index bloat (as long as it’s not a primary key) is easier to solve because you can either just reindex that one index, or you can concurrently create a new index on the same column and then drop the old one when it’s done.

In all cases when you run VACUUM, it’s a good idea to run ANALYZE as well, either at the same time in one command or as two separate commands. This updates the internal statistics that Postgres uses when creating query plans. The number of live and dead rows in a table/index is a part of how Postgres decides to plan and run your queries. It’s a much smaller part of the plan than other statistics, but every little bit can help.

I hope this explanation of what bloat is, and how this tool can help with your database administration, has been helpful.

Written by Keith

May 27th, 2016 at 11:55 am

Posted in PostgreSQL

Tagged with , , ,