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Expression indices
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Function:
Allow to index arbitrary expressions applied to the row values, hence allowing
indexed access paths to be used for expression-based predicates.

Author:
Oleg Loa <loa@mail.ru>
Dmitry Yemanov <yemanov@yandex.ru>

Syntax rules:
CREATE [UNIQUE] [{ASC[ENDING] | DESC[ENDING]}] INDEX <index_name> ON <table_name>
COMPUTED [BY] ( <value_expression> )

Scope:
DSQL (DDL)

Example(s):
1. CREATE INDEX IDX1 ON T1 COMPUTED BY ( UPPER(COL1 COLLATE PXW_CYRL) );
SELECT * FROM T1 WHERE UPPER(COL1 COLLATE PXW_CYRL) = 'ФЫВА'
-- PLAN (T1 INDEX (IDX1))
2. CREATE INDEX IDX2 ON T2 COMPUTED BY ( EXTRACT(YEAR FROM COL2) || EXTRACT(MONTH FROM COL2) );
SELECT * FROM T2 ORDER BY EXTRACT(YEAR FROM COL2) || EXTRACT(MONTH FROM COL2)
-- PLAN (T2 ORDER IDX2)

Note(s):
1. An expression used in the index declaration must match a predicate expression precisely
to allow the engine to choose an indexed access path. Otherwise, the given index won't be used
for a retrieval/sorting.
2. Expression indices have exactly the same features and limitations as usual indices, with the
only exception that, by definition, they cannot be composite (multi-segment).