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43 lines
1.6 KiB
Plaintext
43 lines
1.6 KiB
Plaintext
---------------
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Partial indices
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---------------
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Function:
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Allow to index only a subset of table rows defined by the search condition specified during index creation.
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Author:
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Dmitry Yemanov <dimitr@firebirdsql.org>
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Syntax rules:
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CREATE [UNIQUE] [{ASC[ENDING] | DESC[ENDING]}] INDEX <index_name> ON <table_name>
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{ (<column_list>) | COMPUTED [BY] ( <value_expression> ) }
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WHERE <search_condition>
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Scope:
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DSQL (DDL)
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Example(s):
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1. CREATE INDEX IT1_COL ON T1 (COL) WHERE COL < 100;
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SELECT * FROM T1 WHERE COL < 100;
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-- PLAN (T1 INDEX (IT1_COL))
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2. CREATE INDEX IT1_COL2 ON T1 (COL) WHERE COL IS NOT NULL;
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SELECT * FROM T1 WHERE COL > 100;
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-- PLAN (T1 INDEX IT1_COL2)
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3. CREATE INDEX IT1_COL3 ON T1 (COL) WHERE COL = 1 OR COL = 2;
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SELECT * FROM T1 WHERE COL = 2;
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-- PLAN (T1 INDEX IT1_COL3)
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Note(s):
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1. A partial index definition may include the UNIQUE specification. In this case,
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every key in the index is required to be unique. This allows to enforce uniqueness
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across some subset of table rows.
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2. Partial index is usable only in the following cases:
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a) WHERE condition includes exactly the same boolean expression as the one
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defined for the index;
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b) Search condition defined for the index contains ORed boolean expressions
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and one of them is explicitly included in the WHERE condition;
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c) Search condition defined for the index specifies IS NOT NULL and the
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WHERE condition includes an expression on the same field that is known to
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ignore NULLs.
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