5 Cache Group Operations

Loading and refreshing an explicitly loaded cache group with autorefresh

If the autorefresh state of an explicitly loaded cache group is PAUSED, the autorefresh state is changed to ON after a LOAD CACHE GROUP or REFRESH CACHE GROUP statement issued on the cache group completes.

The following restrictions apply when manually loading or refreshing an explicitly loaded cache group with autorefresh:

• A LOAD CACHE GROUP statement can only be issued if the cache tables are empty.

• The autorefresh state must be PAUSED before you can issue a LOAD CACHE GROUP statement.

• The autorefresh state must be PAUSED before you can issue a REFRESH CACHE GROUP statement.

• A LOAD CACHE GROUP statement cannot contain a WHERE clause.

• A LOAD CACHE GROUP or REFRESH CACHE GROUP statement cannot contain a WITH ID clause.

• A REFRESH CACHE GROUP statement cannot contain a WHERE clause.

• All tables and columns referenced in a WHERE clause when loading the cache group must be fully qualified. For example:

  user_name.table_name and user_name.table_name.column_name

When an autorefresh operation occurs on an explicitly loaded cache group, all committed inserts, updates and deletes on the cached Oracle tables since the last autorefresh cycle are refreshed into the cache tables. New cache instances may be loaded into the cache tables. Cache instances that already exist in the cache tables are updated or deleted if the corresponding rows in the cached Oracle tables have been updated or deleted.

Loading and refreshing a dynamic cache group with autorefresh

If the autorefresh state of a dynamic cache group is PAUSED, the autorefresh state is changed to ON after any of the following events occur:

• Its cache tables are initially empty, and then a dynamic load, a LOAD CACHE GROUP or an unconditional REFRESH CACHE GROUP statement issued on the cache group completes

• Its cache tables are not empty, and then an unconditional REFRESH CACHE GROUP statement issued on the cache group completes

If the autorefresh state of a dynamic cache group is PAUSED, the autorefresh state remains at PAUSED after any of the following events occur:

• Its cache tables are initially empty, and then a REFRESH CACHE GROUP ... WITH ID statement issued on the cache group completes

• Its cache tables are not empty, and then a dynamic load, a REFRESH CACHE GROUP ... WITH ID, or a LOAD CACHE GROUP statement issued on the cache group completes

For a dynamic cache group, an autorefresh operation only refreshes committed updates and deletes on the cached Oracle tables since the last autorefresh cycle into the cache tables because only existing cache instances in the cache tables are refreshed. New cache instances are not loaded into the cache tables. To load new cache instances into the cache tables of a dynamic cache group, use a LOAD CACHE GROUP statement or perform a dynamic load operation. See "Dynamically loading a cache instance" for more information about a dynamic load operation.

The following restrictions apply when manually loading or refreshing a dynamic cache group with automatic refresh:

• The autorefresh state must be PAUSED or ON before you can issue a LOAD CACHE GROUP statement.

• The autorefresh state must be PAUSED before you can issue a REFRESH CACHE GROUP statement.

• A LOAD CACHE GROUP statement that contains a WHERE clause must include a COMMIT EVERY n ROWS clause after the WHERE clause

• A REFRESH CACHE GROUP statement cannot contain a WHERE clause.

• All tables and columns referenced in a WHERE clause when loading the cache group must be fully qualified. For example:

  user_name.table_name and user_name.table_name.column_name

Loading and refreshing a cache group using a WITH ID clause

The WITH ID clause of the LOAD CACHE GROUP or REFRESH CACHE GROUP statement enables you to load or refresh a cache group based on values of the primary key columns without having to use a WHERE clause. The WITH ID clause is more convenient than the equivalent WHERE clause if the primary key contains more than one column. Using the WITH ID clause allows you to load one cache instance at a time. It also enables you to roll back the transaction containing the load or refresh operation, if necessary, unlike the equivalent statement that uses a WHERE clause because using a WHERE clause also requires specifying a COMMIT EVERY n ROWS clause.

LOAD CACHE GROUP recent_orders WITH ID (1756,573);

LOAD CACHE GROUP recent_orders WHERE orderid = 1756 and itemid = 573
  COMMIT EVERY 256 ROWS;

A LOAD CACHE GROUP or REFRESH CACHE GROUP statement issued on an autorefresh cache group cannot contain a WITH ID clause unless the cache group is dynamic.

You cannot use the COMMIT EVERY n ROWS clause with the WITH ID clause.

Initiating an immediate autorefresh

If the Oracle tables have been updated with data that needs to be applied to cache tables without waiting for the next autorefresh operation, you can call the ttCacheAutorefresh built-in procedure. The ttCacheAutorefresh built-in procedure initiates an immediate refresh operation and resets the autorefresh cycle to start at the moment you invoke ttCacheAutorefresh. The refresh operation is full or incremental depending on how the cache group is configured. The autorefresh state must be ON when ttCacheAutorefresh is called.

The autorefresh operation normally refreshes all cache groups sharing the same refresh interval in one transaction in order to preserve transactional consistency across these cache groups. Therefore, although you specify a specific cache group when you call ttCacheAutorefresh, the autorefresh operation occurs in one transaction for all cache groups that share the autorefresh interval with the specified cache group. If there is an existing transaction with table locks on objects that belong to the affected cache groups, ttCacheAutofresh returns an error without taking any action.

You can choose to run ttCacheAutorefresh asynchronously (the default) or synchronously. In synchronous mode, ttCacheAutorefresh returns an error if the refresh operation fails.

After calling ttCacheAutorefresh, you must commit or roll back the transaction before subsequent work can be performed.

Command> call ttCacheAutorefresh('ttuser', 'western_customers');

Loading and refreshing a multiple-table cache group

If you are loading or refreshing a multiple-table cache group while the cached Oracle tables are concurrently being updated, set the isolation level in the TimesTen database to serializable before issuing the LOAD CACHE GROUP or REFRESH CACHE GROUP statement. This causes TimesTen to query the cached Oracle tables in a serializable fashion during the load or refresh operation so that the loaded or refreshed cache instances in the cache tables are guaranteed to be transactionally consistent with the corresponding rows in the cached Oracle tables. After you have loaded or refreshed the cache group, set the isolation level back to read committed for better concurrency when accessing elements in the TimesTen database.

Improving the performance of loading or refreshing a large number of cache instances

You can improve the performance of loading or refreshing a large number of cache instances into a cache group by using the PARALLEL clause of the LOAD CACHE GROUP or REFRESH CACHE GROUP statement. Specify the number of threads to use when processing the load or refresh operation. You can specify 1 to 10 threads. One thread fetches rows from the cached Oracle tables, while the other threads insert the rows into the TimesTen cache tables. Do not specify more threads than the number of CPUs available on your system or you may encounter decreased performance than if you had not used the PARALLEL clause.

REFRESH CACHE GROUP western_customers COMMIT EVERY 256 ROWS PARALLEL 4;

Example of manually loading and refreshing an explicitly loaded cache group

The following is the definition of the Oracle table that will be cached in an explicitly loaded AWT cache group. The Oracle table is owned by the schema user oratt.

CREATE TABLE customer
(cust_num NUMBER(6) NOT NULL PRIMARY KEY,
 region   VARCHAR2(10),
 name     VARCHAR2(50),
 address  VARCHAR2(100));

The following is the data in the oratt.customer cached Oracle table.

CUST_NUM   REGION    NAME              ADDRESS
--------   -------   ---------------   ---------------------------    
       1   West      Frank Edwards     100 Pine St. Portland OR
       2   East      Angela Wilkins    356 Olive St. Boston MA
       3   Midwest   Stephen Johnson   7638 Walker Dr. Chicago IL

The following statement creates an explicitly loaded AWT cache group new_customers that caches the oratt.customer table:

CREATE ASYNCHRONOUS WRITETHROUGH CACHE GROUP new_customers
FROM oratt.customer
 (cust_num NUMBER(6) NOT NULL,
  region   VARCHAR2(10),
  name     VARCHAR2(50),
  address  VARCHAR2(100),
  PRIMARY KEY(cust_num));

The oratt.customer TimesTen cache table is initially empty.

Command> SELECT * FROM oratt.customer;
0 rows found.

The following LOAD CACHE GROUP statement loads the three cache instances from the cached Oracle table into the TimesTen cache table:

Command> LOAD CACHE GROUP new_customers COMMIT EVERY 256 ROWS;
3 cache instances affected.
Command> SELECT * FROM oratt.customer;
< 1, West, Frank Edwards, 100 Pine St. Portland OR >
< 2, East, Angela Wilkins, 356 Olive St. Boston MA >
< 3, Midwest, Stephen Johnson, 7638 Walker Dr. Chicago IL >

Update the cached Oracle table by inserting a new row, updating an existing row, and deleting an existing row:

SQL> INSERT INTO customer
  2  VALUES (4, 'East', 'Roberta Simon', '3667 Park Ave. New York NY');
SQL> UPDATE customer SET name = 'Angela Peterson' WHERE cust_num = 2;
SQL> DELETE FROM customer WHERE cust_num = 3;
SQL> COMMIT;
SQL> SELECT * FROM customer;
CUST_NUM   REGION    NAME              ADDRESS
--------   -------   ---------------   ---------------------------    
       1   West      Frank Edwards     100 Pine St. Portland OR
       2   East      Angela Peterson   356 Olive St. Boston MA
       4   East      Roberta Simon     3667 Park Ave. New York NY

A REFRESH CACHE GROUP statement issued on an explicitly loaded cache group is processed by unloading and then reloading the cache group. As a result, the cache instances in the cache table matches the rows in the cached Oracle table.

Command> REFRESH CACHE GROUP new_customers COMMIT EVERY 256 ROWS;
3 cache instance affected.
Command> SELECT * FROM oratt.customer;
< 1, West, Frank Edwards, 100 Pine St. Portland OR >
< 2, East, Angela Peterson, 356 Olive St. Boston MA >
< 4, East, Roberta Simon, 3667 Park Ave. New York NY >

Example of manually loading and refreshing a dynamic cache group

The following is the definition of the Oracle table that will be cached in a dynamic AWT cache group. The Oracle table is owned by the schema user oratt.

CREATE TABLE customer
(cust_num NUMBER(6) NOT NULL PRIMARY KEY,
 region   VARCHAR2(10),
 name     VARCHAR2(50),
 address  VARCHAR2(100));

The following is the data in the oratt.customer cached Oracle table.

CUST_NUM   REGION    NAME              ADDRESS
--------   -------   ---------------   ---------------------------    
       1   West      Frank Edwards     100 Pine St. Portland OR
       2   East      Angela Wilkins    356 Olive St. Boston MA
       3   Midwest   Stephen Johnson   7638 Walker Dr. Chicago IL

The following statement creates a dynamic AWT cache group new_customers that caches the oratt.customer table:

CREATE DYNAMIC ASYNCHRONOUS WRITETHROUGH CACHE GROUP new_customers
FROM oratt.customer
 (cust_num NUMBER(6) NOT NULL,
  region   VARCHAR2(10),
  name     VARCHAR2(50),
  address  VARCHAR2(100),
  PRIMARY KEY(cust_num));

The oratt.customer TimesTen cache table is initially empty:

Command> SELECT * FROM oratt.customer;
0 rows found.

The following LOAD CACHE GROUP statement loads the three cache instances from the cached Oracle table into the TimesTen cache table:

Command> LOAD CACHE GROUP new_customers COMMIT EVERY 256 ROWS;
3 cache instances affected.
Command> SELECT * FROM oratt.customer;
< 1, West, Frank Edwards, 100 Pine St. Portland OR >
< 2, East, Angela Wilkins, 356 Olive St. Boston MA >
< 3, Midwest, Stephen Johnson, 7638 Walker Dr. Chicago IL >

Update the cached Oracle table by inserting a new row, updating an existing row, and deleting an existing row:

SQL> INSERT INTO customer
  2  VALUES (4, 'East', 'Roberta Simon', '3667 Park Ave. New York NY');
SQL> UPDATE customer SET name = 'Angela Peterson' WHERE cust_num = 2;
SQL> DELETE FROM customer WHERE cust_num = 3;
SQL> COMMIT;
SQL> SELECT * FROM customer;
CUST_NUM   REGION    NAME              ADDRESS
--------   -------   ---------------   ---------------------------    
       1   West      Frank Edwards     100 Pine St. Portland OR
       2   East      Angela Peterson   356 Olive St. Boston MA
       4   East      Roberta Simon     3667 Park Ave. New York NY

A REFRESH CACHE GROUP statement issued on a dynamic cache group only refreshes committed updates and deletes on the cached Oracle tables into the cache tables. New cache instances are not loaded into the cache tables. Therefore, only existing cache instances are refreshed. As a result, the number of cache instances in the cache tables are either fewer than or the same as the number of rows in the cached Oracle tables.

Command> REFRESH CACHE GROUP new_customers COMMIT EVERY 256 ROWS;
2 cache instances affected.
Command> SELECT * FROM oratt.customer;
< 1, West, Frank Edwards, 100 Pine St. Portland OR >
< 2, East, Angela Peterson, 356 Olive St. Boston MA >

A subsequent LOAD CACHE GROUP statement loads one cache instance from the cached Oracle table into the TimesTen cache table because only committed inserts are loaded into the cache table. Therefore, only new cache instances are loaded. Cache instances that already exist in the cache tables are not changed as a result of a LOAD CACHE GROUP statement, even if the corresponding rows in the cached Oracle tables were updated or deleted.

Command> LOAD CACHE GROUP new_customers COMMIT EVERY 256 ROWS;
1 cache instance affected.
Command> SELECT * FROM oratt.customer;
< 1, West, Frank Edwards, 100 Pine St. Portland OR >
< 2, East, Angela Peterson, 356 Olive St. Boston MA >
< 4, East, Roberta Simon, 3667 Park Ave. New York NY >

Dynamic load configuration

Dynamic load can be configured as follows:

• 0 - Disables dynamic load of Oracle data to TimesTen dynamic cache groups for the current connection.

• 1 (default) - Enables dynamic load of Oracle data to a single TimesTen dynamic cache group per statement for the current connection. The statement must reference tables of only one dynamic cache group, which must be referenced in the main query. The statement can also reference non-cache tables. Only one cache instance can be loaded.

• 2 - Enables dynamic load of Oracle data to one or multiple TimesTen dynamic cache groups per statement for the current connection. All cache groups referenced in the main query will be dynamically loaded; any cache groups referenced solely in a subquery will be ignored for dynamic load. The statement can also reference non-cache tables. Only one cache instance can be loaded.

For any dynamic cache group to be considered for a dynamic load, there must be an equality condition with constants and/or parameters on all columns of a primary key or a foreign key of any table of the cache group. If more than one table of a cache group is referenced, each must be connected by an equality condition on the primary and foreign key relationship. For DynamicLoadEnable=1, all tables referenced in the relationship must exist only within a single cache group; for DynamicLoadEnable=2, the referenced tables can exist within multiple cache groups.

Set the appropriate value in the DynamicLoadEnable connection attribute to configure the type of dynamic loading for all cache tables in dynamic cache groups that are accessed within a particular connection.

To enable or disable dynamic loading for a particular transaction, you can set the DynamicLoadEnable optimizer hint. However, the DynamicLoadEnable connection attribute is the only method for configuring what type of dynamic load is enabled.

Set the DynamicLoadEnable optimizer hint with one of the following methods:

• Use the ttIsql utility set dynamicloadenable command.

• Call the ttOptSetFlag built-in procedure with the DynamicLoadEnable flag set to the desired value. The following example sets dynamic loading to 1.

  call ttOptSetFlag('DynamicLoadEnable', 1)
  

Dynamic load guidelines

Dynamic load retrieves at most one cache instance for each cache group referenced in the main query. This section details the guidelines under which dynamic load occurs.

Dynamic load is available only for the following types of statements issued on a cache table in a dynamic cache group:

• When an INSERT statement inserts values into any of the child tables of a cache instance that does not currently exist in the TimesTen tables, the cache instance that the new row belongs to will be dynamic loaded. The insert operation for the new child row is propagated to the cached Oracle table.

• SELECT, UPDATE, or DELETE requires an equality condition with constants and/or parameters on all columns of a primary key or a foreign key of any table of the cache group. If more than one table of a cache group is referenced, each must be connected by an equality condition on the primary and foreign key relationship.

The SELECT, UPDATE, or DELETE statements for which dynamic load is available must satisfy the following conditions:

• If the statement contains a subquery, only the cache group with tables referenced in the main query are considered for a dynamic load.

• If the statement references multiple tables of the cache group, the statement must include an equality join condition between the primary keys and foreign keys for all parent and child relationships.

• The statement cannot contain the UNION, INTERSECT, or MINUS set operators.

• The statement can reference non-cache tables.

• By default, the statement can reference cache tables from only one dynamic cache group. This behavior is enabled when DynamicLoadEnable is set to 1. However, if DynamicLoadEnable is set to 2, the statement can reference cache tables from multiple dynamic cache groups. See "Dynamic load configuration" for more information.

Dynamic load behavior depends on the setting of DynamicLoadEnable. The following describes the rules that are evaluated to determine if a dynamic load occurs. These rules are followed when DynamicLoadEnable is set to either 1 or 2.

• Dynamic load will not occur for a cache group if any table of the cache group is specified more than once in any FROM clause.

• Only the conditions explicitly specified in the query are considered for dynamic load, which excludes any derived conditions.

• If any cache group is referenced only in a subquery, it will not be considered for a dynamic load.

• If the cache group has a time-based aging policy defined, the timestamp in the root table's row must be within the aging policy's lifetime in order for the cache instance to be loaded. See "Implementing aging in a cache group" for information about defining an aging policy on a cache group.

• When using an active standby pair replication scheme, dynamic load cannot occur in any subscriber.

When dynamic load is enabled with setting DynamicLoadEnable to 2, you can include multiple dynamic cache groups in the statement. The rules for this situation that determines if a dynamic load occurs are as follows:

• If multiple cache groups exist within in a query, dynamic load is considered for the cache groups that meet the required conditions for a dynamic load. Dynamic load is not considered for any cache groups that do not meet dynamic load conditions.

• If tables of any dynamic cache group are referenced in the main query, they are considered for dynamic load, even if other tables in any cache group are referenced in the subquery.

The following considerations can affect dynamic load:

• If tables within multiple cache groups or non-cache group tables are specified in the main query, the join order influences if the cache instance is loaded. If during the execution of the query, a dynamic load is possible and necessary to produce the query results, the dynamic load occurs. However, if no rows will be returned, then some or all of the cache instances will not be dynamically loaded.

• If a statement specifies more than the dynamic load condition on tables of a cache group, the cache instance may be dynamically loaded even though the additional conditions are not qualified for the statement.

Examples of dynamically loading a cache instance

The following is the definition of the Oracle tables that will be cached in a dynamic AWT cache group. The Oracle table is owned by the schema user oratt.

CREATE TABLE customer
(cust_num NUMBER(6) NOT NULL PRIMARY KEY,
 region   VARCHAR2(10),
 name     VARCHAR2(50),
 address  VARCHAR2(100))

CREATE TABLE orders
(ord_num      NUMBER(10) NOT NULL PRIMARY KEY,
 cust_num     NUMBER(6) NOT NULL,
 when_placed  DATE NOT NULL,
 when_shipped DATE NOT NULL)

CREATE TABLE orderdetails
 (orderid  NUMBER(10) NOT NULL,
  itemid   NUMBER(8) NOT NULL,
  quantity NUMBER(4) NOT NULL,
  PRIMARY KEY (orderid, itemid))

For example, the following data is in the oratt.customer cached Oracle table.

CUST_NUM   REGION    NAME              ADDRESS
--------   -------   ---------------   ---------------------------    
       1   West      Frank Edwards     100 Pine St., Portland OR
       2   East      Angela Wilkins    356 Olive St., Boston MA
       3   Midwest   Stephen Johnson   7638 Walker Dr., Chicago IL

The following statement creates a dynamic AWT cache group new_customers that caches the oratt.customer, oratt.orders, and oratt.orderdetails tables:

CREATE DYNAMIC ASYNCHRONOUS WRITETHROUGH CACHE GROUP new_customers
FROM oratt.customer
 (cust_num NUMBER(6) NOT NULL,
  region   VARCHAR2(10),
  name     VARCHAR2(50),
  address  VARCHAR2(100),
  PRIMARY KEY(cust_num)),
oratt.orders
 (ord_num      NUMBER(10) NOT NULL,
  cust_num     NUMBER(6) NOT NULL,
  when_placed  DATE NOT NULL,
  when_shipped DATE NOT NULL,
  PRIMARY KEY(ord_num),
  FOREIGN KEY(cust_num) REFERENCES oratt.customer(cust_num)),
oratt.orderdetails
 (orderid  NUMBER(10) NOT NULL,
  itemid   NUMBER(8) NOT NULL,
  quantity NUMBER(4) NOT NULL,
  PRIMARY KEY(orderid, itemid),
  FOREIGN KEY(orderid) REFERENCES oratt.orders(order_num))

The following examples can be used when DynamicLoadEnable is set to 1:

The oratt.customer TimesTen cache table is initially empty:

Command> SELECT * FROM oratt.customer;
0 rows found.

The following SELECT statement with an equality condition on the primary key for the oratt.customer table results in a dynamic load:

Command> SELECT * FROM oratt.customer WHERE cust_num = 1;
< 1, West, Frank Edwards, 100 Pine St., Portland OR >

However, if you do not use an equality condition on the primary key, no dynamic load occurs:

Command> SELECT * FROM oratt.customer WHERE cust_num IN (1,2);

The following example contains equality expressions on all of the primary key columns for a primary key composite. The orderdetails table has a composite primary key of orderid and itemid.

UPDATE oratt.orderdetails SET quantity = 5 WHERE orderid=2280 AND itemid=663

The following example shows an INSERT into the orders child table, which initiates a dynamic load. However, if you tried to insert into the customer table, which is the parent, no dynamic load occurs.

INSERT INTO orders VALUES(1,1, DATE '2012-01-25', DATE '2012-01-30')

The following UPDATE statement dynamically loads one cache instance from the cached Oracle table into the TimesTen cache table, updates the instance in the cache table, and then automatically propagates the update to the cached Oracle table:

Command> UPDATE oratt.customer SET name = 'Angela Peterson' WHERE cust_num = 2;
Command> SELECT * FROM oratt.customer;
< 1, West, Frank Edwards, 100 Pine St., Portland OR >
< 2, East, Angela Peterson, 356 Olive St., Boston MA >

The following is the updated data in the oratt.customer cached Oracle table:

CUST_NUM   REGION    NAME              ADDRESS
--------   -------   ---------------   ---------------------------    
       1   West      Frank Edwards     100 Pine St., Portland OR
       2   East      Angela Peterson   356 Olive St., Boston MA
       3   Midwest   Stephen Johnson   7638 Walker Dr., Chicago IL

The following DELETE statement dynamically loads one cache instance from the cached Oracle table into the TimesTen cache table, deletes the instance from the cache table, and then automatically propagates the delete to the cached Oracle table:

Command> DELETE FROM oratt.customer WHERE cust_num = 3;
Command> SELECT * FROM oratt.customer;
< 1, West, Frank Edwards, 100 Pine St., Portland OR >
< 2, East, Angela Peterson, 356 Olive St., Boston MA >

The following is the updated data in the oratt.customer cached Oracle table.

CUST_NUM   REGION    NAME              ADDRESS
--------   -------   ---------------   ---------------------------    
       1   West      Frank Edwards     100 Pine St., Portland OR
       2   East      Angela Peterson   356 Olive St., Boston MA

The following examples demonstrate how to use dynamic load when referencing tables across multiple cache groups, which is enabled when DynamicLoadEnable is set to 2.

The following statements create multiple dynamic AWT cache groups, each with one or more tables that are cached from the Oracle database.

CREATE DYNAMIC CACHE GROUP cachegrp 
 FROM table1(x1 INT PRIMARY KEY, y1 INT);

CREATE DYNAMIC CACHE GROUP cachegrp2 
 FROM table2(x2 INT PRIMARY KEY, y2 INT), 
 table3(x3 INT PRIMARY KEY, y3 INT, 
 FOREIGN KEY(y3) REFERENCES table2(x2) ;

CREATE DYNAMIC CACHE GROUP cachegrp3 
FROM table4(x4 INT PRIMARY KEY, y4 INT);

CREATE TABLE table5
 (x5 INT PRIMAY KEY,y5 INT);

The following example shows that no dynamic load will occur, even though the optimizer may derive that x1 should be equated to 1:

SELECT * FROM table1, table5 WHERE x5=1 AND x5=x1;

Dynamic load will be considered for the cache instance from table2 within cachegrp2 since table2 is referenced in the main query.

SELECT * FROM table5, table2 
 WHERE x5 IN (SELECT y2 FROM table2, table3 where x2=1 and x2=y3);

The cache instance in the cachegrp2 cache group is not considered for a dynamic load because all of its tables are referenced in the subquery.

SELECT * FROM table5 
 WHERE x5 IN 
 (SELECT y3 FROM table2, table3 WHERE x2=1 AND x2=y3);

In the following example, if the row that would be retrieved from table1 where x1=1 is not already in the cache, whether the cache instance from table1 is loaded depends on the join order. If the join order is 'table5 table1,' the cache instance from table1 will be loaded if and only if there is a row in table5 for which x5=1. If the join order is 'table1 table5', then the cache instance from table1 will always be loaded.

SELECT * FROM table1, table5 WHERE x1=1 AND x5=1;  

A row (1,1) may be loaded for table1, but the SELECT will not return any rows.

SELECT * FROM table1 WHERE x1=1 AND y1>1;

Return dynamic load errors

You can configure TimesTen to return an error if a SELECT, UPDATE or DELETE statement does not meet the requirements stated in "Dynamic load guidelines". The DynamicLoadErrorMode connection attribute controls what happens when an application executes a SQL operation against a dynamic cache group and the SQL operation cannot use dynamic load in a particular connection.

• When DynamicLoadErrorMode is set to a value of 0, dynamic load will happen to any cache group referenced in the query that is qualified for dynamic load. Cache groups that do not qualify will not be dynamically loaded and no errors will be returned. When DynamicLoadEnable=1, no dynamic load will occur if the query references more than one cache group.

• When DynamicLoadErrorMode is set to a value of 1, a query will fail with an error if any dynamic cache group referenced in the query is not qualified for dynamic load. The error indicates the reason why the dynamic load cannot occur.

To set the connection attribute solely for a particular transaction, use one of the following:

• Use the ttIsql utility set dynamicloaderrormode 1 command.

• Call the ttOptSetFlag built-in procedure with the DynamicLoadErrorMode flag and the optimizer value set to 1.

  call ttOptSetFlag('DynamicLoadErrorMode', 1)
  

  Call the ttOptSetFlag built-in procedure with the DynamicLoadErrorMode flag and the optimizer value set to 0 to suppress error reporting when a statement does not comply with dynamic load requirements.

Unloading a cache group across all grid members

You can unload a cache group in all members of a cache grid by setting an optimizer flag. Before executing the UNLOAD CACHE GROUP statement, call the ttOptSetFlag built-in procedure and set the GlobalProcessing optimizer flag to 1:

CALL ttOptSetFlag('GlobalProcessing', 1);

Consider this statement:

UNLOAD CACHE GROUP customer WHERE customer_id=54321;

A local unload operation removes the customer record only if the record exists on the node where the statement is executed. A global unload operation removes the customer record regardless of which node contains the record.

PassThrough=0

PassThrough=0 is the default setting and specifies that all statements are to be executed in the TimesTen database. Figure 5-1 shows that Table A is updated on the TimesTen database. Table F cannot be updated because it does not exist in TimesTen.

Figure 5-1 PassThrough=0

Description of "Figure 5-1 PassThrough=0"

PassThrough=1

Set PassThrough=1 to specify that a statement that references a table that does not exist in the TimesTen database is passed through to the Oracle database for execution. No DDL statements are passed through to the Oracle database.

If TimesTen cannot parse a SELECT statement because it includes keywords that do not exist in TimesTen SQL or because it includes syntax errors, it passes the statement to the Oracle database. If TimesTen cannot parse INSERT, UPDATE or DELETE statements, TimesTen returns an error and the statement is not passed through to the Oracle database.

Figure 5-2 shows that Table A is updated in the TimesTen database, while Table G is updated in the Oracle database because Table G does not exist in the TimesTen database.

Figure 5-2 PassThrough=1

Description of "Figure 5-2 PassThrough=1"

PassThrough=2

PassThrough=2 specifies that INSERT, UPDATE and DELETE statements are passed through to the Oracle database for read-only cache groups and user managed cache groups that use the READONLY cache table attribute. Otherwise, Passthrough=1 behavior applies.

Figure 5-3 shows that updates to Table A and Table G in a read-only cache group are passed through to the Oracle database.

Figure 5-3 PassThrough=2

Description of "Figure 5-3 PassThrough=2"

PassThrough=3

PassThrough=3 specifies that all statements are passed through to the Oracle database for execution, except that INSERT, UPDATE and DELETE statements issued on cache tables in a dynamic AWT global cache group result in a TimesTen error.

Figure 5-4 shows that Table A is updated on the Oracle database for a read-only or updatable cache group. A SELECT statement that references Table G is also passed through to the Oracle database. A SELECT statement that references Table C in a dynamic AWT global cache group is passed through to the Oracle database.

Figure 5-4 PassThrough=3

Description of "Figure 5-4 PassThrough=3"

PassThrough=4

PassThrough=4 specifies that SELECT statements issued on cache tables in a dynamic AWT global cache group that do not satisfy the criteria for a dynamic load query are passed through to the Oracle database for execution. Otherwise, statements are executed in the TimesTen database. See "Dynamic load guidelines" for the criteria for a dynamic load SELECT statement.

Figure 5-5 shows that Table A in an updatable cache group is updated in the TimesTen database. The figure also shows a SELECT statement issued on a dynamic AWT global cache group that does not satisfy the criteria for a dynamic load SELECT statement and is passed through to the Oracle database for execution.

Figure 5-5 PassThrough=4

Description of "Figure 5-5 PassThrough=4"

PassThrough=5

PassThrough=5 specifies that SELECT statements issued on cache tables in a dynamic AWT global cache group that do not satisfy the criteria for a dynamic load query are passed through to the Oracle database for execution when all committed updates on cache tables in dynamic AWT global cache groups by previous transactions within the connection have been propagated to the Oracle database. Otherwise statements are executed in the TimesTen database. See "Dynamic load guidelines" for the criteria for a dynamic load SELECT statement.

Figure 5-6 shows that Table A in an updatable cache group is updated in the TimesTen database. The figure also shows a SELECT statement issued on a dynamic AWT global cache group that does not satisfy the criteria for a dynamic load SELECT statement and is passed through to the Oracle database for execution after all committed updates on cache tables in dynamic AWT global cache groups by previous transactions within the connection have been propagated to the Oracle database.

Figure 5-6 PassThrough=5

Description of "Figure 5-6 PassThrough=5"

Considerations for using passthrough

Passing through update operations to the Oracle database for execution is not recommended when issued on cache tables in an AWT or SWT cache group. Committed updates on cache tables in an AWT cache group are automatically propagated to the cached Oracle tables in asynchronous fashion. However, passing through an update operation to the Oracle database for execution within the same transaction as the update on the cache table in the AWT cache group renders the propagate of the cache table update synchronous, which may have undesired results. Committed updates on cache tables in an SWT cache group can result in self-deadlocks if, within the same transaction, updates on the same tables are passed through to the Oracle database for execution.

A PL/SQL block cannot be passed through to the Oracle database for execution. Also, you cannot pass through to Oracle for execution a reference to a stored procedure or function that is defined in the Oracle database but not in the TimesTen database.

For more information about how the PassThrough connection attribute setting determines which statements are executed in the TimesTen database and which are passed through to the Oracle database for execution and under what circumstances, see "PassThrough" in Oracle TimesTen In-Memory Database Reference.

Changing the passthrough level for a connection or transaction

You can override the current passthrough level using the ttIsql utility's set passthrough command which applies to the current transaction.

You can also override the setting for a specific transaction by calling the ttOptSetFlag built-in procedure with the PassThrough flag. The following procedure call sets the passthrough level to 3:

CALL ttOptSetFlag('PassThrough', 3);

The PassThrough flag setting takes effect when a statement is prepared and it is the setting that is used when the statement is executed even if the setting has changed from the time the statement was prepared to when the statement is executed. After the transaction has been committed or rolled back, the original connection setting takes effect for all subsequently prepared statements.

Dynamic load performance

Dynamic loading based on a primary key search of the root table has faster performance than primary key searches on a child table or foreign key searches on a child table.

Improving AWT throughput

By default, an AWT cache group uses the PL/SQL execution method to apply changes within TimesTen to the Oracle database. AWT bundles all pending operations into a single PL/SQL collection that is sent to the Oracle server to be executed. This execution method is appropriate when there are mixed transactions and network latency between TimesTen and the Oracle server.

Use the cacheAWTMethod first connection attribute to specify SQL array execution to apply changes within TimesTen to the Oracle database. This method is appropriate when the same type of operation is repeated. For example, SQL array execution is very efficient when a user does an update that affects several rows of the table. Updates are grouped together and sent to the Oracle server in one batch.

The PL/SQL execution method transparently falls back to SQL array execution mode temporarily when it encounters one of the following:

• A statement that is over 32761 bytes in length.

• A statement that references a column of type BINARY FLOAT, BINARY DOUBLE and VARCHAR/VARBINARY of length greater than 4000 bytes.

For more information, see "cacheAWTMethod" in Oracle TimesTen In-Memory Database Reference.