Mamta Satoor wrote:
In Derby, there will be two character sets which will have identical
character repertoire (UCS) but they may have different collation
associated with them depending on the value of JDBC url attribute
COLLATION. The 2 character sets will be
1)USER character set - collation of UCS_BASIC/TERRITORY_BASED depending
on the value of jdbc url attribute COLLATION specified at create
database time.
2)SQL_IDENTIFIER character set - collation of UCS_BASIC.
In general I think this looks good, thanks for the work on this Mamta
and a nice summary. I think there are some finer points to be decided
but the basic design is good. This finer points are on how a collation
is derived for certain expressions, that doesn't change the overall
design, just the input to the collation decision making. E.g. all of
this could be coded and then changing the collation for string literals
would not affect the rest of the design.
As per SQL spec, Section 11.1<schema definition>, there is an optional
syntax to associate a character set for a schema at create schema time.
Syntax Rule 5 says that if a character set is not specified by the user,
then the character set associated with schema is implementation defined.
In Derby 10.3, system schemas will be associated with SQL_IDENTIFER
character set and all the user schemas will be associated with USER
character set. Futher on, General Rule 3 specifies that the character
set associated with schema is used as the default character set for all
<column definitions>. Based on this, all the user character columns will
pick up the collation associated with USER character set and all the
system character columns will pick the up the collation associated with
SQL_IDENTIFIER character set.
The character set specification for string literals is not as well
defined as for <column definitions> but my proposal here will work
within SQL spec boundaries. SQL spec Section 5.3<literal>, Syntax Rule
14b says that if the character set is not specified for character string
literal, then character string literal's character set will be the
character set of the SQL-client module. Derby does not implement
SQL-client module, but definition of SQL-client module in Section 13.1
says that SQL-client module definition has mandatory <module name
clause> which is defined in Section 13.2 <module name clause>. The
Syntax Rule 4 in this section says that if a character set is not
specified for the SQL-client module, then it's character set is
implementation-defined. I think we can use this implementation-defined
character set for a SQL-client module to our advantage. We can define
Derby's implementation-defined character set for SQL-client module as
current schema's character set and hence the current schema's character
set will become string literal's character set.
Interesting. The thing that jumped out at me is that this effectively
means that the character set for the SQL-client module depends on the
session's state (its current schema). That just seems strange.
I came across a couple more pieces of information from the SQL Spec that
may or may not help. :-)
4.37.3
"An SQL-session has a default character set name" that is
implementation defined.
18.7
"Set the default character set name for <character string literal>s in
<preparable statement>s that are prepared
in the current SQL-session ..."
Not sure that helps, but does allow us to pick a single default
character set for a session (though it couldn't change based upon
current schema).
Then the SQL session also has a collation, 4.37.3 again:
"For each character set known to the SQL-implementation, an
SQL-session has at most one SQL-session collation
for that character set, to be used when the rules of Subclause 9.13,
“Collation determination”, are applied. There
are no SQL-session collations at the start of an SQL-session."
but that there are "no SQL-session collations at the start" means that
it's pointless without a <set session collation statement>, i.e. 9.13
SR3c) does not apply to Derby.
3)<character string type> (SQL spec section 6.1 <data type> Syntax Rule
3b and 16) - Rule 3b says that collation type of character string type
is the character set's collation AND rule 16 says that if <character
string type> is not contained in a <column definition>, then an
implementation-defined character set is associated with the <character
string type>. We can define Derby's implementation-defined character
set for such <character string type> to be current schema's character
set. The collation derivation will be implicit.
Does 3) cover JDBC parameters as well, (ie. ?) where the type of the
parameter is a character type?
6)CHAR, VARCHAR functions do not look like they are defined in the SQL
spec. But based on 5) [TRIM etc.] above, the result character string type's
collation can be considered same as the first argument's collation type
if the first argument to CHAR/VARCHAR function is a character string
type. If the first argument is not character string type, then the
result character string of CHAR/VARCHAR will have the same collation as
current schema's character set. The collation derivation will be implicit.
This approach means that CHAR(varchar_col, 20) behaves differently to
CAST (varchar_col AS CHAR(20)). Not sure if that's good or bad, but they
might be implemented today using the same code path.
Dan.
