Reusing a Cursor
Reusing a cursor means compiling a SQL statement just once, but executing it more than once. When a SQL statement is compiled, the database checks the syntax and chooses a query path. By doing this only once, but executing this statement several times, you can obtain an improvement in performance.
In PeopleCode, you can reuse a cursor either by using the ReuseCursor property or by following certain restrictions in your code.
If you specify the ReuseCursor property as True, the cursor isn't closed until either the SQL object is explicitly closed or re-opened. This provides greater control over the cursor associated with your SQL object. However, when you set ReuseCursor to True, you're essentially pledging to do the right thing in your program. There are some considerations for how you use this property:
In a Application Engine program, if your program can be restarted, you mustcheck for a restart after a checkpoint by testing to see if the SQL object is open after the checkpoint. If it isn't open, that means a restart has happened, and you must reopen the SQL object. In most cases, on checkpoints, your open SQL objects aren't closed, saving the overhead of re-establishing the SQL object after the checkpoint. If the SQL object is open on a select statement at a checkpoint it isn't restored to the open state because you cannot reliably establish the state of the execute-fetch sequence.
In the following example the SQL object is established on a select statement which is executed twice with different bind parameters but is compiled only once. Without the ReuseCursor property the SQL object would be closed after the first fetch cycle completes.
Local SQL &Sql; Local String &Company1 = "CCB"; Local String &Company2 = "CCF"; &Sql = CreateSQL("SELECT A.COMPANY, %Timeout(A.EFFDT) FROM %Table(COMPANY_TBL) A WHERE A.COMPANY=:1", &Company1); /* commenting this out should make the subsequent Execute fail */ &Sql.ReuseCursor = True; While &Sql.Fetch(&Company1, &Effdt) /* processing for the first Company */ End-While; &Sql.Execute(&Company2); While &Sql.Fetch(&Company1, &Effdt) /* processing for the second company */ End-While; &Sql.Close();
If you want to reuse a cursor in your program, there are several conditions:
You have to use the SQL object. Only SQL objects retain SQL cursor information.
For INSERT, DELETE, and UPDATE statements, you automatically reuse the cursor as long as you don't change the SQL statement as part of the binding process. If the SQL statement is textually the same, so only the binds have changed, you get reuse. For example, you won't get reuse if you first bind one kind of record object to %Insert(), then bind another, different kind of record object.
For SELECTs, you mustuse the meta-SQL shortcuts (%SelectAll, %SelectByKey, or %SelectByKeyEffDt), and the CreateSQL or GetSQL functions without supplying any bind or buffer parameters. The bind parameters are supplied in the Execute function. The Fetch parameter must be the fetch buffer, and be the same as the first Execute parameter. You can supply WHERE clauses, ORDER-BY, and so on, on the end of the SQL string containing the meta-SQL.
Note: BulkMode doesn't reuse the cursor in the same way as the SQL object. BulkMode requests that, when the system can reuse the cursor, it also holds all the changes so they can be communicated to the database in batches.
By using one of the forms of the Select meta-SQL, you're guaranteeing the resulting fetched values are all put into one record object (buffer). This means the implementation doesn't have to ask the database for the length and type of each column: the record buffer is already defined. So this sample code uses the SQL object to maintain the state of the connection with the database, and the record object, to maintain a series of fields suitable for database operations.
Local SQL &SQL; Local Record &REC; &REC = CreateRecord(Record.KP_KPI_DFN); /* start with select statement, no bind refs, no */ /* bind parameters */ &SQL = CreateSQL("%Selectall(:1) Where SETID = :2 and KPI_ID = :3 and EFFDT = (SELECT MAX(EFFDT) FROM PS_KP_KPI_DFN WHERE SETID = :4 AND KPI_ID = :5 AND EFFDT <= %DateIn(:6))");
/* bind and execute the statement */ &SQL.Execute(&REC, &SETID_KPI, &COMPID, &SETID_KPI, &COMPID, &EFFDT); /* Note record parameter for Fetch statement must be the same as the first Execute parameter the results are in this record */ If &SQL.Fetch(&REC) Then /*process this record */ End-If;
&SQL.Close(); /* there is no implicit close on a Fetch returning False */
The following example comes from a Application Engine program. Because the loop goes in and out of PeopleCode, you must declare the SQL object as Global. This example is in three parts.
Program called by Application Engine before the loop:
Global SQL &SQL; &SQL = CreateSQL("INSERT INTO %Table(MY_WORK) (TRANS , REGISTERWRITE) VALUES (:1, :2)"); &SQL.BulkMode = True; /* not required for reuse, but will get better performance on platforms that support bulk insert */
Program called in the Application Engine loop on SELECT 'X' FROM PSRECDEFN WHERE RECNAME LIKE '%A':
Global SQL &SQL; &var1 = "X"; &var2 = "Y"; &SQL.Execute(&var1, &var2);
Program called by Application Engine after the loop:
Global SQL &SQL; &SQL.Close();