Indiscriminate use of CTEs considered harmful

Common Table Expressions are a wonderful thing. Not only are they indespensible for creating recursive queries, but they can be a powerful tool in creating complex queries that are comprehensible. It's very easy to get lost in a fog of sub-sub-sub-queries, so using CTEs as a building block can make things a lot nicer.

However, there is one aspect of the current implementation of CTEs that should make you pause. Currently CTEs are in effect materialized before they can be used. That is, Postgres runs the query and stashes the data in a temporary store before it can be used in the larger query. There are a number of consequences of this.

First, this can be a good thing. I have on a number of occasions used this fact to good effect to get around problems with poorly performing query plans. It's more or less the same effect as putting "offset 0" on a subquery.

However, it can also result in some very inefficient plans. In particular, if CTEs return a lot of rows they can result in some spectacularly poorly performing plans at the point where you come to use them. Note also that any indexes on the underlying tables will be of no help to you at all at this point, since you are no longer querying against those tables but against the intermediate result mentioned above, which has no indexes at all.

This was brought home to me forcefully on Friday and Saturday when I was looking at a very poorly performing query. After some analysis and testing, the simple act of inlining two CTEs in the query in question resulted in the query running in 4% of the time it had previously taken. Indiscriminate use of CTEs had made the performance of this query 25 times worse.

So the moral is: be careful in using CTEs. They are not just a convenient tool for abstracting away subqueries.

There has been some discussion about removing this aspect of the implementation of CTEs. It's not something that is inherent in CTEs, it's simply a part of the way we have implemented them in PostgreSQL. However, for now, you need to be familiar with the optimization effects when using them, or you might face the same problem I was dealing with above.

Windows XP - PostgreSQL's long goodbye.

I just committed a patch to allow building with Visual Studio 2015. Due to a change in the way we need to detect locales with this compiler, it will not be possible to use binaries built with it on Windows XP. Despite the fact that Microsoft declared Windows XP to be completely out of support well over a year ago, it still lives on in a huge number of installations. My own instance still gets occasional updates from Microsoft. And you can still build and run the very latest PostgreSQL on Windows XP. But you can't use it on Windows XP if it's built with Visual Studio 2015 or later.

I will keep my instance (whose only job is to run several buildfarm members) running as long as it doesn't require any significant tweaks. But a day will eventually come when it does require such tweaks, or just won't build and run successfully any more,  and at that stage I'll shut it down.

Building an extension with VS2015

I needed to get a DLL prepared of my tiny closed format ranges extension, and so I tried following the excellent instructions Craig Ringer gave a couple of years ago. I was using Visual Studio 2015, on a Windows 10 box, targeting PostgreSQL 9.4, 64bit.

But I ran into some problems. I could compile the thing quite cleanly, but Postgres couldn't find the functions when I tried to create the extension. I had a short and very helpful email exchange with Craig, and came up with this solution: the exported functions needed to be declared with a prototype before the call to PG_FUNCTION_INFO_V1, and both the prototype and the function definition needed to be decorated with PGDLLEXPORT. Once I had done that, the DLL loaded when I ran "create extension", and the types worked as expected.

I don't know if this is required with any earlier versions of Visual Studio, or even of it works with them.  When I get a chance I will see if I can find out.

Based on this, I think it's probably worth trying at some stage to put together a script to build the project and solution files for extensions. I don't have time to do that right now, however.

Weird stuff happens

Five days ago, my buildfarm animal jacana suddenly started getting an error while trying to extract typedefs. It had been happily doing this for ages, and suddenly the quite reproducible error started. For now I have disabled its typedef analysis, but I will need to get to the bottom of it. It's bad enough to crash the buildfarm client leaving the build directory dirty and not able to process further builds until I clean it up. I'm assuming it's probably something that changed in the source code, as nothing else has changed at all. These are the commits that took place between a good run and the first appearance of the error.

• 9a83564 Allow SSL server key file to have group read access if owned by root
• 6eb2be1 Fix stupid omission in c4901a1e.
• 07aed46 Fix missed update in _readForeignScan().
• ff0a7e6 Use yylex_init not yylex_init_extra().
• a3e39f8 Suppress FLEX_NO_BACKUP check for psqlscanslash.l.
• 0ea9efb Split psql's lexer into two separate .l files for SQL and backslash cases.

 I don't know for dead certain that any of these has caused an issue, but finding out what the problem is is just one more way for me to spend my copious free time.

GIN indexing an array of enums

GIN indexes of arrays can be pretty useful. And arrays of enums can also be pretty useful. Unfortunately, there is no built in GIN indexing of arrays of enums, and in fact there is no pseudo-type for arrays of enums. So, while we can declare an operator class for supporting a type of "anyenum", which I did in my recent work on btree_gin and btree_gist support for enums, we can't declare one for support of "anyenum[]".

However, we can declare one for support of a concrete enum type's array type. And all the pieces are already there.

After a lot of reading and experimentation, Here's what I found that seems to work just like the builtin GIN array operator classes.

Given an enum type of animal, you can declare an operator class for its array type like this (note how almost completely generic this is):

create operator class _animal_ops 
    default for type public.animal[] 
    using gin 
    family array_ops as  
        function 1 enum_cmp(anyenum,anyenum), 
        function 2 pg_catalog.ginarrayextract(anyarray, internal), 
        function 3 ginqueryarrayextract(anyarray, internal, smallint, internal, 
                                        internal, internal, internal), 
        function 4 ginarrayconsistent(internal, smallint, anyarray, integer, 
                                      internal, internal, internal, internal),
        function 6 ginarraytriconsistent(internal, smallint, anyarray, integer, 
                                         internal, internal, internal), 
    storage oid ;

and here it is working:

andrew=# \d animal_classes
 Table "public.animal_classes"
 Column  |   Type   | Modifiers 
---------+----------+-----------
 class   | integer  | 
 animals | animal[] | 

andrew=# select * from animal_classes where '{lion}' <@ (animals);
 class |   animals    
-------+--------------
     1 | {lion,zebra}

andrew=# set enable_seqscan = off;
SET
andrew=# create index animal_classes_animals_idx on animal_classes using gin(animals);
CREATE INDEX
andrew=# explain (costs off) select * from animal_classes where '{lion}' <@ (animals);
                      QUERY PLAN                       
-------------------------------------------------------
 Bitmap Heap Scan on animal_classes
   Recheck Cond: ('{lion}'::animal[] <@ animals)
   ->  Bitmap Index Scan on animal_classes_animals_idx
         Index Cond: ('{lion}'::animal[] <@ animals)
(4 rows)

andrew=# select * from animal_classes where '{lion}' <@ (animals);
 class |   animals    
-------+--------------
     1 | {lion,zebra}
(1 row)

andrew=# 

Gist and Gin support for enums

Recently I submitted a patch (too late for release 9.6, unfortunately) to add support for enum data types to the btree_gist and btree_gin additional modules. This came out of a desire to be able to use an enum field in an exclusion constraint. That really only requires GiST support, but since I was adding enum support to btree_gist it seemed a good idea to add it to btree_gin as well, so that, for example, enum fields can be used in multi-column GIN indexes.

Here's an exclusion constraint example using enums, adapted from the Postgres docs.

andrew=# create extension btree_gist;
CREATE EXTENSION
andrew=# \dx
                           List of installed extensions
    Name    | Version |   Schema   |                  Description                  
------------+---------+------------+-----------------------------------------------
 btree_gist | 1.2     | public     | support for indexing common datatypes in GiST
 plpgsql    | 1.0     | pg_catalog | PL/pgSQL procedural language
(2 rows)

andrew=# create type animal as enum ('zebra','lion');
CREATE TYPE
andrew=# create table zoo (cage int, exhibit animal, exclude using gist(cage with =, exhibit with <>));
CREATE TABLE
andrew=# INSERT INTO zoo VALUES(123, 'zebra');
INSERT 0 1
andrew=# INSERT INTO zoo VALUES(123, 'zebra');
INSERT 0 1
andrew=# INSERT INTO zoo VALUES(123, 'lion');
ERROR:  conflicting key value violates exclusion constraint "zoo_cage_exhibit_excl"
DETAIL:  Key (cage, exhibit)=(123, lion) conflicts with existing key (cage, exhibit)=(123, zebra).
andrew=# 

Json dates, times, and binary data

If I had to pick one thing I would change about the JSON spec it's this: they should allow timestamps and dates as primitive values. It's a very common complaint. Another thing they should have provided for is binary data. This gets a bit annoying if you convert a record containing a bytea to json and hand it to the client. They see a string like '\x71951ce4c84c0b4de861377f27a4938a'. If you can, it's probably nicer to encode your bytea before converting it to json, using encode(mybytea,'base64') or encode(mybytea,'hex'). Then your client might see a string for which they have a standard routine to convert to binary, rather than having to strip off the the initial '\x' before converting it.

Release 4.17 of the PostgreSQL Buildfarm client

I have just cut release 4.17 of the PostgreSQL Buildfarm client. It is available at http://www.pgbuildfarm.org/downloads/latest-client.tgz.

Changes of note:

• use PGCTLTIMEOUT instead of hardcoded timeout settings
• shipped config file is renamed to build-farm.conf.sample to avoid
• overwriting a default config file
• use configured make command in make_bin_installcheck
• restrict restart sleeps to Windows platforms
• fix trigger_exclude pattern in build-farm.conf
• protect git checkout with a lock if using git_use_workdirs

For the most part, the thing of significance it the PGCTLTIMEOUT change, which follows up on some changes in core code. The new code defaults this setting to 120, but it can be set in the config file. If you don't need any of the other changes (most people probably won't) then just adding something like

PGCTLTIMEOUT => '120',

to the build_env stanza of your config file should do the trick, and you can happily wait for the next release.

Fixed length record files

I've had a few requests recently for support for the Fixed Length File FDW that I created as an experiment about 5 years ago. Apparently people are still getting data from COBOL programs or some such source. The code has bitrotted some in the meanwhile, so I'll try to grab a little time to update it, add some test cases and docco, etc.

Buildfarm server moving

Today we're moving the buildfarm server to a new machine and more modern Postgres. Here is yesterday's (corrrected) announcement:

Apologies for the late notice.

Tomorrow, January 19th, at 4.00 pm US East Coast time (UT - 5.0) we will be moving the buildfarm server from its current home at CommandPrompt, where we have been ever since we started, to a machine that is part of the standard core infrastructure. In doing so we will be moving to a) a more modern and supported PostgreSQL version, and b) a machine with more disk space so that our current severe pace shortage will be alleviated. In addition, the community would be much better placed to maintain the buildfarm if either JD or I were to fall under a bus.

The outage is expected to last about 4 hours or less, and we will sent out notifications when this is complete.

Buildfarm owners who want to avoid getting reporting failures should disable their animals during that time. We don't have an avalanche of commits right now either, but it might also be nice if committers were to refrain from adding changes in the hours leading up to this and until we announce that we're back online, for the benefit of those owners who don't see this message in time.

Thanks in advance for your help and understanding.

And many thanks to CommandPrompt for their constant support over the many years we've been in operation.

In a few hours I will start disabling my 12 buildfarm members.

Using PostgreSQL 9.5's IMPORT FOREIGN SCHEMA

This is a pretty nice feature in 9.5. The other day I needed to look at some data from a WordPress site that's stored in MySQL. I had a dump of the data, but I'm not terribly proficient at using MySQL tools or their SQL dialect, so I thought I would try out just importing the schema to PostgreSQL using the new IMPORT FOREIGN SCHEMA command. It worked pretty well. First I installed the Postgres rpms from the community repository. Unfortunately, the repository doesn't have the latest mysql foreign data wrapper for 9.5, so I cloned it from github and built and installed it without difficulties. Then I restored the backup into the test database on local mysql instance. Then there was a small glitch. When I tried to import the schema it complained that it didn't know about the type "tinytext". So I created a domain for this that just mapped it to text. and then re-ran the import, and it worked just fine. Here is the whole import session:

[andrew@dino mysql_fdw]$ psql wp
psql (9.5.0)
Type "help" for help.

wp=# create extension mysql_fdw;
CREATE EXTENSION
wp=# CREATE SERVER mysql_server
         FOREIGN DATA WRAPPER mysql_fdw
         OPTIONS (host '127.0.0.1', port '3306');
CREATE SERVER
wp=# create user mapping for andrew server mysql_server;
CREATE USER MAPPING
wp=# import FOREIGN SCHEMA test from server mysql_server into public;
ERROR:  type "tinytext" does not exist
LINE 4:   comment_author tinytext NOT NULL,
                         ^
QUERY:  CREATE FOREIGN TABLE wp_comments (
  "comment_ID" bigint NOT NULL,
  "comment_post_ID" bigint NOT NULL,
  comment_author tinytext NOT NULL,
  comment_author_email varchar NOT NULL,
  comment_author_url varchar NOT NULL,
  "comment_author_IP" varchar NOT NULL,
  comment_date timestamp NOT NULL,
  comment_date_gmt timestamp NOT NULL,
  comment_content text NOT NULL,
  comment_karma int NOT NULL,
  comment_approved varchar NOT NULL,
  comment_agent varchar NOT NULL,
  comment_type varchar NOT NULL,
  comment_parent bigint NOT NULL,
  user_id bigint NOT NULL
) SERVER mysql_server OPTIONS (dbname 'test', table_name 'wp_comments');

CONTEXT:  importing foreign table "wp_comments"
wp=# create domain tinytext as text;
CREATE DOMAIN
wp=# import FOREIGN SCHEMA test from server mysql_server into public;
IMPORT FOREIGN SCHEMA
wp=# 

PostgreSQL 9.5 fills in lots of gaps

The two big features from my point of view in the long-awaited PostgreSQL 9.5, officially released today, are the INSERT ... ON CONFLICT DO NOTHING / UPDATE feature, and addition of CUBE, ROLLUP and more generalized GROUPING SETS. These are two very important developments that go a long way to filling in the gaps in our feature set.

My contribution has been a bit more more modest than previously for this release. Here are the things I was involved with, along with others:

• Add jsonb functions jsonb_set() and jsonb_pretty()
• Allow text, text array, and integer values to be subtracted from jsonb documents
• Add jsonb || operator
• Add json_strip_nulls() and jsonb_strip_nulls() functions to remove JSON null values from documents
• Add jsonb generator functions to_jsonb(), jsonb_object(), jsonb_build_object(), jsonb_build_array(), jsonb_agg(), and jsonb_object_agg()
• Add \pset option pager_min_lines to control pager invocation in psql
• Improve psql's line counting used when deciding to invoke the pager
• Make psql's \sf and \ef commands honor ECHO_HIDDEN
• Add statistics for minimum, maximum, mean, and standard deviation times to pg_stat_statements

Original work for the first three of these was done by Dmitry Dolgov, and for the last by Mitsumasa Kondo.

This is the fourth straight release where I've done a substantial amount of work on JSON features, and it will be the last for a while at least.  We've come a long way since Robert Haas and I snuck some JSON stuff into release 9.2 at the very last minute. Despite a somewhat rocky road, I venture to say that if we hadn't done that we would not have made as much progress as we have since then.