From da2322883b9e50b1aac70a3b6eaf2a4f0e486469 Mon Sep 17 00:00:00 2001
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: Thu, 22 Jun 2017 17:12:27 -0400
Subject: [PATCH] Fix typos in README.dependencies
There was a logic error in a formula, reported by Atsushi Torokoshi.
Ashutosh Bapat furthermore recommended to change notation for a variable
that was re-using a letter from a previous formula, though his proposed
patch contained a small error in attributing what the new letter is for.
Also, instead of his proposed d' I ended up using e, to avoid confusing
the reader with quotes which are used differently in the explaining
prose.
Bugs appeared in commit 2686ee1b7ccfb9214064d4d2a98ea77382880306.
Reported-by: Atsushi Torikoshi, Ashutosh Bapat
Discussion: https://postgr.es/m/CAFjFpRd03YojT4wyuDcjhCfYuygfWfnt68XGn2CKv=rcjRCtTA@mail.gmail.com
---
src/backend/statistics/README.dependencies | 11 ++++++-----
1 file 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
--
GitLab