Consider Removing psql type specifiers when Comparing Computed Constructs

[tcommandeur]

Describe the bug
I am using a Computed column in a Postgresql database and created a migration. The next migration I create with automigration returns a warning: alembic\autogenerate\compare.py:1034: UserWarning: Computed default on table.search_vector cannot be modified

The string generated for comparison by _normalize_computed_default:
setweightto_tsvectorenglish,title,a
setweightto_tsvectorenglish::regconfig,title::text,a::char

The code responsible for this:

alembic/alembic/autogenerate/compare.py

Lines 979 to 986 in 44965f0

	def _normalize_computed_default(sqltext: str) -> str:
	"""we want to warn if a computed sql expression has changed. however
	we don't want false positives and the warning is not that critical.
	so filter out most forms of variability from the SQL text.
	"""
	return re.sub(r"[ \(\)'\"`\[\]\t\r\n]", "", sqltext).lower()

Proposed solution
Remove groups of text starting with :: by adding another regex substitution.

 def _normalize_computed_default(sqltext: str) -> str: 
     """we want to warn if a computed sql expression has changed.  however 
     we don't want false positives and the warning is not that critical. 
     so filter out most forms of variability from the SQL text. 
     """ 
  
    normalized_sqltext = re.sub(r"[ \(\)'\"`\[\]]", "", sqltext).lower()
    return re.sub(r"::\w+", "", normalized_sqltext)

[CaselIT]

Hi,

seems like a sensible solution. There is already some code that's trying to compare the index expression in the postgresql dialect, so maybe that could be re-used.

Could you provided a PR for it?