Obtaining Query Count Without executing a Query in Oracle D2k
Obtaining a count of records that will be retrieved by EXECUTE_QUERY before actually performing it in a database block is especially useful when the requirement is to prevent navigation to a block when query hits are zero. A typical scenario of such a situation is when the detail block records exist on a separate canvas not visible on Form startup and the user is required to click a Details button to see them. Giving an alert message such as No Details exist when the user clicks the Details button is more meaningful than displaying a blank details screen, when no details exist for the chosen parent record.
The technique given here avoids two performance issues. First, you do not want to perform a SELECT COUNT(*) from the corresponding base table mainly for performance reasons. Second, using :SYSTEM.LAST_QUERY and executing it dynamically using DBMS_SQL cause a bottleneck by executing the query on the server side explicitly, thus involving more trips.
The solution is to do a COUNT_QUERY and get the QUERY_HITS for the corresponding block immediately following the COUNT_QUERY. The following function does the job:
FUNCTION query_count (p_block_name VARCHAR2) RETURN NUMBER
IS
cnt NUMBER;
BEGIN
GO_BLOCK(p_block_name);
COUNT_QUERY;
cnt := GET_BLOCK_PROPERTY(p_block_name, QUERY_HITS);
IF FORM_SUCCESS THEN
RETURN (cnt);
ELSE
MESSAGE(‘Error in getting Query Hits for block ‘||:SYSTEM.CURRENT_BLOCK);
RAISE FORM_TRIGGER_FAILURE;
END IF;
END;
The preceding function can be called in the appropriate trigger, such as WHEN-BUTTEN-PRESSED, to achieve the desired functionality.
The following WHEN-BUTTON-PRESSED trigger is defined for the Details button. It initially invokes the above query_count function to obtain the count of detail records for a particular master record. If this count is zero it throws an alert to indicate No Details exist. Otherwise, control navigates to the detail block and does an EXECUTE_QUERY.
WHEN-BUTTON-PRESSED trigger of ‘Details‘button
DECLARE
v_cnt NUMBER;
BEGIN
v_cnt := query_count(<detail block name>);
IF (v_cnt = 0) THEN
p_show_alert(‘No Details exist.‘);
ELSE
GO_BLOCK(<detail block name>);
EXECUTE_QUERY;
END IF;
END;
This technique involves two tasks:
- COUNT_QUERY is necessary to initiate the QUERY_HITS property of the block and should be immediately before the GET_BLOCK_PROPERTY statement.
- Oracle Forms displays the message FRM-40355: Query will display 0 records when the query hits are zero as obtained by a call to COUNT_QUERY. This should be suppressed in an ON-MESSAGE trigger by using the following code:
if message_type = ‘FRM‘and message_code = 40355 then null; else message(message_type||‘-‘||to_char(message_code)||‘: ‘||message_text); end if;