MasterZiv безусловно прав, разбиение большой таблицы на несколько меньших в IQ особого смысла не имеет.

Для начала неплохо бы плучить план запроса для обоих случаев:


и


Еще можно почитать:

Performance and Tuning Guide
Optimizing queries that reference UNION ALL views
All partitions in a UNION ALL view must have a complete set of indexes
defined for optimization to work.
Queries with DISTINCT will tend to run more slowly using a UNION ALL
view than a base table.
• Split group by over union all view
• Pushdown join into union all view
Should you need to adjust performance for queries that reference UNION ALL
views, you may want to set the Join_Preference database option, which affects
joins between UNION ALL views. For details of these options, see Chapter 5,
“Database Options” in the Sybase IQ Reference Manual.


Reference Manual
JOIN_PREFERENCE option
Function Controls the choice of algorithms when processing joins.
Allowed values -7 to 7
Alphabetical list of options
80 Sybase IQ
Scope DBA permissions are not required to set this option. Can be set temporary, for
an individual connection, or for the PUBLIC group. Takes effect immediately.
Default 0
Description For joins within a query, the IQ optimizer has a choice of several algorithms
for processing the join. This option allows you to override the optimizer's costbased
decision when choosing the algorithm to use. It does not override
internal rules that determine whether an algorithm is legal within the query
engine. If you set it to any non-zero value, it affects every join in a query; it
cannot be used to selectively modify one join out of several in a query.
This option is normally used for internal testing, and only experienced DBAs
should use it. The following table describes the valid values for this option and
their action.
Table 2-13: JOIN_PREFERENCE values
JOIN_SIMPLIFICATION_THRESHOLD option
Function Controls the minimum number of tables being joined together before any join
optimizer simplifications are applied.
Allowed values Integer from 1 to 64
Value Action
0 Let the optimizer choose
1 Prefer sort/merge
2 Prefer nested loop
3 Prefer nested loop push-down
4 Prefer hash
5 Prefer hash push-down
6 Prefer prejoin
7 Prefer sort/merge push-down
-1 Avoid sort/merge
-2 Avoid nested loop
-3 Avoid nested loop push-down
-4 Avoid hash
-5 Avoid hash push-down
-6 Avoid prejoin
-7 Avoid sort/merge push-down