User-supplied estimates

The Sybase IQ query optimizer uses educated guesses to help decide the best strategy for executing a query. For each table in a potential execution plan, the optimizer must estimate the number of rows that will be part of the results. If you know that a condition has a success rate that differs from the optimizer rule, you can tell the database this information by using an estimate in the search condition.

The estimate is a percentage, which can be a positive integer or can contain fractional values.

Examples

The following query provides an estimate that one percent of the ship_date values will be later than 1994/06/30:
```
SELECT  ship_date
FROM  sales_order_items
WHERE ( ship_date > '1994/06/30', 1 )
ORDER BY ship_date DESC
```
The following query estimates that half a percent of the rows will satisfy the condition:
```
SELECT *
FROM customer c, sales_order o
WHERE (c.id = o.cust_id, 0.5)
```

Fractional values enable more accurate user estimates for joins, particularly for large tables.

Compatibility

Adaptive Server Enterprise does not support explicit estimates.