Updatable Views - What we have and what we want

Views The Rule Rewrite System Further Development Further Development Conclusion

Updatable ViewsWhat we have and what we want

Bernd Helmle

29. Juni 2009

Bernd Helmle Updatable Views What we have and what we want


Agenda

1 Views

2 The PostgreSQL Rule Rewrite System

3 Further Development

4 Conclusions



Views (1)

“Virtual Relation”

Abstraction Layer, user specific view of the data model

Materialized, not materialized

Independency of the logical data model (to some extent)



Views (2)

Updatable views defined in SQL92

Simple views, one relation only

No Joins

DISTINCT, GROUP BY, aggregates or UNIONs not allowed

No constant or function expressions in target list



Views (3)

Updatable Views defined in SQL99

Joins allowed (e.g. one-to-one relationships)

Notion of preserved keys

potentially updatable

updatable

simply updatable

insertable



Views (4)

CHECK OPTION forces view integrity

Qualification of the view definition becomes view constraint

Violation forces transaction error

CASCADED or LOCAL CHECK OPTION

CREATE VIEW v f o o AS SELECT i d FROM f o oWHERE i d BETWEEN 1 AND 10WITH CHECK OPTION ;



Rules

Instructs the rewriter to rewrite query trees

More like “macros”

Many tasks are better done with TRIGGERs



Rules

CREATE OR REPLACE RULE f o o u p d r u l eASON UPDATETO f o oDO ALSOINSERT INTO f o o l o g VALUES(NEW. id ,

l o c a l t i m e s t a m p ) ;



Rewriting Queries(1)

Views are implemented via rules

CREATE VIEW creates a “dummy relation”

ON SELECT rule with the view definition

Rule has no qualification

Rule is defined as INSTEAD




CREATE VIEW v f o oASSELECT

∗FROM

f o oWHERE

i d BETWEEN 1 AND 1 0 ;




RETURN rule for ON SELECT event on relation vfoo

SELECTf o o . i d

FROMf o o ∗NEW∗ , f o o ∗OLD∗ , f o o f o o

WHERE f o o . i d BETWEEN 1 AND 1 0 ;




1 SELECTing from a view becomes effectively a subquery

2 Rewriter scans Range Table Entries and substitutes RETURNrules

3 Directly folded into the original query tree

SELECT v t e s t . i d FROM (SELECT

f o o . i dFROM

f o o ∗NEW∗ , f o o ∗OLD∗ , f o o f o oWHERE f o o . i d BETWEEN 1 AND 10) AS v t e s t

;



Updatable Views(1)

Idea: Use rules for update actions, too

Rules for DELETE, UPDATE and INSERT actions

Don’t modify the query tree in place

Need to be INSTEAD

Conditional rules produce more than one query tree



Updatable Views(2)

Example of a view UPDATE rule.

CREATE OR REPLACE RULE ” UPDATE” ASON UPDATE TO v f o o DO INSTEAD UPDATE ONLY

f o o SET i d = NEW. i dWHERE

CASEWHEN o l d . i d I S NOT NULL THEN o l d .

i d = f o o . i dELSE s e r i a l t e s t . i d I S NULL

END

This just *works* for simple views.



Updatable Views(3)

CHECK OPTION needs additional thoughts

Obvious solution: Use conditional rules

check function checks update action for integrity

Rule should abort transaction when new tuple violatesCHECK OPTION

Unfortunately, this doesn’t work



Updatable Views(4)

CREATE OR REPLACE FUNCTIONdummy check opt ion ( p c o n s t r a i n t INBOOLEAN)

RETURNS BOOLEANLANGUAGE p l p g s q lAS$$BEGIN

IF p c o n s t r a i n t I S FALSE THENr a i s e exception ’ Update v i o l a t e s

v iew c o n d i t i o n ’ ;END IF ;

RETURN TRUE;END;$$ ;



Updatable Views(5)

CREATE OR REPLACE RULE ” INSERT”AS

ON INSERT TO v s e r i a l t e s tWHERE

dummy check opt ion ( new . i d >= 1 AND new .i d <= 10)

DO INSTEADINSERT INTO s e r i a l t e s t ( i d ) VALUES (

new . i d ) ;

Fails due to double evaluation of VOLATILE expressions.



Further Development(1)

Idea: Create update rules automatically

Need to check query tree if updatable or not after parsing(checkTree())

Transform SELECT -Rule into corresponding update action:1 Range table entries2 Target list3 Join tree4 Command type5 Result relation



Further Development(2)

Patch to create update rules automatically exists

Implements simple view updates as described in SQL92

CHECK OPTION not completed



Conclusions

Rules works with simple view updates

Much more work needed to support JOINed views etc.

CHECK OPTION needs executor support, e.g. push-downfacility for check option expressions from rewriter to executor

Other databases (e.g. DB2) are using INSTEAD OF triggersfor view updates


Technology

Updatable Views - What we have and what we want