/*
* dependency_is_compatible_clause
* Determines if the clause is compatible with functional dependencies
*
* Only clauses that have the form of equality to a pseudoconstant, or can be
* interpreted that way, are currently accepted. Furthermore the variable
* part of the clause must be a simple Var belonging to the specified
* relation, whose attribute number we return in *attnum on success.
*/
static bool
dependency_is_compatible_clause(Node *clause, Index relid, AttrNumber *attnum)
{
RestrictInfo *rinfo = (RestrictInfo *) clause;
Var *var;
if (!IsA(rinfo, RestrictInfo))
return false;
/* Pseudoconstants are not interesting (they couldn't contain a Var) */
if (rinfo->pseudoconstant)
return false;
/* Clauses referencing multiple, or no, varnos are incompatible */
if (bms_membership(rinfo->clause_relids) != BMS_SINGLETON)
return false;
if (is_opclause(rinfo->clause))
{
/* If it's an opclause, check for Var = Const or Const = Var. */
OpExpr *expr = (OpExpr *) rinfo->clause;
/* Only expressions with two arguments are candidates. */
if (list_length(expr->args) != 2)
return false;
/* Make sure non-selected argument is a pseudoconstant. */
if (is_pseudo_constant_clause(lsecond(expr->args)))
var = linitial(expr->args);
else if (is_pseudo_constant_clause(linitial(expr->args)))
var = lsecond(expr->args);
else
return false;
/*
* If it's not an "=" operator, just ignore the clause, as it's not
* compatible with functional dependencies.
*
* This uses the function for estimating selectivity, not the operator
* directly (a bit awkward, but well ...).
*
* XXX this is pretty dubious; probably it'd be better to check btree
* or hash opclass membership, so as not to be fooled by custom
* selectivity functions, and to be more consistent with decisions
* elsewhere in the planner.
*/
if (get_oprrest(expr->opno) != F_EQSEL)
return false;
/* OK to proceed with checking "var" */
}
else if (not_clause((Node *) rinfo->clause))
{
/*
* "NOT x" can be interpreted as "x = false", so get the argument and
* proceed with seeing if it's a suitable Var.
*/
var = (Var *) get_notclausearg(rinfo->clause);
}
else
{
/*
* A boolean expression "x" can be interpreted as "x = true", so
* proceed with seeing if it's a suitable Var.
*/
var = (Var *) rinfo->clause;
}
/*
* We may ignore any RelabelType node above the operand. (There won't be
* more than one, since eval_const_expressions has been applied already.)
*/
if (IsA(var, RelabelType))
var = (Var *) ((RelabelType *) var)->arg;
/* We only support plain Vars for now */
if (!IsA(var, Var))
return false;
/* Ensure Var is from the correct relation */
if (var->varno != relid)
return false;
/* We also better ensure the Var is from the current level */
if (var->varlevelsup != 0)
return false;
/* Also ignore system attributes (we don't allow stats on those) */
if (!AttrNumberIsForUserDefinedAttr(var->varattno))
return false;
*attnum = var->varattno;
return true;
}
/*
* clauselist_selectivity -
* Compute the selectivity of an implicitly-ANDed list of boolean
* expression clauses. The list can be empty, in which case 1.0
* must be returned. List elements may be either RestrictInfos
* or bare expression clauses --- the former is preferred since
* it allows caching of results.
*
* See clause_selectivity() for the meaning of the additional parameters.
*
* Our basic approach is to take the product of the selectivities of the
* subclauses. However, that's only right if the subclauses have independent
* probabilities, and in reality they are often NOT independent. So,
* we want to be smarter where we can.
* Currently, the only extra smarts we have is to recognize "range queries",
* such as "x > 34 AND x < 42". Clauses are recognized as possible range
* query components if they are restriction opclauses whose operators have
* scalarltsel() or scalargtsel() as their restriction selectivity estimator.
* We pair up clauses of this form that refer to the same variable. An
* unpairable clause of this kind is simply multiplied into the selectivity
* product in the normal way. But when we find a pair, we know that the
* selectivities represent the relative positions of the low and high bounds
* within the column's range, so instead of figuring the selectivity as
* hisel * losel, we can figure it as hisel + losel - 1. (To visualize this,
* see that hisel is the fraction of the range below the high bound, while
* losel is the fraction above the low bound; so hisel can be interpreted
* directly as a 0..1 value but we need to convert losel to 1-losel before
* interpreting it as a value. Then the available range is 1-losel to hisel.
* However, this calculation double-excludes nulls, so really we need
* hisel + losel + null_frac - 1.)
*
* If either selectivity is exactly DEFAULT_INEQ_SEL, we forget this equation
* and instead use DEFAULT_RANGE_INEQ_SEL. The same applies if the equation
* yields an impossible (negative) result.
*
* A free side-effect is that we can recognize redundant inequalities such
* as "x < 4 AND x < 5"; only the tighter constraint will be counted.
*
* Of course this is all very dependent on the behavior of
* scalarltsel/scalargtsel; perhaps some day we can generalize the approach.
*/
Selectivity
clauselist_selectivity(PlannerInfo *root,
List *clauses,
int varRelid,
JoinType jointype,
SpecialJoinInfo *sjinfo,
bool use_damping)
{
Selectivity s1 = 1.0;
Selectivity *rgsel = NULL;
RangeQueryClause *rqlist = NULL;
ListCell *l;
int pos = 0;
int i = 0;
/* allocate array to hold all selectivity factors */
rgsel = (Selectivity *) palloc(sizeof(Selectivity) * list_length(clauses));
/*
* If there's exactly one clause, then no use in trying to match up pairs,
* so just go directly to clause_selectivity().
*/
if (list_length(clauses) == 1)
return clause_selectivity(root, (Node *) linitial(clauses),
varRelid, jointype, sjinfo, use_damping);
/*
* Initial scan over clauses. Anything that doesn't look like a potential
* rangequery clause gets directly added as selectivity factor. Anything that
* does gets inserted into an rqlist entry.
*/
foreach(l, clauses)
{
Node *clause = (Node *) lfirst(l);
RestrictInfo *rinfo;
Selectivity s2;
/* Always compute the selectivity using clause_selectivity */
s2 = clause_selectivity(root, clause, varRelid, jointype, sjinfo, use_damping);
/*
* Check for being passed a RestrictInfo.
*
* If it's a pseudoconstant RestrictInfo, then s2 is either 1.0 or
* 0.0; just use that rather than looking for range pairs.
*/
if (IsA(clause, RestrictInfo))
{
rinfo = (RestrictInfo *) clause;
if (rinfo->pseudoconstant)
{
rgsel[pos++] = s2;
continue;
}
clause = (Node *) rinfo->clause;
}
else
rinfo = NULL;
/*
* See if it looks like a restriction clause with a pseudoconstant on
* one side. (Anything more complicated than that might not behave in
* the simple way we are expecting.) Most of the tests here can be
* done more efficiently with rinfo than without.
*/
if (is_opclause(clause) && list_length(((OpExpr *) clause)->args) == 2)
{
OpExpr *expr = (OpExpr *) clause;
bool varonleft = true;
bool ok;
if (rinfo)
{
ok = (bms_membership(rinfo->clause_relids) == BMS_SINGLETON) &&
(is_pseudo_constant_clause_relids(lsecond(expr->args),
rinfo->right_relids) ||
(varonleft = false,
is_pseudo_constant_clause_relids(linitial(expr->args),
rinfo->left_relids)));
}
else
{
ok = (NumRelids(clause) == 1) &&
(is_pseudo_constant_clause(lsecond(expr->args)) ||
(varonleft = false,
is_pseudo_constant_clause(linitial(expr->args))));
}
if (ok)
{
/*
* If it's not a "<" or ">" operator, just merge the
* selectivity in generically. But if it's the right oprrest,
* add the clause to rqlist for later processing.
*/
switch (get_oprrest(expr->opno))
{
case F_SCALARLTSEL:
addRangeClause(&rqlist, clause,
varonleft, true, s2);
break;
case F_SCALARGTSEL:
addRangeClause(&rqlist, clause,
varonleft, false, s2);
break;
default:
/* Just merge the selectivity in generically */
rgsel[pos++] = s2;
break;
}
continue; /* drop to loop bottom */
}
}
/* Not the right form, so treat it generically. */
rgsel[pos++] = s2;
}
/*
* clauselist_selectivity -
* Compute the selectivity of an implicitly-ANDed list of boolean
* expression clauses. The list can be empty, in which case 1.0
* must be returned.
*
* See clause_selectivity() for the meaning of the additional parameters.
*
* Our basic approach is to take the product of the selectivities of the
* subclauses. However, that's only right if the subclauses have independent
* probabilities, and in reality they are often NOT independent. So,
* we want to be smarter where we can.
* Currently, the only extra smarts we have is to recognize "range queries",
* such as "x > 34 AND x < 42". Clauses are recognized as possible range
* query components if they are restriction opclauses whose operators have
* scalarltsel() or scalargtsel() as their restriction selectivity estimator.
* We pair up clauses of this form that refer to the same variable. An
* unpairable clause of this kind is simply multiplied into the selectivity
* product in the normal way. But when we find a pair, we know that the
* selectivities represent the relative positions of the low and high bounds
* within the column's range, so instead of figuring the selectivity as
* hisel * losel, we can figure it as hisel + losel - 1. (To visualize this,
* see that hisel is the fraction of the range below the high bound, while
* losel is the fraction above the low bound; so hisel can be interpreted
* directly as a 0..1 value but we need to convert losel to 1-losel before
* interpreting it as a value. Then the available range is 1-losel to hisel.
* However, this calculation double-excludes nulls, so really we need
* hisel + losel + null_frac - 1.)
* If the calculation yields zero or negative, however, we chicken out and
* use a default estimate; that probably means that one or both
* selectivities is a default estimate rather than an actual range value.
* Of course this is all very dependent on the behavior of
* scalarltsel/scalargtsel; perhaps some day we can generalize the approach.
*/
Selectivity
clauselist_selectivity(Query *root,
List *clauses,
int varRelid,
JoinType jointype)
{
Selectivity s1 = 1.0;
RangeQueryClause *rqlist = NULL;
List *clist;
/*
* Initial scan over clauses. Anything that doesn't look like a
* potential rangequery clause gets multiplied into s1 and forgotten.
* Anything that does gets inserted into an rqlist entry.
*/
foreach(clist, clauses)
{
Node *clause = (Node *) lfirst(clist);
Selectivity s2;
/*
* See if it looks like a restriction clause with a pseudoconstant
* on one side. (Anything more complicated than that might not
* behave in the simple way we are expecting.)
*
* NB: for consistency of results, this fragment of code had better
* match what clause_selectivity() would do in the cases it
* handles.
*/
if (is_opclause(clause) &&
(varRelid != 0 || NumRelids(clause) == 1))
{
OpExpr *expr = (OpExpr *) clause;
if (length(expr->args) == 2)
{
bool varonleft = true;
if (is_pseudo_constant_clause(lsecond(expr->args)) ||
(varonleft = false,
is_pseudo_constant_clause(lfirst(expr->args))))
{
Oid opno = expr->opno;
RegProcedure oprrest = get_oprrest(opno);
s2 = restriction_selectivity(root, opno,
expr->args, varRelid);
/*
* If we reach here, we have computed the same result
* that clause_selectivity would, so we can just use
* s2 if it's the wrong oprrest. But if it's the
* right oprrest, add the clause to rqlist for later
* processing.
*/
switch (oprrest)
{
case F_SCALARLTSEL:
addRangeClause(&rqlist, clause,
varonleft, true, s2);
break;
case F_SCALARGTSEL:
addRangeClause(&rqlist, clause,
varonleft, false, s2);
break;
default:
/* Just merge the selectivity in generically */
s1 = s1 * s2;
break;
}
continue; /* drop to loop bottom */
}
}
}
/* Not the right form, so treat it generically. */
s2 = clause_selectivity(root, clause, varRelid, jointype);
s1 = s1 * s2;
}
