Oracle - Using bind variables with dynamic SELECT INTO clause in PL
SQL Bind Variables/Parameters in databases - Use The Index, Luke!
Selecting ten times as many rows will elevate the cost value by that factor. The overall cost using the index is then even higher than a full table scan. The optimizer will therefore select the other execution plan for the bigger subsidiary. Subsidiary_id Using a bind parameter: if (stmt mysqli- prepare select first_name, last_name". " from employees". " where subsidiary_id? stmt- bind_param i subsidiary_id stmt- execute else / handle SQL error / See also: mysqli_stmt:bind_param class documentation and Prepared statements and stored procedures in the PDO documentation. The quandary is that the database does not know if the full optimization cycle delivers a different execution plan without actually doing the full optimization. Database vendors try to solve this dilemma with heuristic methodsbut with very limited success. From this perspective, it is a little bit paradoxical that bind parameters can improve performance if not using bind parameters enables the optimizer to always opt for the best execution plan. Question marks are positional parameters. That means the question marks are numbered from left to right. To bind a value to a particular question mark, you have to specify its number.
The optimizer uses it to determine the frequency of the subsidiary ID mentioned in the SQL query. Consequently it gets two different row count estimates for both queries. The subsequent cost calculation will therefore result in two different cost values.
Using bind variables in select statement
Naturally there are exceptions, for example if the affected data volume depends on the actual values: SELECT first_name, last_name FROM employees. WHERE subsidiary_id Id Operation Name Rows Cost SELECT STATEMENT TABLE ACCESS BY INDEX ROWID EMPLOYEES INDEX RANGE SCAN EMPLOYEE _PK Predicate Information (identified by.
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