When creating a materialized view, the definition for the materialized view must define column names explicitly; you cannot
SELECT * construct as part of the column definition.
Do not include columns defined as TIMESTAMP WITH TIME ZONE in the materialized view. The value of the time_zone_adjustment option varies between connections based on their location and the time of year, resulting in incorrect results and unexpected behavior.
When creating a materialized view, the definition for the materialized view cannot contain:
The following database options must have the specified settings when a materialized view is created; otherwise, an error is returned. These database option values are also required for the view to be used by the optimizer:
The following database option settings are stored for each materialized view when it is created. The current option values for the connection must match the stored values for a materialized view for the view to be used in optimization:
When a view is refreshed, the connection settings for all the options listed in the bullets above are ignored. Instead, the database option settings (which must match the stored settings for the view) are used.
Materialized views are similar to base tables in that the rows are not stored in any particular order; the database server orders the rows in the most efficient manner when computing the data. Therefore, specifying an ORDER BY clause in a materialized view definition has no impact on the ordering of rows when the view is materialized. Also, the ORDER BY clause in the view's definition is ignored by the optimizer when performing view matching.
The following restrictions are checked when changing a manual view to an immediate view. An error is returned if the view violates any of the restrictions:
You can use the sa_materialized_view_can_be_immediate system procedure to find out if a manual view is eligible to become an immediate view.
The view must be uninitialized.
If the view does not contain outer joins, then the view must have a unique index on non nullable columns. If the view contains outer joins, the view must have a unique index on non nullable columns, or a unique index declared as WITH NULLS NOT DISTINCT on nullable columns.
If the view definition is a grouped query, the unique index columns must correspond to SELECT LIST items that are not aggregate functions.
The view definition cannot contain:
The view definition must be a single select-project-join or grouped-select-project-join query block, and the grouped-select-project-join query block cannot contain a HAVING clause.
The grouped-select-project-join query block must contain COUNT ( * ) in the SELECT list, and is allowed only the SUM and COUNT aggregate functions.
An aggregate function in the SELECT list cannot be referenced in a complex expression. For example,
SUM( expression ) + 1 is not allowed in the SELECT list.
If the SELECT list contains the
SUM(expression) aggregate function and expression is a nullable expression, then the SELECT list must include a
COUNT(expression) aggregate function.
If the view definition contains outer joins (LEFT OUTER JOIN, RIGHT OUTER JOIN, FULL OUTER JOIN) then the view definition must satisfy the following extra conditions:
If a table, T, is referenced in an ON condition of an OUTER JOIN as a preserved side, then T must have a primary key and the
primary key columns must be present in the SELECT list of the view. For example, the immediate materialized view V defined
SELECT T1.pk, R1.X FROM T1, T2 LEFT OUTER JOIN (R1 KEY JOIN R2) ON T1.Y = R.Y has the preserved table, T1, referenced in the ON clause and its primary key column, T1.pk, is in the SELECT list of the
immediate materialized view, V.
For each NULL-supplying side of an outer join, there must be at least one base table such that one of its non-nullable columns
is present in the SELECT list of the immediate materialized view. For example, for the immediate materialized view, V, defined
SELECT T1.pk, R1.X FROM T1, T2 LEFT OUTER JOIN (R1 KEY JOIN R2) ON T1.Y = R1.Y, the NULL-supplying side of the left outer join is the table expression
(R1 KEY JOIN R2). The column R1.X is in the SELECT list of the V and R1.X is a non nullable column of the table R1.
If the view is a grouped view and the previous condition does not hold, then for each NULL-supplying side of an outer join,
there must be at least one base table, T, such that one of its non-nullable columns, T.C, is used in the aggregate function
COUNT( T.C ) in the SELECT list of the immediate materialized view. For example, for the immediate materialized view, V, defined as
SELECT T1.pk, COUNT( R1.X ) FROM T1, T2 LEFT OUTER JOIN ( R1 KEY JOIN R2 ) ON T1.Y = R1.Y GROUP BY T1.pk, the NULL-supplying side of the left outer join is the table expression
( R1 KEY JOIN R2 ). The aggregate function
COUNT( R1.X ) is in the SELECT list of the V and R1.X is a non-nullable column of the table R1.
The following conditions must be satisfied by the predicates of the views with outer joins:
The ON clause predicates for LEFT, RIGHT, and FULL OUTER JOINs must refer to both preserved and NULL-supplying table expression.
T LEFT OUTER JOIN R ON R.X = 1 does not satisfy this condition as the predicate R.X=1 references only the NULL-supplying side R.
Any predicate must reject NULL-supplied rows produced by a nested outer join. In other words, if a predicate refers to a table expression which is NULL-supplied by a nested outer join, then it must reject all rows which have nulls generated by that outer join.
For example, the view V1
SELECT T1.pk, R1.X FROM T1, T2 LEFT OUTER JOIN (R1 KEY JOIN R2) ON (T1.Y = R1.Y) WHERE R1.Z = 10 has the predicate
R1.Z=10 referencing the table R1 which can be NULL-supplied by the
T2 LEFT OUTER JOIN (R1 KEY JOIN R2), hence it must reject any NULL-supplied rows. This is true because the predicate evaluates to UNKNOWN when the column R1.Z
However, the view V2
SELECT T1.pk, R1.X FROM T1, T2 LEFT OUTER JOIN (R1 KEY JOIN R2) ON (T1.Y = R1.Y) WHERE R1.Z IS NULL does not have this property. The predicate
R1.Z IS NULL references the NULL-supplying side R1 but it evaluates to TRUE when the table R1 is NULL-supplied (that is, the R1.Z column
is null). The method of rejecting NULL-supplied rows is not as restrictive as a NULL-intolerant property. For example, the
R.X IS NOT DISTINCT FROM T.X and rowid(T) IS NOT NULL is not NULL-intolerant on the table T as it evaluates to TRUE when T.X is NULL. However, the predicate rejects all the rows
which are NULL-supplied on the base table T.
Discuter à propos de cette page dans DocCommentXchange.
|Copyright © 2012, iAnywhere Solutions, Inc. - SQL Anywhere 12.0.1|