База данных / IS_to_STUD / Soft / 2.1.0.17798-Final / Firebird-2.1.0.17798-0_Win32 / doc / README.Optimizer

Optimizer enhancements in FB2.0

The Optimizer has two different optimizer "modules" depending on the ODS of the
database. The reason for this is that in ODS 11 segment-selectivity is introduced.


Improvements for every ODS:

A) Deliver allowable comparisons from HAVING clause to WHERE clause so that indexes
could be used. Only simple comparisons are checked if they are deliverable. At
least 1 side of the comparison should be a field contained in the GROUP BY clause.
Note that WHERE clauses on aggregate views are also put in the HAVING clause.

SELECT Count(*) FROM RelationCategories GROUP BY CategoryID HAVING CategoryID = 5

B) Try to deliver conjuntions made on a UNION inside every single stream.

SELECT Count(*) FROM
(SELECT RelationID FROM Relations
UNION ALL
SELECT CategoryID FROM Categories) dt (ID)
WHERE
dt.ID = 1

C) Cardinality calculation has been improved for a better estimated value.

D) MERGE/SORT operation may now be generated for inner joins using equality
comparison on expressions.

SELECT Count(*) FROM
Relations r
JOIN RelationCategories rc ON (rc.RelationID + 0 = r.RelationID + 0)
WHERE
rc.CategoryID = 3

E) When a UNIQUE index used on a relation can be fully matched (equality), use this
index and forget any possible indexes.

F) Indexed order read with an OUTER JOIN.

SELECT * FROM
Relations r
LEFT JOIN RelationCategories rc ON (rc.RelationID = r.RelationID and rc.CategoryID = 3)
ORDER BY
r.LASTNAME

Note: Patch provided by <Radek Palmowski>


Improvements for ODS 11:

G) Ignore NULL values in index scan for operators other than IS NULL and Equivalent.
In previous versions with a NULL-able index NULL-keys could be put in the bitmap
for record retrieval.

Note: This works currently only at single segment indexes.


H) Calculate with more accuracy selectivity for compound indexes which are partially
matched with conjunctions. Due this we can now use cost-based approach also for
partially matched indexes.

Example:
SELECT * FROM RelationCategories WHERE CategoryID = 1

I) IS NULL can be used inside compound indexes on every segment (assuming previous
segments are matched too).

Example:
SELECT * FROM Relations WHERE LastName = 'Goebel' and MiddleName IS NULL

J) STARTING WITH can be used inside compound indexes on every segment, but only the
last matching segment.

Example:
SELECT * FROM Relations WHERE FirstName = 'Joe' and LastName STARTING WITH 'D'

K) Use AND conjunctions also inside OR conjunctions to help with matching on a
compound index. The same calculation for every left and right node of an OR
conjunction is used as for the AND conjunctions.

Example:
SELECT * FROM RelationCategories WHERE CategoryID = 3 and RelationID IN (123, 720, 708)

Above select will use 3 times the index created for the primary key.

L) JOIN order calculation doesn't prefer primary indexes, but uses cost based
calculation for every relation (selectivity, cardinality and number of indexes).
Selectivity for a node is calculated over all selectivities used in that node.
Indexes matched with IS NULL and STARTING WITH are now also considered in this
calculation. Less JOIN order combinations are being calculated.

SELECT * FROM
Relations r
JOIN RelationCategories rc ON (rc.RelationID = r.RelationID and rc.CATEGORYID = 1)
WHERE
r.FIRSTNAME LIKE 'J%'


Example metadata:

CREATE TABLE Relations (
RelationID INT NOT NULL PRIMARY KEY,
FirstName VARCHAR(35),
MiddleName VARCHAR(35),
LastName VARCHAR(35)
);

CREATE INDEX IDX_FIRSTNAME ON Relations (FirstName);
CREATE INDEX IDX_LASTNAME ON Relations (LastName);
CREATE INDEX IDX_FIRST_LASTNAME ON Relations (FirstName, LastName);
CREATE INDEX IDX_LAST_MIDDLENAME ON Relations (MiddleName, LastName);

CREATE TABLE Categories (
CategoryID INT NOT NULL PRIMARY KEY,
Description VARCHAR(30)
);

CREATE TABLE RelationCategories (
RelationID INT NOT NULL,
CategoryID INT NOT NULL,
CONSTRAINT FK_RC_RELATIONS FOREIGN KEY (RelationID)
REFERENCES Relations (RelationID),
CONSTRAINT FK_RC_CATEGORIES FOREIGN KEY (CategoryID)
REFERENCES Categories (CategoryID),
PRIMARY KEY (CategoryID, RelationID)
);