我是否应该在重复查询中使用prepareStatement，其中where子句谓词经常更改，从而导致所选计划的更改

Question

I have a Java application which is executing queries on PostgreSQL 9.3 Server using JDBC. In my java application, I had to execute same query many times(in thousands) with different arguments in 'where' clause predicates alone. I have been using Statement class till now. I recently read about PreparedStatement class somewhere and I am thinking should I use it to speedup processing. But my doubt is this. Since my query executes each time with different values in Where clause predicates, the selectivity will change and hence plan chosen by the db server will change. In that case, will using PreparedStatement speedup the processing? Is the plan chosen when Preparedstatement is created or plan is chosen only when execute is called on the preparedstatement object? If plan is chosen when preparedstatement is created itself, how is it done since optimizer chooses plans based on selectivity calculated using actual predicate values.

My Query is a complex one involving many tables. Template is like, select something from tables where predicate1 and predicate2 and price < X and date < Y; where X and Y varies for each query .

Answer 1

From PostgreSQL doc :

PREPARE creates a prepared statement. A prepared statement is a server-side object that can be used to optimize performance. When the PREPARE statement is executed, the specified statement is parsed, analyzed, and rewritten. When an EXECUTE command is subsequently issued, the prepared statement is planned and executed. This division of labor avoids repetitive parse analysis work, while allowing the execution plan to depend on the specific parameter values supplied.

moe was right : preparing a query will only remove the overhead of reparsing it again and again. The planing is done only when you will execute the prepared query with its parameters.

Answer 2

In 9.3, it uses a heuristic. It does something like planning the query with the specific bind values the first 5 times the prepared statement is executed. If none of those plans turn out to be substantially better than the generic plan, then it stops the individual planning and justs uses the generic plan from then on.

But there is another wrinkle in that just because your code told the driver to use a prepared statement doesn't mean driver is actually doing so. A lot of drivers do weird things.

The real answer is test, test, test.