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79 lines
2.8 KiB
Plaintext
79 lines
2.8 KiB
Plaintext
---------------------------
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Universal triggers (FB 1.5)
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---------------------------
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Function:
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Allows triggers that handle multiple row operations.
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Author:
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Dmitry Yemanov <yemanov@yandex.ru>
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Syntax rules:
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CREATE TRIGGER name FOR table
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[ACTIVE | INACTIVE]
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{BEFORE | AFTER} <multiple_action>
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[POSITION number]
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AS trigger_body
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<multiple_action> ::= <single_action> [OR <single_action> [OR <single_action>]]
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<single_action> ::= {INSERT | UPDATE | DELETE}
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Example(s):
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1. CREATE TRIGGER TRIGGER1 FOR TABLE1 BEFORE INSERT OR UPDATE AS ...;
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2. CREATE TRIGGER TRIGGER2 FOR TABLE2 AFTER INSERT OR UPDATE OR DELETE AS ...;
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ODS:
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Encoding of field RDB$TRIGGER_TYPE (relation RDB$TRIGGERS) has been extended
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to allow complex trigger actions. Coding scheme is shown below (in C syntax):
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// trigger type prefixes
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#define TRIGGER_BEFORE 0
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#define TRIGGER_AFTER 1
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// trigger type suffixes
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#define TRIGGER_INSERT 1
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#define TRIGGER_UPDATE 2
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#define TRIGGER_DELETE 3
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// that's how trigger action types are encoded
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/*
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bit 0 = TRIGGER_BEFORE/TRIGGER_AFTER flag,
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bits 1-2 = TRIGGER_INSERT/TRIGGER_UPDATE/TRIGGER_DELETE (slot #1),
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bits 3-4 = TRIGGER_INSERT/TRIGGER_UPDATE/TRIGGER_DELETE (slot #2),
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bits 5-6 = TRIGGER_INSERT/TRIGGER_UPDATE/TRIGGER_DELETE (slot #3),
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and finally the above calculated value is decremented
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example #1:
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TRIGGER_AFTER_DELETE =
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= ((TRIGGER_DELETE << 1) | TRIGGER_AFTER) - 1 =
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= ((3 << 1) | 1) - 1 =
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= 0x00000110 (6)
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example #2:
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TRIGGER_BEFORE_INSERT_UPDATE =
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= ((TRIGGER_UPDATE << 3) | (TRIGGER_INSERT << 1) | TRIGGER_BEFORE) - 1 =
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= ((2 << 3) | (1 << 1) | 0) - 1 =
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= 0x00010001 (17)
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example #3:
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TRIGGER_AFTER_UPDATE_DELETE_INSERT =
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= ((TRIGGER_INSERT << 5) | (TRIGGER_DELETE << 3) | (TRIGGER_UPDATE << 1) | TRIGGER_AFTER) - 1 =
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= ((1 << 5) | (3 << 3) | (2 << 1) | 1) - 1 =
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= 0x00111100 (60)
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*/
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Note(s):
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1. One-action triggers are fully compatible at ODS level with FB 1.0.
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2. RDB$TRIGGER_TYPE encoding is order-dependant, i.e.
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BEFORE INSERT OR UPDATE and BEFORE UPDATE OR INSERT will be coded differently,
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although they have the same semantics and will be executed exactly the same way.
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3. In multiple-action triggers both OLD and NEW contexts are available. If the
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trigger invocation forbids one of them (e.g. OLD context for INSERT operation),
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then all fields of that context will evaluate to NULL. If you assign to
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improper context, runtime exception will be thrown.
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4. You may use new context variables INSERTING/UPDATING/DELETING to check the
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operation type at runtime.
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See also:
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README.context_variables
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