Tag Archives: postgresql

Pyrseas contributions solicited

Do you use PostgreSQL and truly believe it’s “the world’s most advanced open source database” and that its upcoming 9.3 release will make it even more awesome?

Do you also use Python and believe it’s “an easy to learn, powerful programming language” with “elegant syntax” that makes it an ideal language for developing applications and tools around PostgreSQL, such as Pyrseas?

Then we could use your help. For starters, we want to add support for the MATERIALIZED VIEWs and EVENT TRIGGERs coming up in PG 9.3.

We have also been requested to add the capability to load and maintain “static data” (relatively small, unchanging tables) as part of yamltodb, so that it can be integrated more easily into database version control workflows.

And for the next release, Pyrseas 0.7, we’d like to include the first version of the database augmentation tool which will support declarative implementation of business logic in the database–starting off with audit trail columns. Some work has been done on this already, but it needs integration with the current code and tests.

Or perhaps coding is not your forte, but you’re really good at explaining and documenting technical “stuff”. Then you could give us a hand with revamping the docs, maybe writing a tutorial so that users have a smooth ride using our tools.

Or maybe you have your own ideas as to how improve the PostgreSQL version control experience. We’d love to hear those too.

If you’d like to help, you can fork the code on GitHub, join the mailing list and introduce yourself, or leave a comment below.

Tuples in the Pythonic, TTM-inspired interface to PostgreSQL

The Third Manifesto formally describes tuple types (RM prescription 6), tuple values (prescription 9), tuple variables (prescription 12) as well as other tuple-related elements. As mentioned in the previous post, a tuple value is a set of ordered triples each consisting of attribute name, type and value.

Class Tuple of the TTM-inspired interface to PostgreSQL models TTM tuples as Python lists of TTM Attribute objects. Lists were used rather than sets because for many practical purposes the order of the attributes is useful (or has “meaning”), e.g., the first attribute listed is most often –even in purist relational theory presentations– the primary key or part of the primary key.

The interface stores the Tuple heading as a (Python) n-tuple of name-type tuples, in the “internal use” _heading attribute. The n-tuple was chosen over a list due to its immutability. The interface also sets each Attribute as a Python attribute of the Tuple object. Thus, if you define a Tuple variable as follows:

film = Tuple([
    Attribute('id', int, sysdefault=True),
    Attribute('release_year', int)])

You can then assign or access an Attribute using simple Python syntax:

film.title = "Seven Samurai"
if film.year == 1954:
    do something

The interface also stores two other internal use lists, one for nullable attributes and another for attributes that allow default values. These are to be used by upstream classes such as RelVar.

Class Tuple has a __setattr__ method tailored to deal with assignment to TTM Attributes. It disallows assignment to internal attributes after initialization, with one exception: the _tuple_version attribute (used by RelVar for optimistic concurrency). It also doesn’t allow assignment to undefined Attributes, e.g., given the film variable above, attempting to assign to film.length will raise an AttributeError. Finally, the assignment is “filtered” through class Attribute, so that an attempt such as film.title = 8.5 will result in a ValueError from that class.

The pyrseas.relation.tuple module defines a standalone function: tuple_values_dict. This is used to generate a dictionary of attribute values suitable for passing to Psycopg’s cursor.execute method. For INSERT, a single currtuple argument is expected. For UPDATE, the modified Tuple is passed as a second argument and tuple_values_dict will return a dictionary solely for the attribute and values that have changed.

Attributes in the Pythonic, TTM-inspired interface to PostgreSQL

The Third Manifesto‘s Relational Model (RM) prescription 9 defines a relation heading as “a set of ordered pairs or attributes of the form <A,T>,” where A is the name of the attribute and T is the name of the declared type of the attribute. It then defines a tuple value (or tuple for short) as a set of ordered triples of form <A,T,v> where v is an arbitrary value of type T, called the attribute value.

Class Attribute of the TTM-inspired interface to PostgreSQL models the latter ordered triple rather than the ordered pair. I considered implementing the pair, say as AttribType, and deriving Attribute from it, but opted for the leaner, no-hierarchy solution for now.

An Attribute object should be initialized with a name, Python type and a value. However, only the name is required. The default type is str and for Python 2, unicode is treated almost as a synonym for str (I realize this goes against everything that your mother taught you, but I’m hoping that widespread Python 3 adoption will make this moot soon).

I wholehearteadly agree with TTM RM proscription 5 banning attributes that do not have a value, i.e., like SQL NULLs. However, I hope the interface is useful on existing databases, so in the interest of practicality, class Attribute has two additional, optional arguments: nullable and sysdefault. The former is to specify that an attribute allows NULLs (or None in Python). The latter can be used to indicate the corresponding table column has an SQL DEFAULT specification (this includes those defined as SERIAL or BIGSERIAL).

If an attribute is not nullable and not sysdefault, a value must be specified. If omitted, a suitable default is created based on the type (if possible), i.e., an empty string for str, 0 for int, 0.0 for float. If the attribute is nullable, empty string, 0 or 0.0 are converted to None. This approach is to facilitate dealing with empty HTML form fields, i.e., if the user skips a nullable field, the attribute should end up as a NULL in the database.

If a value is provided, the code raises ValueError if the value does not agree with the specified (or defaulted) Python type. The only exceptions are that an int value is cast to float, and a unicode value is allowed if the type is str and we’re working in Python 2.x.

A Pythonic, TTM-inspired interface to PostgreSQL – Requirements

Several moons ago, I started a series of posts about “designing and implementing a generic end user interface for PostgreSQL.” After a while, the series got sidetracked by other issues.

More recently, I have returned to the original endeavor. Partly from reading Database Explorations: Essays on The Third Manifesto and related topics by C.J. Date and Hugh Darwen, I decided to use relational concepts as presented in The Third Manifesto (TTM) in my implementation. This post provides an overview of the requirements.

Limited Scope

The interface is not a full-blown replacement for an object-relational mapper (ORM) (although in theory it could eventually grow in that direction). The interface is intended to assist with two typical needs of a database “admin” application: browsing and CRUD.

Browsing refers to presenting a subset of rows (tuples) of a table (relation variable or relvar) for subsequent editing. The relvar will typically be normalized so it may be necessary to join it to other relvars. Browsing will usually display a limited number of columns (attributes) so relational projection will be needed.

CRUD refers to the ability to create, read, update and delete single tuples in a relvar. The interface should only support relvars with a properly defined, possibly composite primary key.


The user (developer) should have to define only the attributes of each relvar together with the key, and for browsing, the projected attributes plus a JOIN specification if multiple relvars are involved. The definitions should be simple enough so that most of them could be (at a later date) derived automatically from the database catalogs.

From the definitions, the interface should generate all necessary SQL commands to INSERT a single tuple (possibly returning a generated key value), retrieve, UPDATE or DELETE a single tuple using the key, and fetch subsets of projected/joined tuples in a given order.

Optimistic Concurrency Control

The interface should take advantage of PostgreSQL features to implement optimistic locking when handling updates or deletes, as described in a previous series of posts.

Query by Example Support

The interface should facilitate querying of the browsed tuples using something similar to Query-By-Example. For example, when browsing movies if the argument release_year is passed as “>= 1969“, the results should only include films released on that year or later. This feature was not discussed in a post but had been committed to the tutorial repository.


The interface should follow the TTM guidelines when possible. For example, although implemented in Python, assignment to a relvar attribute defined as int should not be allowed if the value is of type str, and duplicate attribute names in a join expression should not be permitted. However, since the interface ought to be usable against existing SQL databases, allowance should be made for certain SQL features such as nullable attributes.

The implementation has been committed to the Pyrseas repository and changes were made to the DBUI tutorial to use the new interface. Subsequent posts will cover the interface in more detail.

A couple of Pyrseas enhancements

Based on feedback from users and contributors, Pyrseas now sports two enhancements.

Multi-line String Formatting

Up to Pyrseas 0.6, long textual elements such as view definitions, function source text and long object comments, would usually be shown in the YAML output as quoted strings with embedded newlines. Here are two examples from the autodoc database:

schema product:
  description: "This schema stores a list of products and information\n about the\
    \ product"
schema warehouse:
  view products:
    definition: " SELECT DISTINCT product.product_id, product.product_code, product.product_description\n\
      \   FROM warehouse.inventory\n   JOIN product.product USING (product_id);"

As you can imagine, this was particularly unsatisfactory for complex functions and views. Thanks to preliminary work by Andrey Popp, Pyrseas 0.7 will be able to format these elements in YAML block style. The above elements will be shown as follows:

schema product:
  description: |-
    This schema stores a list of products and information
     about the product
schema warehouse:
  view products:
    definition: |2-
       SELECT DISTINCT product.product_id, product.product_code, product.product_description
         FROM warehouse.inventory
         JOIN product.product USING (product_id);

Thanks to testing by Josep Martínez, 0.7 will also properly display and handle such strings even when they include non-ASCII characters such as accented characters. For example, in 0.6, “Martínez” would be shown as “Mart\xEDnez”. In 0.7, the output will be the original UTF-8 string.

Directory of Database Objects

Pyrseas 0.6 has a single format for output by dbtoyaml or input into yamltodb: a single YAML-formatted file. This becomes a problem when your database has hundreds or more tables, functions, etc (let alone 409,994 tables and counting!). Furthermore, as dbtoyaml and yamltodb are intended to assist with database version control, your team may want to store individual object specifications in your version control system, or you may want to diff individual objects.

The 0.7 --directory option to dbtoyaml and yamltodb allows you to split the YAML spec into multiple files in a directory (or folder) tree. For example, using the dbtoyaml -d option on the autodoc database results in the following tree (shown under Linux using ls -RF):

schema.inherit/      schema.public/      schema.warehouse/
schema.inherit.yaml  schema.public.yaml  schema.warehouse.yaml
schema.product/      schema.store/
schema.product.yaml  schema.store.yaml

table.tab1b.yaml  table.tab1.yaml  table.taba.yaml  table.tabb.yaml

function.worker.yaml  sequence.product_product_id_seq.yaml  table.product.yaml


sequence.store_store_id_seq.yaml  table.inventory.yaml  table.store.yaml

function.worker.yaml                      table.warehouse.yaml
sequence.warehouse_warehouse_id_seq.yaml  view.products.yaml

As can be seen, each schema gets its own directory wherein are stored each object belonging to that schema. In addition to schemas, the root level also stores non-schema owned objects such as foreign data wrappers and extensions (the latter can be placed in a schema, but are not owned by it).

The directory tree and multi-line string formats are still under review, so I’d like to encourage you to test both enhancements and provide feedback.

Testing Python and PostgreSQL on Windows, Part 6

Alliterative locales, languages, collations.

A tox on all your houses (test combinations).

The last item to fix in the Pyrseas unit tests so that they run on Windows is related to the PostgreSQL 9.1 COLLATION feature. When creating the tests, I was influenced by the examples in the documentation, i.e., I created a collation with ‘fr_FR.utf8’ LC_COLLATE and LC_CTYPE. On Linux, it’s fairly straightforward to add such a locale to your system (although perhaps Windows users may disagree :-)), so the tests worked as expected.

On Windows, however, most collation tests failed with

DataError: could not create locale "fr_FR.utf8": No error

Unfortunately, the PG documentation doesn’t seem to provide any hints on what is the Windows equivalent of ‘fr_FR.utf8’ (or similar Linux locales). Eventually I figured it out by looking at the output of \l (list databases) in psql. This showed the Collation and Ctype (in my case) where ‘English_United States.1252’ so I assumed what was needed was ‘French.France.1252’. Here is the procedure to set that up:

Open the Control Panel, select Date, Time, Language, and Regional Options, then Regional and Language Options (or Add other languages), click on the Advanced tab in the dialog and then choose “French (France)” from the dropdown. Finally, click OK and respond to any subsequent prompts to install the locale, including rebooting the machine.

Aside: For comparison, on Debian Linux, the equivalent procedure involves running sudo dpkg-reconfigure locales, selecting fr_FR.UTF-8 UTF-8 from a list, accepting the default locale and waiting for the locales to be generated (no reboot necessary). Second aside: On Linux, you can deselect a locale to remove it from your system, but Windows doesn’t appear to allow for language removals.

To test, make sure you have the latest Pyrseas code from GitHub, which includes a change to fix the COLLATION tests to run on Windows.

Finally, we’re ready to install Tox and run all the unit tests with a single command. First, run pip install tox under both Python 2.7 and 3.2. Next, define (set) the environment variables PG84_PORT, PG90_PORT, PG91_PORT, and PG92_PORT to point to the corresponding PostgreSQL ports.

Then simply invoke tox from the Python 2.7 environment. Thanks to the Pyrseas tox.ini, this will install Python 2.7 and 3.2 virtualenvs, under a .tox subdirectory in the Pyrseas tree, install Psycopg2, PyYAML and Pyrseas into each virtualenv and run the unit tests eight times, once for each combination of Python and PostgreSQL.

If you have been following along, the only test failure should be in test_extension.py, in test_map_lang_extension, when attempting to CREATE EXTENSION plperl, due to the missing PERL514.DLL (see previous post). The error will only occur under PG 9.1 and 9.2.

The only problem I noticed with tox is that when there are errors it may get confused in its summary report.

___________________________________ summary ___________________________________
  py27pg90: commands succeeded
  py27pg91: commands succeeded
  py27pg92: commands succeeded
ERROR:   py32pg91: commands failed
  py27pg84: commands succeeded
ERROR:   py32pg84: commands failed
ERROR:   py32pg90: commands failed
ERROR:   py32pg92: commands failed

The errors actually occurred in the *pg91 and *pg92 environments but tox reports that all py32* tests failed, which was not the case. This is a minor issue considering all that tox accomplishes, with very little setup or configuration.

Testing Python and PostgreSQL on Windows, Part 5

I’ve got the Perl on Windows blues …

Aside from PL/pgSQL, the base distribution of PostgreSQL supports three procedural languages: Perl, Python and Tcl. When creating Pyrseas unit tests for languages (before they became EXTENSIONs), PL/Perl seemed like the “natural” choice. Perl is available on virtually all Linux and BSD base distributions, and (in contrast to Python) PL/Perl is available in both trusted and untrusted versions—a distinguishing attribute that the unit tests ought to check.

Alas, when testing on Windows I discovered that Perl isn’t as ubiquitous or as easy to deal with as it is on Linux.

The preliminary research prompted me to install Active Perl, specifically its Community Edition. Currently, the only two versions available are 5.14 and 5.16. Strangely enough, if you install one of those versions and then attempt to create the PL/Perl language under PG 8.4 or 9.0, the command succeeds, but when you try to create a function using plperl, you’ll see something like:

ERROR:  could not load library "C:/Program Files/PostgreSQL/8.4/lib/plperl.dll":
 The specified module could not be found.

Against PG 9.1 and 9.2, if you installed Perl 5.16, you’ll see the error message when issuing the CREATE EXTENSION (or LANGUAGE) statement. If you installed Perl 5.14, there should be no error.

When I first saw the error message, I was a bit puzzled since the plperl.dll libraries had all been installed and were located in the paths show in the messages. What “specified module” was missing?

Some web searching pointed me to Dependency Walker (depends.exe), a tool that appears to be indispensable if you’re going to be testing with multiple executable and DLL versions. It is analogous to Linux ldd. Depends.exe showed that plperl.dll in the 8.4 and 9.0 installations was linked with PERL510.DLL and in 9.1 and 9.2 with PER514.DLL.

Unfortunately, Active Perl has no Perl 5.10 Community Edition available, so off I was looking for an alternative. Thus I found Strawberry Perl.

The downside of Strawberry Perl’s installers is that they install it in C:\strawberry so you can’t have both Perl 5.10 and 5.14 at the same time. Someone on IRC explained that it is possible to install it in two separate paths (but it ain’t easy). For now, I chose to only install Perl 5.10. This allowed me to test Pyrseas using Python 2.7 and 3.2 against PostgreSQL 8.4, 9.0, 9.1 and 9.2, with only one Perl-related test faling (under PG 9.1 and 9.2, due to the absence of Perl 5.14).

A note of caution: Strawberry Perl installs GCC (3.4.5 in the Perl 5.10 version). If you have a pydistutils.cfg specifying a mingw32 compiler (as mentioned in my previous post), that may cause problems if you try to install or upgrade psycopg2 (or some other C extension module).