One of the neat little features that arrived at PostgreSQL 9.4 is the WITH ORDINALITY ANSI-SQL construct. What this construct does is to tack an additional column called ordinality as an additional column when you use a set returning function in the FROM part of an SQL Statement.

Since the last time I talked about it, I had quite a few feedbacks, bug issues, pull requests, and so on. Many issues were fixed, the last of it tonight.

I also added two new reports. I had the idea while working on my customers' clusters.

One of them had a lot of writes on their databases, and I wanted to know how much writes occured in the WAL files. vmstat would only tell me how much writes on all files, but I was only interested in WAL writes. So I added a new report that grabs the current XLOG position, and diff it with the previous XLOG position. It gives something like this with a pgbench test:

$ ./pgstat -s xlog 
-------- filename -------- -- location -- ---- bytes ----
 00000001000000000000003E   0/3EC49940        1053071680
 00000001000000000000003E   0/3EC49940                 0
 00000001000000000000003E   0/3EC49940                 0
 00000001000000000000003E   0/3EC875F8            253112
 00000001000000000000003E   0/3ED585C8            856016
 00000001000000000000003E   0/3EE36C40            910968
 00000001000000000000003E   0/3EEFCC58            811032
 00000001000000000000003E   0/3EFAB9D0            716152
 00000001000000000000003F   0/3F06A3C0            780784
 00000001000000000000003F   0/3F0E79E0            513568
 00000001000000000000003F   0/3F1354E0            318208
 00000001000000000000003F   0/3F1F6218            789816
 00000001000000000000003F   0/3F2BCE00            814056
 00000001000000000000003F   0/3F323240            418880
 00000001000000000000003F   0/3F323240                 0
 00000001000000000000003F   0/3F323240                 0

That's not big numbers, so it's easy to find it writes at 253K/s, but if the number were bigger, it might get hard to read. One of my co-worker, Julien Rouhaud, added a human readable option:

$ ./pgstat -s xlog -H
-------- filename -------- -- location -- ---- bytes ----
 00000001000000000000003F   0/3F32EDC0      1011 MB
 00000001000000000000003F   0/3F32EDC0      0 bytes
 00000001000000000000003F   0/3F32EDC0      0 bytes
 0000000100000000

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Long have CASE statements been a double-edged sword in the database world. They’re functional, diverse, adaptive, and simple. Unfortunately they’re also somewhat bulky, and when it comes to using them to categorize aggregates, something of a hack. This is why I wanted to cry with joy when I found out that PostgreSQL 9.4 introduced a feature I’ve always wanted, but found difficult to express as a need. I mean, CASE statements are fine, right? Well, yes they are, but now we have something better. Now, we have the FILTER aggregate expression.

I always like working with examples, so let’s create some test data to illustrate just what I’m talking about.

CREATE TABLE sys_order
(
    order_id     SERIAL     NOT NULL,
    product_id   INT        NOT NULL,
    item_count   INT        NOT NULL,
    order_dt     TIMESTAMP  NOT NULL DEFAULT now()
);

INSERT INTO sys_order (product_id, item_count)
SELECT (a.id % 100) + 1, (random()*10)::INT + 1
  FROM generate_series(1, 1000000) a(id);

ALTER TABLE sys_order ADD CONSTRAINT pk_order_order_id
      PRIMARY KEY (order_id);

We now have a table for tracking fake orders, using 100 nonexistent products. I added the primary key after loading the table as a well known DBA trick. Doing this after data loading means the index can be created as a single step, which is much more efficient than repeatedly extending an existing index.

With that out of the way, let’s do a basic product order count, since that’s something many people are already familiar with:

SELECT sum(item_count) AS total
  FROM sys_order;

-[ RECORD 1 ]--
total | 1000000

No surprises here. But what happens when Jeff from Accounting wants to know how many people ordered five specific products as a column list? In the old days, we might do something like this:

SELECT sum(CASE WHEN product_id = 1 THEN item_count ELSE 0 END) AS horse_mask_count,
       sum(CASE WHEN product_id = 7 THEN item_count ELSE 0 END) AS eyeball_count,
       sum(CASE WHEN product_id = 13 THEN item_count ELSE 0 END) AS badger_count,
       sum(C

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Mark & I left David Wheeler in charge of the PDXPUG February meeting while we were at SCALE last week.

Here’s David’s report:
—
This week Dave Kerr discussed using Bucardo mutli-master replication to gradually migrate a production database from EC2 to RDS. This work allowed his team to switch back and forth between the two systems with the assurance that the data would be the same on both. It also allowed them a fallback in case the RDS database didn’t work out: the S3 system would still be there. The discussion allowed those present to complain about Bucardo, EC2, RDS, and the French.
—
Thanks!

worker_ltt.naptime = 60

worker_ltt.database = 'yourdb'

worker_ltt.user = 'youruser'

worker_ltt.function = 'move_longtail'

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Up to Postgres 9.4, when a node in recovery checks for the availability of WAL from a source, be it a WAL stream, WAL archive or local pg_xlog and that it fails to obtain what it wanted, it has to wait for a mount of 5s, amount of time hardcoded directly in xlog.c. 9.5 brings more flexibility with a built-in parameter allowing to control this interval of time thanks to this commit:

commit: 5d2b45e3f78a85639f30431181c06d4c3221c5a1
author: Fujii Masao <fujii@postgresql.org>
date: Mon, 23 Feb 2015 20:55:17 +0900
Add GUC to control the time to wait before retrieving WAL after failed attempt.

Previously when the standby server failed to retrieve WAL files from any sources
(i.e., streaming replication, local pg_xlog directory or WAL archive), it always
waited for five seconds (hard-coded) before the next attempt. For example,
this is problematic in warm-standby because restore_command can fail
every five seconds even while new WAL file is expected to be unavailable for
a long time and flood the log files with its error messages.

This commit adds new parameter, wal_retrieve_retry_interval, to control that
wait time.

Alexey Vasiliev and Michael Paquier, reviewed by Andres Freund and me.

wal_retrieve_retry_interval is a SIGHUP parameter (possibility to update it by reloading parameters without restarting server) of postgresql.conf that has the effect to control this check interval when a node is in recovery. This parameter is useful when set to values shorter than its default of 5s to increase for example the interval of time a warm-standby node tries to get WAL from a source, or on the contrary a higher value can help to reduce log noise and attempts to retrieve a missing WAL archive repetitively when for example WAL archives are located on an external instance which is priced based on the amount of connections attempted or similar (note as well that a longer interval can be done with some timestamp control using a script that is kicked by restore_command, still it is good to have a built-in option to do it instea

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shared_preload_libraries = 'plan_filter'
plan_filter.statement_cost_limit = 100000.0

New repository

It’s been almost a year now that I wrote the first version of the btree bloat estimation query. Then, came the first fixes, the bloat estimation queries for tables, more fixes, and so on. Maintaining these queries as gists on github was quite difficult and lack some features: no documented history, multiple links, no doc, impossible to fork, etc.

So I decided to move everything to a git repository you can fork right away: http://github.com/ioguix/pgsql-bloat-estimation. There’s already 10 commits for improvements and bug fixes.

Do not hesitate to fork this repo, play with the queries, test them or make pull requests. Another way to help is to discuss your results or report bugs by opening issues. This can lead to bug fixes or the creation of a FAQ.

Changelog

Here is a quick changelog since my last post about bloat:

• support for fillfactor! Previous versions of the queries were considering any extra space as bloat, even the fillfactor. Now, the bloat is reported without it. So a btree with the default fillfactor and no bloat will report a bloat of 0%, not 10%.
• fix bad tuple header size for tables under 8.0, 8.1 or 8.2.
• fix bad header size computation for varlena types.
• fix illegal division by 0 for the btrees.
• added some documentation! See README.md.

In conclusion, do not hesitate to use this queries in your projects, contribute to them and make some feedback!

• This builds against PostgreSQL 9.4 and OpenBabel 2.3.2 on Linux.
• It contains all fixes and contributions of the previous versions.
• Windows is not supported anymore - and since it builds and runs way better on Linux, probably never will be again.
• Depiction functions have been removed. Their run time was too unpredictable to be run inside a database server.
• Theoretical isotope pattern generation with MERCURY7 is now available with isotopes for 39 elements.

After VanLUG I spent the next morning at the Vancouver Aquarium with my ladies. Nothing like beautiful weather, dolphins and jelly fish to brighten the week. Once back in Bellingham, we moved on to a WhatcomPUG meeting where I presented, "Practical PostgreSQL: AWS Edition". It was the inaugural meeting but was attended by more than just the founders which is a great start!

I got to rest from community work on Wednesday and instead dug my head into some performance problems on a client High Availability Cluster. It is amazing that even with proper provisioning how much faster ASYNC rep is over SYNC rep. Some detailed diagnosis and proving data demonstrated, we switched to ASYNC rep and all critical problems were resolved.

On Thursday it was off to Southern California Linux Expo where I presented, "Suck it! Webscale is dead; long live PostgreSQL!". The room was packed, people laughed and for those who might have been offended, I warned you. Your offense is your problem. Look inside yourself for your insecurities! All my talks are PG-13 and it is rare that I will shy away from any topic. My disclosure aside, I had two favorite moments:

1. When someone was willing to admit they hadn't seen Terminator. I doubt that person will ever raise his hand to one of my questions again.
2. When Berkus (who knew the real answer) suggested it was Elton John that wrote the lyrics at the end of the presentation.

After I spent the evening with JimmyM (BigJim, my brother), Joe Conway of SDPUG/Credativ , Jim Nasby of the

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So ... Prague PostgreSQL Developer Day 2015, the local PostgreSQL conference, happened about two weeks ago. Now that we collected all the available feedback, it's probably the right time for a short report and sketch of plans for next year.

The first year of Prague PostgreSQL Developer Day (P2D2) happened in 2008, and from the very beginning was organized as a community event for developers - from students of software engineering to people who use PostgreSQL at work.

We've changed the venue a few times, but in most cases we've just moved from one university / faculty to another one, and the same happened this year for capacity reasons. The previous venue at Charles University served us well, but we couldn't stuff more than 120 people in, and we usually reached that limit within a week after opening the registration. The new venue, located at Czech Technical University can handle up to ~180 people, which should be enough for the near future - this year we registered 150 people, but a few more ended on a wait list.

The most obvious change was adding a full day of trainings on February 11 (i.e. the day before the main conference day), similarly to what happens at pgconf.eu and various other conferences. The feedback to this is overwhelmingly good, so we're pretty sure we'll preserve this for the next years.

The main conference (on February 12) consisted of 9 talks, not counting the initial "welcome" speech. We had the usual mixture of talks, from a brief talk about features introduced in 9.4, talks about using PostgreSQL in actual projects, to a talk about Bi-Directional Replication.

Although the conference is aimed at local users, and thus the majority of talks is either in Czech or Slovak, every year we invite a few foreign speakers to give talks in english. This year we had the pleasure to welcome Marc Balmer, who gave a talk "Securing your PostgreSQL applications", and Keith Fiske explaining that "When PostgreSQL Can't You Can".

In the early years of the conference we've been getting "too many talks in english"

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(picture by Steve Wilson)

Way back in 2005 I added the ON_ERROR_ROLLBACK feature to psql, the Postgres command line client. When enabled, any errors cause an immediate rollback to just before the previous command. What this means is that you can stay inside your transaction, even if you make a typo (the main error-causing problem and the reason I wrote it!). Since I sometimes see people wanting to emulate this feature in their application or driver, I thought I would explain exactly how it works in psql.

First, it must be understood that this is not a Postgres feature, and there is no way you can instruct Postgres itself to ignore errors inside of a transaction. The work must be done by a client (such as psql) that can do some voodoo behind the scenes. The ON_ERROR_ROLLBACK feature is available since psql version 8.1.

Normally, any error you make will throw an exception and cause your current transaction to be marked as aborted. This is sane and expected behavior, but it can be very, very annoying if it happens when you are in the middle of a large transaction and mistype something! At that point, the only thing you can do is rollback the transaction and lose all of your work. For example:

greg=# CREATE TABLE somi(fav_song TEXT, passphrase TEXT, avatar TEXT);
CREATE TABLE
greg=# begin;
BEGIN
greg=# INSERT INTO somi VALUES ('The Perfect Partner', 'ZrgRQaa9ZsUHa', 'Andrastea');
INSERT 0 1
greg=# INSERT INTO somi VALUES ('Holding Out For a Hero', 'dx8yGUbsfaely', 'Janus');
INSERT 0 1
greg=# INSERT INTO somi BALUES ('Three Little Birds', '2pX9V8AKJRzy', 'Charon');
ERROR:  syntax error at or near "BALUES"
LINE 1: INSERT INTO somi BALUES ('Three Little Birds', '2pX9V8AKJRzy'...
greg=# INSERT INTO somi VALUES ('Three Little Birds', '2pX9V8AKJRzy', 'Charon');
ERROR:  current transaction is aborted, commands ignored until end of transaction block
greg=# rollback;
ROLLBACK
greg=# select count(*) from somi;
 count
-------
     0

When ON_ERROR_ROLLBACK is enabled, psql will issue a SAVEPOINT before every command you sen

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Some of our customers really like writing their business logic inside of PostgreSQL. While this is really cool that PostgreSQL is capable of handling, trying to performance tune large amounts of PL/pgSQL code becomes unwieldy. If your functions are small enough, it’s possible add some logging statements, but that is not possible with hundreds or even thousands lines of legacy code.

Several years ago as part of the PL/pgSQL debugger, Korry Douglas wrote a PL/pgSQL profiler, but over the years, it seems to have suffered from bit rot. A profiler for PL/pgSQL code helps solve a lot of problems and gives us insight into how your server side code is running.

Below is an example output from the profiler showing how many times each line of code executed and what was the time taken for each line.

The plprofiler has not been tested is many different environments yet, so be careful in rolling it out to production servers. Check it out and let me know if you find any issues.

https://bitbucket.org/openscg/plprofiler

Not later than this week a bug regarding pg_dump and compression with zlib when dumping data has been reported here.

The issue was that when calling -Fd, the compression level specified by -Z was ignored, making the compressed dump having the same size for Z > 0. For example with a simple table:

=# CREATE TABLE dump_tab AS SELECT random() as a,
                                   random() as b
   FROM generate_series(1,10000000);
SELECT 10000000

A dump keeps the same size whatever the compression level specified:

$ for num in {0..4}; do pg_dump -Fd -t dump_tab -f \
    level_$num.dump -Z num ; done
$ ls -l level_?.dump/????.dat.gz level_0.dump/????.dat
-rw-r--r--  1 michael  staff  419999247 Feb 20 22:13 level_0.dump/2308.dat
-rw-r--r--  1 michael  staff  195402899 Feb 20 22:13 level_1.dump/2308.dat.gz
-rw-r--r--  1 michael  staff  195402899 Feb 20 22:14 level_2.dump/2308.dat.gz
-rw-r--r--  1 michael  staff  195402899 Feb 20 22:15 level_3.dump/2308.dat.gz
-rw-r--r--  1 michael  staff  195402899 Feb 20 22:16 level_4.dump/2308.dat.gz

After a couple of emails exchanged, it was found out that a call to gzopen() missed the compression level: for example to do a compression of level 7, the compression mode (without a strategy) needs to be something like "w7" or "wb7" but the last digit was simply missing. An important thing to note is how quickly the bug has been addressed, the issue being fixed within one day with this commit (that will be available in the next series of minor releases 9.4.2, 9.3.7, etc.):

commit: 0e7e355f27302b62af3e1add93853ccd45678443
author: Tom Lane <tgl@sss.pgh.pa.us>
date: Wed, 18 Feb 2015 11:43:00 -0500
Fix failure to honor -Z compression level option in pg_dump -Fd.

cfopen() and cfopen_write() failed to pass the compression level through
to zlib, so that you always got the default compression level if you got
any at all.

In passing, also fix these and related functions so that the correct errno
is reliably returned on failure; the original coding supposes that free()
cannot change

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• SCALE, Los Angeles, this week: 2-day Postgres track, booth.  Use code "SPEAK" if you still haven't registered for a small discount.  I'm speaking on 9.4 (Friday), and PostgreSQL on AWS (Sunday).
• March 10, Burlingame, CA: pgDay SF 2015 Running the event, and a lightning talk.
• March 25-27, NYC, NY: pgConf NYC: speaking on PostgreSQL on PAAS: a comparison of all the big ones.
• April 25-26, Bellingham, WA: LinuxFest NorthWest, tentatively.  Talks haven't been chosen yet.  If I go, I'll also be working a booth no doubt.  I understand there are plans to have a bunch of Postgres stuff at this event.
• June 16-20, Ottawa, Canada: pgCon of course.
• July 20-24, Portland, OR: OSCON (tentatively, talks not selected).  Postgres talk of some sort, and probably booth duty.