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https://issues.apache.org/jira/browse/IMPALA-3531?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16787164#comment-16787164
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Greg Rahn commented on IMPALA-3531:
-----------------------------------
A few comments on grammar specifics:
The ANSI/ISO SQL spec specifically covers deferrable and optionally enforced
constraints using the following grammar which should be supported as part of
this effort:
{noformat}
<constraint name definition> ::=
CONSTRAINT <constraint name>
<constraint characteristics> ::=
<constraint check time> [ [ NOT ] DEFERRABLE ] [ <constraint enforcement> ]
| [ NOT ] DEFERRABLE [ <constraint check time> ] [ <constraint enforcement> ]
| <constraint enforcement>
<constraint check time> ::=
INITIALLY DEFERRED
| INITIALLY IMMEDIATE
<constraint enforcement> ::=
[ NOT ] ENFORCED
{noformat}
Adding support for Oracle's grammar makes sense for compatibility reasons:
{noformat}
ENABLE | DISABLE
Specifies whether the constraint is enabled or disabled.
VALIDATE | NOVALIDATE
Specifies whether to validate existing data in the table when a constraint is
created. Only used in conjunction with the ENABLE | DISABLE property.
RELY | NORELY
Specifies whether a constraint in NOVALIDATE mode is taken into account for
query optimizations.
{noformat}
In addition to grammar, I'll also highlight
information_schema.table_constraints uses the following columns to show the
features:
- is_deferrable - YES if the constraint is deferrable, NO if not
- initially_deferred - YES if the constraint is deferrable and initially
deferred, NO if not
- enforced - YES if the constraint is enforced, NO if not
See also:
- https://www.postgresql.org/docs/11/infoschema-table-constraints.html
> Implement FK/PK "rely novalidate" constraints for better CBO
> ------------------------------------------------------------
>
> Key: IMPALA-3531
> URL: https://issues.apache.org/jira/browse/IMPALA-3531
> Project: IMPALA
> Issue Type: New Feature
> Components: Catalog, Frontend, Perf Investigation
> Affects Versions: Impala 2.5.0, Impala 2.6.0
> Environment: CDH
> Reporter: Ruslan Dautkhanov
> Assignee: Anurag Mantripragada
> Priority: Minor
> Labels: CBO, performance, ramp-up
>
> Oracle has "RELY NOVALIDATE" option for constraints.. Could be easier for
> Hive to start with something like that for PK/FK constraints. So CBO has more
> information for optimizations. It does not have to actually check if that
> constraint is relationship is true; it can just "rely" on that constraint.
> https://docs.oracle.com/database/121/SQLRF/clauses002.htm#sthref2289
> So it would be helpful with join cardinality estimates, and with cases like
> IMPALA-2929.
> https://docs.oracle.com/database/121/DWHSG/schemas.htm#DWHSG9053
> "Overview of Constraint States":
> - Enforcement
> - Validation
> - Belief
> So FK/PK with "rely novalidate" will have Enforcement&Validate disabled but
> Belief = RELY as it is possible to do in Oracle and now in Hive (HIVE-13076).
> It opens a lot of ways to do additional ways to optimize execution plans.
> As exxplined in Tom Kyte's "Metadata matters"
> http://www.peoug.org/wp-content/uploads/2009/12/MetadataMatters_PEOUG_Day2009_TKyte.pdf
> pp.30 - "Tell us how the tables relate and we can remove them from the
> plan...".
> pp.35 - "Tell us how the tables relate and we have more access paths
> available...".
> Also it might be helpful when Impala is being integrated with Kudu as the
> latter have to have a PK.
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