Complete the ORM Function Library

[Matthew Pava]

I was surprised to learn that we didn't have a StrIndex function until version 2, and yet we had Substr since at least version 1.8. I wonder how users were using Substr without also finding a use for StrIndex this whole time. Anyway, since we seem to be adding these functions one at a time, why don't we work on trying to get the built-ins implemented in one sweep instead?

We may even want to split the documentation page (​https://docs.djangoproject.com/en/dev/ref/models/database-functions/) into further categories with String functions and Numeric functions.

This is just a sample checklist, with corresponding attributes to which backend has them available.

Comparison
Name	Description	PostgreSQL	Oracle	MySQL	SQLite	PR
NULLIF()	Returns NULL if the first argument equals the second.	X	X	X	X	-

Text
Name	Description	PostgreSQL	Oracle	MySQL	SQLite	PR
ASCII()	Returns numeric value of left-most character	X	X	X	as UNICODE()	-
CHR()	Character with the given code	X	X	as CHAR()	as CHAR()	-
LEFT()	Returns the leftmost number of characters as specified	X	-	X	-	-
LPAD()	Returns the string argument, left-padded with the specified string	X	X	X	-	-
MD5()	Calculates the MD5 hash of string, returning the result in hexadecimal	X	-	-	-	-
REPEAT()	Repeats a string the specified number of times	X	-	X	-	-
REPLACE()	Replaces occurrences of a specified string	X	X	X	X	-
REVERSE()	Reverse the characters in a string	X	-	X	-	-
RIGHT()	Returns the specified rightmost number of characters	X	-	X	-	-
RPAD()	Appends string the specified number of times	X	X	X	-	-
LTRIM()	Removes leading spaces	X	X	X	X	​9220
RTRIM()	Removes trailing spaces	X	X	X	X	​9220
TRIM()	Removes leading and trailing spaces	X	X	X	X	​9220

Math
Name	Description	PostgreSQL	Oracle	MySQL	SQLite	PR
ABS()	Returns the absolute value.	X	X	X	X	-
ACOS()	Returns the arccosine.	X	X	X	-	-
ASIN()	Returns the arcsine.	X	X	X	-	-
ATAN()	Returns the arctangent.	X	X	X	-	-
ATAN2()	Returns the arctangent of the two variables passed to it.	X	X	X	-	-
CEILING()	Returns the smallest integer value that is not less than a numeric expression	X	as CEIL()	X	-	-
COS()	Returns the cosine expressed in radians.	X	X	X	-	-
COT()	Returns the cotangent.	X	X	X	-	-
DEGREES()	Returns a numeric expression converted from radians to degrees.	X	X	X	-	-
EXP()	Returns the base of the natural logarithm (e) raised to the power of a numeric expression.	X	X	X	-	-
FLOOR()	Returns the largest integer value that is not greater than a numeric expression.	X	X	X	-	-
LOG()	Returns the natural logarithm of a numeric expression.	X	X	X	-	-
MOD()	Returns the remainder of one expression by diving by another expression.	X	X	X	-	-
PI()	Returns the value of pi	X	-	X	-	-
POWER()	Returns the value of one expression raised to the power of another expression	X	X	X	-	-
RADIANS()	Returns the value of an expression converted from degrees to radians.	X	X	X	-	-
ROUND()	Returns a numeric expression rounded to an integer. Can be used to round an expression to a number of decimal points	X	X	X	X	-
SIN()	Returns the sine given in radians.	X	X	X	-	-
SQRT()	Returns the square root.	X	X	X	-	-
TAN()	Returns the tangent expressed in radians.	X	X	X	-	-

[Josh Smeaton]

I had hoped that the 3rd party community would take care of providing "Function Packs" for each of the backends. But functions are fairly easy to create in your own project if you need them so I guess few people have bothered to group them into a distributable package.

I would prefer that the django ecosystem had a standard group of function expressions that can be used across backends, rather than a bunch of similar but not quite the same implementations. Then 3rd party libraries can depend on the known good versions and everyone is better off. Since the community hasn't seemed to provide such a library (that I'm aware of), I think it's fine for Django to do so.

Splitting the module up into string/numeric/date types sounds fine. Let's take inspiration from postgres and other db vendor docs so navigating them is familiar. Let's begin with functions that have standard support across our 4 backends, then look into what's left over. If we can approximate support by combining other functions we can do that. Otherwise if a particular function only has support for one or two backends we can consider ignoring it or implementing in a contrib module.

[Josh Smeaton]

For whoever wants to begin here, I think a good plan of attack will be a PR per function or per a small set of functions. Then the commit message would be something like "Refs #28643 -- Added X, Y, Z functions".

That way we can get through smaller pieces at a time, without such a large burden being placed on any one individual. We could also get some newer contributors to help in this way. Each function on its own should be a relatively easy thing to implement. A higher level task of re-organising the layout into functions/string functions/number may be helpful to get the ticket rolling.

[Matthew Pava]

I did find this third-party utility that handles specific PostgreSQL functions. I wonder if there are other backends that they could be ported to.
​https://github.com/hypertrack/django-pg-utils

And I also wonder how all of this connects with specific PostgreSQL aggregate functions already builtin to Django:
​https://docs.djangoproject.com/en/1.11/ref/contrib/postgres/aggregates/

[Mads Jensen]

Replying to Matthew Pava:

I did find this third-party utility that handles specific PostgreSQL functions. I wonder if there are other backends that they could be ported to.
​https://github.com/hypertrack/django-pg-utils

From a quick look, these things are already supported by Django.

[Mariusz Felisiak]

I prepared patch to reorganize database functions docs and code (​PR). I think we should organize code and doc as follows (bolded functions doesn't exist):

• docs/ref/models/database-functions.txt section Comparison and conversion functions (django/db/models/functions/comparison.py):
  • Cast, Coalesce, NullIf, Greatest, Least;
• docs/ref/models/database-functions.txt section Date Functions (django/db/models/functions/datetime.py):
  • Extract, ExtractDay, ExtractHour, ExtractMinute, ExtractMonth, ExtractQuarter, ExtractSecond, ExtractWeek, ExtractWeekDay, ExtractYear, Now, Trunc, TruncDate, TruncDay, TruncHour, TruncMinute, TruncMonth, TruncQuarter, TruncSecond, TruncTime, TruncYear";
• docs/ref/models/database-functions.txt section Math Functions (django/db/models/functions/math.py):
  • Abs, ACos, ASin, ATan, ATan2, Ceil, Cos, Cot, Exp, Floor, Log, Mod, Power, Round, Sin, Sqrt, Tan;
• docs/ref/models/database-functions.txt section Text Functions (django/db/models/functions/text.py):
  • Ascii, Chr, Concat, ConcatPair, Length, LPad, Lower, LTrim, Replace, RPad, RTrim, StrIndex, Substr, Trim, Upper;
• docs/ref/models/database-functions.txt section Window Functions (django/db/models/functions/window.py):
  • CumeDist, DenseRank, FirstValue, Lag, LastValue, Lead, NthValue, Ntile, PercentRank, Rank, RowNumber.

Headers and functions in docs and code should be organized alphabetically. Thanks Tim Graham for suggestions.

I'm not convince that following functions should be implemented because there not supported on all databases:

• LEFT(), MD5(), REPEAT(), REVERSE(), RIGHT(), DEGREES(), PI(), RADIANS().

[Tim Graham]

The "Miscellaneous" group is sort of bothering me.

Cast, Coalesce, Greatest, Least sound like candidates for a section called something like "Comparison and conversion functions"

Ascii and Chr sound like they could be "text" related.

Now sounds like a candidate for "Date Functions".

[Mariusz Felisiak]

Agreed. I updated above comment and both PRs.

[Matthew Pava]

In regards to Left() and Right(), they are available in PostgreSQL and MySQL, but not in SQLite or Oracle. However, you can emulate their functionality using the corresponding Substr function in the database backend. I would hate to see us leave those out for that reason alone. At the same time, developers could use the Substr across all databases instead of using Left and Right.

I was also wondering about the use of Trim. Python doesn't use that term; instead, it uses strip. I wonder if that really matters. Some developers may look for a strip functionality in Django ORM not realizing that the databases use the term Trim. Perhaps that could be clarified in the documentation.

[Shai Berger]

Replying to Matthew Pava:

I was also wondering about the use of Trim. Python doesn't use that term; instead, it uses strip. I wonder if that really matters. Some developers may look for a strip functionality in Django ORM not realizing that the databases use the term Trim. Perhaps that could be clarified in the documentation.

Unless there is some other use for the term strip in the databases, I'd consider "documenting" this by giving an alias:

class Trim(Transform):
    # ...

Strip = Trim  # Give Trim a more Pythonic name

[GitHub <noreply@…>]

In ad8036d7:

Refs #28643 -- Reorganized database functions docs.

Thanks Tim Graham for the review.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 8b42a18b:

[2.0.x] Refs #28643 -- Reorganized database functions docs.

Thanks Tim Graham for the review.

Backport of ad8036d715d4447b95d485332511b4edb1a40c0e from master

[GitHub <noreply@…>]

In 4f27e475:

Refs #28643 -- Reorganized database functions.

Thanks Tim Graham for the review.

[Mariusz Felisiak]

​PR - Ltrim, Rtrim, and Trim with aliases Lstrip, Rstrip, and Strip, respectively.

[Matthew Pava]

And, of course, there is a Python lstrip and rstrip as well.

Lstrip = Ltrim  # Give Ltrim a more Pythonic name
Rstrip = Rtrim  # Give Rtrim a more Pythonic name

[Mariusz Felisiak]

Replying to Matthew Pava:

And, of course, there is a Python lstrip and rstrip as well.

Lstrip = Ltrim  # Give Ltrim a more Pythonic name
Rstrip = Rtrim  # Give Rtrim a more Pythonic name

I added these aliases.

[Matthew Pava]

I formatted the list into a table so we could see better what we can do to address this ticket. It looks like SQLite is missing much functionality that the other backends support, especially in the math department.

I also suggest implementing Left and Right, but for the unsupported backends to use the corresponding Substr function.

[Nick Pope]

Updated description and comment:9 to include NullIf.

See the following links:

• ​http://modern-sql.com/concept/null#nullif
• ​http://modern-sql.com/feature/case#nullif
• ​http://modern-sql.com/feature/case#compatibility