1berry/postgres
1
0
Fork 0
Code Issues Packages Projects Releases Wiki Activity

Fix typos in README.dependencies

Browse Source 
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
This commit is contained in:
Alvaro Herrera 2017-06-22 17:12:27 -04:00
parent 82c1507e30
commit da2322883b
1 changed files with 6 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

11
src/backend/statistics/README.dependencies
View File

@ -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