Recognize some OR clauses as compatible with functional dependencies
Since commit 8f321bd16c functional dependencies can handle IN clauses, which however introduced a possible (and surprising) inconsistency, because IN clauses may be expressed as an OR clause, which are still considered incompatible. For example a IN (1, 2, 3) may be rewritten as (a = 1 OR a = 2 OR a = 3) The IN clause will work fine with functional dependencies, but the OR clause will force the estimation to fall back to plain per-column estimates, possibly introducing significant estimation errors. This commit recognizes OR clauses equivalent to an IN clause (when all arugments are compatible and reference the same attribute) as a special case, compatible with functional dependencies. This allows applying functional dependencies, just like for IN clauses. This does not eliminate the difference in estimating the clause itself, i.e. IN clause and OR clause still use different formulas. It would be possible to change that (for these special OR clauses), but that's not really about extended statistics - it was always like this. Moreover the errors are usually much smaller compared to ignoring dependencies. Author: Tomas Vondra Reviewed-by: Dean Rasheed Discussion: https://www.postgresql.org/message-id/flat/13902317.Eha0YfKkKy%40pierred-pdoc
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Tomas Vondra
parent
dbe0d9892c
commit
ccaa3569f5
@ -753,11 +753,11 @@ pg_dependencies_send(PG_FUNCTION_ARGS)
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static bool
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dependency_is_compatible_clause(Node *clause, Index relid, AttrNumber *attnum)
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{
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RestrictInfo *rinfo = (RestrictInfo *) clause;
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Var *var;
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if (!IsA(rinfo, RestrictInfo))
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return false;
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if (IsA(clause, RestrictInfo))
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{
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RestrictInfo *rinfo = (RestrictInfo *) clause;
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/* Pseudoconstants are not interesting (they couldn't contain a Var) */
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if (rinfo->pseudoconstant)
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@ -767,10 +767,13 @@ dependency_is_compatible_clause(Node *clause, Index relid, AttrNumber *attnum)
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if (bms_membership(rinfo->clause_relids) != BMS_SINGLETON)
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return false;
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if (is_opclause(rinfo->clause))
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clause = (Node *) rinfo->clause;
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}
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if (is_opclause(clause))
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{
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/* If it's an opclause, check for Var = Const or Const = Var. */
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OpExpr *expr = (OpExpr *) rinfo->clause;
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OpExpr *expr = (OpExpr *) clause;
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/* Only expressions with two arguments are candidates. */
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if (list_length(expr->args) != 2)
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@ -801,10 +804,10 @@ dependency_is_compatible_clause(Node *clause, Index relid, AttrNumber *attnum)
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/* OK to proceed with checking "var" */
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}
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else if (IsA(rinfo->clause, ScalarArrayOpExpr))
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else if (IsA(clause, ScalarArrayOpExpr))
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{
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/* If it's an scalar array operator, check for Var IN Const. */
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ScalarArrayOpExpr *expr = (ScalarArrayOpExpr *) rinfo->clause;
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ScalarArrayOpExpr *expr = (ScalarArrayOpExpr *) clause;
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/*
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* Reject ALL() variant, we only care about ANY/IN.
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@ -839,13 +842,43 @@ dependency_is_compatible_clause(Node *clause, Index relid, AttrNumber *attnum)
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/* OK to proceed with checking "var" */
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}
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else if (is_notclause(rinfo->clause))
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else if (is_orclause(clause))
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{
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BoolExpr *expr = (BoolExpr *) clause;
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ListCell *lc;
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/* start with no attribute number */
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*attnum = InvalidAttrNumber;
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foreach(lc, expr->args)
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{
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AttrNumber clause_attnum;
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/*
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* Had we found incompatible clause in the arguments, treat the
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* whole clause as incompatible.
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*/
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if (!dependency_is_compatible_clause((Node *) lfirst(lc),
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relid, &clause_attnum))
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return false;
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if (*attnum == InvalidAttrNumber)
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*attnum = clause_attnum;
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if (*attnum != clause_attnum)
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return false;
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}
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/* the Var is already checked by the recursive call */
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return true;
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}
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else if (is_notclause(clause))
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{
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/*
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* "NOT x" can be interpreted as "x = false", so get the argument and
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* proceed with seeing if it's a suitable Var.
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*/
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var = (Var *) get_notclausearg(rinfo->clause);
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var = (Var *) get_notclausearg(clause);
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}
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else
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{
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@ -853,7 +886,7 @@ dependency_is_compatible_clause(Node *clause, Index relid, AttrNumber *attnum)
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* A boolean expression "x" can be interpreted as "x = true", so
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* proceed with seeing if it's a suitable Var.
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*/
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var = (Var *) rinfo->clause;
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var = (Var *) clause;
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}
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/*
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@ -458,6 +458,32 @@ SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE
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3 | 400
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(1 row)
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-- OR clauses referencing the same attribute
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SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE (a = 1 OR a = 51) AND b = ''1''');
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estimated | actual
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-----------+--------
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2 | 100
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(1 row)
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SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE (a = 1 OR a = 51) AND (b = ''1'' OR b = ''2'')');
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estimated | actual
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-----------+--------
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4 | 100
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(1 row)
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SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE (a = 1 OR a = 2 OR a = 51 OR a = 52) AND (b = ''1'' OR b = ''2'')');
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estimated | actual
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-----------+--------
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8 | 200
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(1 row)
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-- OR clauses referencing different attributes
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SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE (a = 1 OR b = ''1'') AND b = ''1''');
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estimated | actual
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-----------+--------
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3 | 100
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(1 row)
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-- ANY
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SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE a = ANY (ARRAY[1, 51]) AND b = ''1''');
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estimated | actual
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@ -592,6 +618,32 @@ SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE
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400 | 400
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(1 row)
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-- OR clauses referencing the same attribute
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SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE (a = 1 OR a = 51) AND b = ''1''');
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estimated | actual
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-----------+--------
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99 | 100
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(1 row)
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SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE (a = 1 OR a = 51) AND (b = ''1'' OR b = ''2'')');
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estimated | actual
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-----------+--------
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99 | 100
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(1 row)
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SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE (a = 1 OR a = 2 OR a = 51 OR a = 52) AND (b = ''1'' OR b = ''2'')');
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estimated | actual
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-----------+--------
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197 | 200
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(1 row)
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-- OR clauses referencing different attributes are incompatible
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SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE (a = 1 OR b = ''1'') AND b = ''1''');
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estimated | actual
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-----------+--------
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3 | 100
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(1 row)
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-- ANY
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SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE a = ANY (ARRAY[1, 51]) AND b = ''1''');
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estimated | actual
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@ -286,6 +286,16 @@ SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE
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SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE a IN (1, 2, 26, 27, 51, 52, 76, 77) AND b IN (''1'', ''2'', ''26'', ''27'') AND c IN (1, 2)');
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-- OR clauses referencing the same attribute
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SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE (a = 1 OR a = 51) AND b = ''1''');
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SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE (a = 1 OR a = 51) AND (b = ''1'' OR b = ''2'')');
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SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE (a = 1 OR a = 2 OR a = 51 OR a = 52) AND (b = ''1'' OR b = ''2'')');
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-- OR clauses referencing different attributes
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SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE (a = 1 OR b = ''1'') AND b = ''1''');
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-- ANY
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SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE a = ANY (ARRAY[1, 51]) AND b = ''1''');
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@ -338,6 +348,16 @@ SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE
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SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE a IN (1, 2, 26, 27, 51, 52, 76, 77) AND b IN (''1'', ''2'', ''26'', ''27'') AND c IN (1, 2)');
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-- OR clauses referencing the same attribute
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SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE (a = 1 OR a = 51) AND b = ''1''');
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SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE (a = 1 OR a = 51) AND (b = ''1'' OR b = ''2'')');
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SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE (a = 1 OR a = 2 OR a = 51 OR a = 52) AND (b = ''1'' OR b = ''2'')');
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-- OR clauses referencing different attributes are incompatible
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SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE (a = 1 OR b = ''1'') AND b = ''1''');
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-- ANY
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SELECT * FROM check_estimated_rows('SELECT * FROM functional_dependencies WHERE a = ANY (ARRAY[1, 51]) AND b = ''1''');
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