Constraints and Triggers

Contents 1 Introduction 2 Naming 3 Constraints 3.1 Messages 3.2 Backwards compatibility 4 Indexes 4.1 Operator Classes 4.2 Function based indexes 4.3 Partial indexes 4.3.1 Not Null Partial Indexes On Nullable Columns 4.4 Indexes for Contains Search 5 Triggers 5.1 Syntax 5.2 OB Conventions 5.2.1 Exceptions and messages 5.2.2 Oracle's laziness 5.2.3 Soft disabling 5.2.4 Object returning

Introduction

Both check constraints and triggers are objects defined physically in database. This document will not explain the basis for triggers and constraints but just the particularities Openbravo ERP has in their usage.

Naming

When adding a check constraint, triggers and indexes modularity naming rules have to be taken into account. This because triggers and indexes are global objects for a database. The modularity naming rule is as follows: the constraint /index/trigger name must start with the DB Prefix of the module the constraint belongs to. For instance, MYMODULEDBPREFIX_CONSTRAINTNAME.

In the case of indexes and constraints, if the index/constraint is added to a table of another module then an additional EM_ prefix is required: EM_MYMODULEDBPREFIX_CONSTRAINTNAME.

By following this naming rule the index/trigger/constraint is exported to the module directory and packaged with the module.

Constraints

Check constraints do not have any particularity in Openbravo, except how they should be named and how the back-end treats them to show messages.

A step by step HowTo on How to add a Constraint can be found: here

Messages

It is possible to define a message to be shown when the rule defined by the constraint is not satisfied. How to do that is explained in the Messages documentation.

Backwards compatibility

Modules should allow compatibility for other ones built on top of them at least between minor versions, additionally there could be user data already in the application if it is in a productive environment. This means that user data or other module's could rely in the current database model and in case a new constraint is added or an existent one is modified to be more restrictive than it was, backwards compatibility could be broken. Therefore it should be avoided to add new constraints or to modify existent ones to make them more restrictive during between versions.

Indexes

Operator Classes

In PostgreSQL Operator Classes certain operator classes (text_pattern_ops, varchar_pattern_ops, and bpchar_pattern_ops) enables using indexes in queries involving pattern matching expressions. For instance, the following query:

SELECT name
FROM c_bpartner
WHERE name LIKE 'John%'

would not use an index created like this:

CREATE INDEX c_bpartner_name
  ON c_bpartner
  USING btree
  (name COLLATE pg_catalog."default");

but would use an index defined with an operator class:

CREATE INDEX c_bpartner_name
  ON c_bpartner
  USING btree
  (name COLLATE pg_catalog."default" varchar_pattern_ops);

Operator Classes are not needed in Oracle to use an index in the previously defined query, in that case if the index column defines an operator class, the operator class will have no effect.

Function based indexes

Openbravo supports the use of functions in indexes. For instance, this, along with the use of an operator class would enable the use of indexes in case insensitive queries that use the iStartsWith operator, like this one:

SELECT name
FROM c_bpartner
WHERE UPPER(name) LIKE 'JOHN%'

The following index could be used by the previous query:

CREATE INDEX c_bpartner_name
  ON c_bpartner
  USING btree
  (upper(name) COLLATE pg_catalog."default" varchar_pattern_ops);

Any function can be used in the indexes, as long as it is deterministic. Even needed functions are supported:

CREATE INDEX c_bpartner_name
  ON c_bpartner
  USING btree
  (UPPER(REPLACE(name, 'a', 'b')));

Until 3.0PR17Q2 only one function expression can be used per index, so the following index it was not supported, starting from 3.0PR17Q2 it is valid.

CREATE INDEX c_bpartner_name_id
  ON c_bpartner
  USING btree
  (UPPER(REPLACE(name, 'a', 'b')),
   UPPER(c_bpartner_id));

Partial indexes

PostgreSQL supports the definition of partial indexes. A partial index is an index where it is possible to specify the rows that are indexed. This kind of indexes are useful for commonly used WHERE conditions that use constant values.

Thus, with a partial index it is possible to index just the table data that is most commonly used, helping to reduce the amount of disk space used by the index.

A partial index can be created as follows:

CREATE INDEX a_amortization_active 
  ON a_amortization (isactive)
  WHERE isactive = 'Y';

For the moment Oracle does not support the creation of partial indexes in a explicit way. For this reason, if a partial index is found in the Openbravo XML model when using an Oracle database, the partial index definition will not be taken into account and it will be created as a regular index.

Not Null Partial Indexes On Nullable Columns

In an Oracle database, it does not include rows in an index if the indexed columns are NULL. That means that for the case where we are indexing a nullable foreign key column every index is a partial index.

This is not the behavior in PostgresSQL databases, where we will need to define the index as partial to get the same behavior. For example:

CREATE INDEX c_order_return_reason 
  ON c_order (c_return_reason_id)
  WHERE c_return_reason_id IS NOT NULL;

Indexes for Contains Search

The indexes for contains search, are those intended to provide fast searching of sub-strings within the values stored in a particular database column.

In PostgresSQL we can define a contains search index as follows:

CREATE INDEX c_bpartner_value_basic ON c_bpartner USING gin (value gin_trgm_ops);

Note that to define this kind of indexes we have to make use of the gin access method together with the gin_trgm_ops operator class for the indexed column. Both elements are available thanks to the pg_trgm extension which is included in Openbravo distribution by default.

Besides, this feature allows to define a function based index to improve icontains (case insensitive) searching:

CREATE INDEX c_bpartner_value_basic ON c_bpartner USING gin (UPPER(value) gin_trgm_ops);

For the moment, this kind of indexes are not supported in Oracle: if they are present in the XML model, they will be created as regular indexes in the database.

Triggers

A step by step HowTo on How to add a Trigger can be found: here

Syntax

Triggers, as the rest of PL code in Openbravo ERP, should be written following some restrictions in order to make them compatible between PostgreSQL and Oracle and to make it possible to correctly export and import them using DBSourceManager. These rules are detailed in the PL-SQL code rules document.

OB Conventions

Exceptions and messages

Triggers can raise exceptions (it is the common practice to abort a transaction), the back-end captures that exception in order to show a proper message. Read in the messags documentation how to do it.

Oracle's laziness

Whereas oracle evaluates expressions in a lazy manner PostgreSQL does not do so. This specially important for triggers that are for insert and delete because in the first case there are :new variables and in the second one there are :old ones, so when writing if clauses (even in Oracle if we want to do compatible code) this must be taken into account writing two clauses instead of just one.

Soft disabling

All triggers in Openbravo ERP must be able to be disabled softly. This means not disabling it in database but just in a logic way. This is used by DAL when it is importing data: triggers must be disabled in order to allow data imporation without triggering them, but that only should affect to the session that it is executing the importation. To add this capability to triggers, the first lines of each of them should look like:

In Oracle

 
    IF AD_isTriggerEnabled()='N' THEN 
      RETURN;
    END IF;

In PostgreSQL

 
     IF AD_isTriggerEnabled()='N' THEN
        IF TG_OP = 'DELETE' THEN RETURN OLD; ELSE RETURN NEW; END IF;
     END IF;

Object returning

PostgreSQL trigger function must explicitly take care of returning the trigger object, also depending on the type of the trigger. This means that the last line of the trigger function must be something like:

 
     IF TG_OP = 'DELETE' THEN RETURN OLD; ELSE RETURN NEW; END IF;

Category: Concepts