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Joins
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An inner equi-join query is a query where rows from each table are matched to specific rows
in other tables using equal predicates.
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Outer joins include LEFT OUTER JOIN or FULL OUTER JOIN.
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In a cross join, not all predicates are given so each row of a table is matched with all the rows
of the other table.
Restrictions for CREATE MATERIALIZED VIEW
The following restrictions specify what materialized view types can be defined as incremental:
MJV (Materialized Join View)
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Only inner equi-joins are supported; outer joins and cross joins are not supported.
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Must be a single block SELECT-FROM-WHERE query (for example, UNION, ORDER BY
are not allowed).
MAV (Materialized Aggregate View) on a Single Table
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The MAV can also be defined on a previously defined MV.
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The HAVING clause is not allowed.
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The DISTINCT function is not supported.
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Aggregates must be top-most functions and they cannot contain nested aggregates (for
example, no AVG(X)+1).
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Under certain conditions when the MAX or MIN function is used in a MAV, the incremental
refresh of the MV returns an error and the MV needs to be recomputed. This is because if a
row that contains the MAX or MIN value of its group is deleted from the base table, a new
value cannot be incrementally computed. The failure of an incremental refresh operation of
a MAV, because of the deletion of the MIN or MAX value, can be avoided:
— if an even greater value was inserted, it is the new MAX or MIN value
— if the base table is in insert-only table, mark it as INSERTLOG
— add an index on the base table on the columns that are used as the MVs group by
columns. When such a supporting index exists, the new MAX or MIN value can be
computed without a full scan on the base table, keeping the REFRESH incremental.
If none of these options is used, the MV has to be computed.
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The columns in the GROUP BY clause cannot exceed the maximum key length limit.
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Must be a single block SELECT-FROM-WHERE-GROUPBY query (for example, ORDER
BY is not allowed). There is one exception to this rule; nested blocks are allowed when the
inner block's purpose is to extract a group by column (using a built-in function like substring
or extract).
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Non-repeatable expressions (for example, current_time) are not supported for all types
of aggregate materialized views.
DML Operations on Materialized Views
All update operations (INSERT, UPDATE, DELETE) are blocked on all materialized views except
for the DELETE operation on ON REQUEST materialized views.
Example of CREATE MATERIALIZED VIEW
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This example creates a materialized view:
CREATE MATERIALIZED VIEW PUBSCH.DETAIL_MV REFRESH ON REQUEST INITIALIZE ON REFRESH
AS SELECT PUBSCH.DETAIL_TABLE.ORDERNUM, SUM(PUBSCH.DETAIL_TABLE.QUANTITY) AS
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SQL Statements
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