[FIX] expression: table alias reaching 64 characters limits

Postgresql has a limit of 64 characters for tables, columns names
as well as for aliases names.

When generating an alias name, e.g. for group by and order
by clauses, if the alias is longer than 64 characters,
use hashing to force the alias length to be within this
64 chars limit.

Fixes #8094
Closes #8142

openerp/osv/expression.py

@@ -135,6 +135,7 @@ import collections
 
 import logging
 import traceback
+from zlib import crc32
 
 import openerp.modules
 from . import fields
@@ -343,7 +344,16 @@ def generate_table_alias(src_table_alias, joined_tables=[]):
         return '%s' % alias, '%s' % _quote(alias)
     for link in joined_tables:
         alias += '__' + link[1]
-    assert len(alias) < 64, 'Table alias name %s is longer than the 64 characters size accepted by default in postgresql.' % alias
+    # Use an alternate alias scheme if length exceeds the PostgreSQL limit
+    # of 63 characters.
+    if len(alias) >= 64:
+        # We have to fit a crc32 hash and one underscore
+        # into a 63 character alias. The remaining space we can use to add
+        # a human readable prefix.
+        alias_hash = hex(crc32(alias))[2:]
+        ALIAS_PREFIX_LENGTH = 63 - len(alias_hash) - 1
+        alias = "%s_%s" % (
+            alias[:ALIAS_PREFIX_LENGTH], alias_hash)
     return '%s' % alias, '%s as %s' % (_quote(joined_tables[-1][0]), _quote(alias))