comment out the changes in
src/backend/utils/cache/plancache.c
// /* process session variables */
// if (OidIsValid(parsetree->resultVariable))
// {
// if (acquire)
// LockDatabaseObject(VariableRelationId, parsetree->resultVariable,
// 0, AccessShareLock);
// else
// UnlockDatabaseObject(VariableRelationId,
parsetree->resultVariable,
// 0, AccessShareLock);
// }
// else if (IsA(node, Param))
// {
// Param *p = (Param *) node;
// if (p->paramkind == PARAM_VARIABLE)
// {
// if (acquire)
// LockDatabaseObject(VariableRelationId, p->paramvarid,
// 0, AccessShareLock);
// else
// UnlockDatabaseObject(VariableRelationId, p->paramvarid,
// 0, AccessShareLock);
// }
// }
the regress tests are still successful, that means these code changes
don't have related tests.
the locking is tested by isolation test in patch05
SetSessionVariable(Oid varid, Datum value, bool isNull)
{
create_sessionvars_hashtables();
svar = (SVariable) hash_search(sessionvars, &varid, HASH_ENTER, &found);
if (!found)
setup_session_variable(svar, varid);
/* if this fails, it won't change the stored value */
set_session_variable(svar, value, isNull);
}
after set_session_variable,
we want to make sure that svar->is_valid is true,
svar->value = value and svar->isnull= isNull.
Based on this, I've simplified the function set_session_variable,
refer v1-0001-minor-refactoring-set_session_variable.no-cfbot
Unfortunately, I don't think it is possible. See comments
/*
* Assign a new value to the session variable. It is copied to
* SVariableMemoryContext if necessary.
*
* **** If any error happens, the existing value won't be modified. ****
*/
So the variable cannot be released before memory for new content is allocated
The current code is a little bit more complex, but significantly reduces the risk so the variable will hold unexpected value.
The current code ensures stability of LET command - when it is successful, then value is changed, or when it is not successful, then variable is not changed.
Proposed code breaks it. There is risk of palloc exception, and that means, after unsuccessful LET the variable can be initialized. So there are three ending states, not just two.
pfree theoretically can raise an exception, but in this case it should be code our design error (using wrong allocator)
we use PlannerGlobal
{
Oid basenodeSessionVarid;
Bitmapset *checkSelectPermVarids;
}
to solve the self-assigned corner case SELECT privilege.
(let v1.a =v1.a; in this case, we need have SELECT priv for v1.a
but let v1.a = 1, we don't need SELECT priv for v1.a).
i found out these two field value(information) most case is the same
as PlannerGlobal.sessionVariables;
I came up with another solution, introduce a bool (Query.is_Variable_assigned),
and get rid of PlannerGlobal.basenodeSessionVarid,
PlannerGlobal.checkSelectPermVarids.
not sure it make sense to you, refer
v1-0002-refactoring-LET-statement-self-assign-privileg.no-cfbot
It is true, so bitmapset checkSelectPermVarids contains almost all the same data like sessionVariables.
But checkSelectPermVarids allows fast checking if a variable is used already, and sessionVariables list ensures necessary order.
My implementation needs more memory (one bitmapset), but I think it is very simple and less error prone (doesn't depend on iteration order).
In this case and this moment I prefer my bitmapset based solution. It can be optimized maybe later - I thought about a dedicated item in sessionVariables for the basenode parameter.
This should be a clear winner for value passed types, but for varlena types (for long varlena types), it can force passing content of some session variable twice.
Regards
Pavel