Just a Theory

By David E. Wheeler

Committed: pgTAP Result Set Assertion Functions

Regular readers will know that I’ve been thinking a lot about testing SQL result sets and how to how to name result testing functions, and various implementation issues. I am very happy to say that I’ve now committed the first three such test functions to the Git repository. They’ve been tested on 8.4 and 8.3. Here’s what I came up with.

I had a pretty good idea how to compare sets and how to compare ordered bags, but ordered sets and unordered bags of results escaped me. During two days of intense hacking and experimentation, I quickly wrote set_eq(), which performs a set comparison of the results of two queries, and obag_eq(), which performs a row-by-row comparison of the results of two queries. I then set to work on bag_eq(), which would do a set comparison but require the same number of duplicate rows between the two queries. set_eq() was easy because I just needed to create temporary tables of the two queries and then execute two EXCEPT queries against them to see where they differ, if at all. bag_eq() was getting kind of hairy, though, so I asked about it on the Freenode #postgresql channel, where depesz looked at my example and pointed out that EXCEPT ALL would do just want I needed.

Hot damn, all it took was the addition a single extra word to the same queries used by set_eq() and I was set. This made me very happy, and such well-thought-out features are the reason I love PostgreSQL. My main man depesz made my day.

But oset_eq(), which was to compare ordered sets of results was proving much harder. The relational operators that operate on sets don’t care about order, so I would have to write the code to care myself. And because dupes needed to be ignored, it got even harder. In fact, it proved just not worth the effort. The main reason I abandoned this test function, though, was not difficulties of implementation (which were significant), but ambiguity of interpretation. After all, if duplicates are allowed but ignored, how does one deal with their effect on order? For example, say that I have two queries that order people based on name. One query might order them like so:

select * from people order by name;
  name  | age 
 Damian |  19
 Larry  |  53
 Tom    |  35
 Tom    |  44
 Tom    |  35

Another run of the same query could give me a different order:

select * from people order by name;
  name  | age 
 Damian |  19
 Larry  |  53
 Tom    |  35
 Tom    |  35
 Tom    |  44

Because I ordered only on “name,” the database was free to sort records with the same name in an undefined way. Meaning that the rows could be in different orders. This is known, if I understand correctly, as a “Partially ordered set,” or “poset.” Which is all well and good, but from my point of view makes it damn near impossible to be able to do a row-by-row comparison and ignore dupes, because they could be in different orders!

So once I gave up on that, I was down to three functions instead of four, and only one depends on ordering. So I also dropped the idea of having the “o” in the function names. Instead, I changed obag_eq() to results_eq(), and now I think I have three much more descriptive names. To summarize, the functions are:

Compares two result sets row by row, meaning that they must be in the same order and have the same number of duplicate rows in the same places.
Compares result sets to ensure they have the same rows, without regard to order or duplicate rows.
Compares result sets without regard to order, but each must have the same duplicate rows.

I’m very happy with this, because I was able to give up on the stupid function names with the word “order” included or implicit in them. Plus, I have different names for functions that are similar, which is nicely in adherence to the principle of distinction. They all provide nice diagnostics on failure, as well, like this from results_eq():

# Failed test 146
#     Results differ beginning at row 3:
#         have: (1,Anna)
#         want: (22,Betty)

Or this from set_eq() or bag_eq()

# Failed test 146
#     Extra records:
#         (87,Jackson)
#         (1,Jacob)
#     Missing records:
#         (44,Anna)
#         (86,Angelina)

set_eq() and bag_eq() also offer up useful diagnostics when the data types of the rows vary:

# Failed test 147
#     Columns differ between queries:
#         have: (integer,text)
#         want: (text,integer)

results_eq() doesn’t have access to such data, though if I can find some tuits (got any to give me?), I’ll write a quick C function that can return an array of the data types in a record object.

Now, as for the issue of arguments, what I settled on is, like Epic, passing strings of SQL to these functions. However, unlike Epic, if you pass in a simple string with no spaces, or a double-quoted string, pgTAP assumes that it’s the name of a prepared statement. The documentation now recommends prepared statements, which you can use like this:

PREPARE my_test AS SELECT * FROM active_users() WHERE name LIKE 'A%';
PREPARE expect AS SELECT * FROM users WHERE active = $1 AND name LIKE $2;
SELECT results_eq('my_test', 'expect');

This allows you to keep your SQL written as SQL, keeping your test, um, SQLish. But in those cases where you have some really simple SQL, you can just use that, too:

SELECT set_eq(
    'SELECT * FROM active_users()',
    'SELECT * FROM users ORDER BY id'

This feels like a good compromise to me, allowing the best of both worlds: keeping things in pure SQL to avoid quoting ugliness in SQL strings, while letting users pass in SQL strings if they really want to.

It turns out that I wasn’t able to support cursors for set_eq() or bag_eq(), because they use the statements passed to them to create temporary tables and then compare the records in those temporary tables. But results_eq() uses cursors internally. And it turns out that there’s a data type for cursors, refcursor. So it was easy to add cursor support to results_eq() for those who want to use it:

DECLARE cwant CURSOR FOR SELECT * FROM active_users();
SELECT results_eq('cwant'::refcursor, 'chave'::refcursor );

Neat, huh? As I said, I’m very pleased with this approach overall. There are a few caveats, such as less strict comparisons in results_eq() on 8.3 and lower, and less useful diagnostics for data type differences in results_eq(), but overall, I think that the implementation is pretty good, and that these functions will be really useful.

So what do you think? Please clone the Git repository and take the functions for a test drive on 8.3 or 8.4. Let me know what you think!

In the meantime, before releasing a new version, I still plan to add:

  • set_includes() - Set includes records in another set.
  • set_excludes() - Set excludes records in another set.
  • bag_includes() - Bag includes records in another bag.
  • bag_excludes() - Bag excludes records in another bag.
  • col_eq() - Single column result set equivalent to an array of values.
  • row_eq() - Single row form a query equivalent to a record.
  • rowtype_is() - The data type of the rows in a query is equivalent to an array of types.

Hopefully I can find some time to work on those next week. The only challenging one is row_eq(), so I may skip that one for now.

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