diff options
| -rw-r--r-- | src/backend/statistics/README.dependencies | 11 |
1 files changed, 6 insertions, 5 deletions
diff --git a/src/backend/statistics/README.dependencies b/src/backend/statistics/README.dependencies index 59f9d576578..702d34e3f80 100644 --- a/src/backend/statistics/README.dependencies +++ b/src/backend/statistics/README.dependencies @@ -79,20 +79,21 @@ to break the consistency. Clause reduction (planner/optimizer) ------------------------------------ -Applying the functional dependencies is fairly simple - given a list of +Applying the functional dependencies is fairly simple: given a list of equality clauses, we compute selectivities of each clause and then use the degree to combine them using this formula P(a=?,b=?) = P(a=?) * (d + (1-d) * P(b=?)) -Where 'd' is the degree of functional dependence (a=>b). +Where 'd' is the degree of functional dependency (a => b). With more than two equality clauses, this process happens recursively. For -example for (a,b,c) we first use (a,b=>c) to break the computation into +example for (a,b,c) we first use (a,b => c) to break the computation into - P(a=?,b=?,c=?) = P(a=?,b=?) * (d + (1-d)*P(b=?)) + P(a=?,b=?,c=?) = P(a=?,b=?) * (e + (1-e) * P(c=?)) -and then apply (a=>b) the same way on P(a=?,b=?). +where 'e' is the degree of functional dependency (a,b => c); then we can +apply (a=>b) the same way on P(a=?,b=?). Consistency of clauses |