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Looking at Oracle's docs on the Function Index, what's the meaning of rivers in the following?

CREATE INDEX area_index ON rivers (area(geo));

SELECT id, geo, area(geo), desc
     FROM rivers     
     WHERE Area(geo) >5000;

What's the importance of rivers with respect to this query?

Could dual have just been used?

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  • rivers is the table name.
    – user1822
    Commented Nov 23, 2016 at 22:22

1 Answer 1

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It is the name of the table that the index is created on.

This:

CREATE INDEX area_index ON rivers (area(geo));

.. creates a functional index on the rivers table, that uses the function area() on the column geo.

In simple terms, Oracle creates an index that pre-calculates the value of area(geo), thus making any lookups faster.

The query you have posted:

SELECT id, geo, area(geo), desc
FROM rivers     
WHERE Area(geo) >5000;

... can use the area_index functional index to select the pre-calculated value(s) of area(geo) without having to call the area() function for each row in the table.

Essentially, the query can look like this in pseudo-code:

SELECT id, geo, area_index_value, desc
FROM rivers     
WHERE area_index_value >5000;

The Oracle documentation explains this well.

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  • What if the area Oracle Function took a second argument, y, that was not in the rivers table? Could a function index not, then, be created? Or, what if y came from another table, e.g. mountains? Commented Nov 23, 2016 at 22:29
  • If Y was a constant, then it could be specified in the index definition. It cannot be read from a different table. Commented Nov 23, 2016 at 23:47

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