This does not belong to any specific language, but logic. I just use SQL as an example: I have 2 clauses:
(1) WHERE A1 AND A2
(2) WHERE A1 OR A2
In the (1) case, would query stop right after checking A1 is false?
In the (2) case, would query stop right after checking A1 is true?
Please mark this one as duplicated if someone asked before.
Thank you
You have the wrong idea. SQL queries represent the result set, not the processing that takes place. So, the where clause does not represent a specific set of instructions. The clauses can be executed in any order.
That said, most databases do short-circuit such evaluations. So execution usually stops with the first clause that determines the condition (true or false).
What you're describing is called "short-circuiting", and I'm sure it varies by language, but I know JavaScript and ColdFusion both behave the way you describe.
SQL is implementation dependent:
https://stackoverflow.com/a/909770/2913684
Lets say I have a function, POINT_IN_SHAPE('shape_name', x, y) that is an expensive-ish call, so I would want to reduce the amount of times it would be called. One way to prevent calling the function is to draw a bounding box around the shape. For example, if a point's x is greater than the maximum x value in the shape, then it's not inside.
if(x > max_X) return outside;
With this in mind, would adding that condition to my WHERE clause speed up the query by reducing the amount of rows that POINT_IN_SHAPE() needs to run on? Let's say there in an index on x, and POINT_IN_SHAPE() does not do a bounding box check.
select * from points where x <= max_X and POINT_IN_SHAPE(...);
vs
select * from points where POINT_IN_SHAPE(...);
I'm hoping the conditions reduce the set of rows for other conditions, but I'm not sure how SQL works. I'm also hoping that it'll be smart enough to reduce rows first using the cheaper condition.
I don't really care if the ordering of the conditions matters, I'm just wondering if one condition would reduce the "search space" for another condition. From the comments, answers, links to other threads, and a bit of testing, it looks like the conditions do reduce the search space for other conditions in the where clause.
In your example, it is almost certain that the optimizer will use the condition x <= max_x first. Why? Because some conditions are "access predicates" (conditions the engine uses to determine which rows will be looked at in the first place), while other conditions are "filter predicates" (conditions used simply to decide which rows should be selected and which should be discarded). "Access predicates" are evaluated before "filter predicates" (obviously, if you think about what that means).
In your case the optimizer will decide to use x <= max_x as access predicate, especially with an index on the x column. So it will use that condition first.
Gordon shows the standard way to "force" the order of evaluation of conditions separated by AND. (It would work the same for conditions separated by OR.) Do remember this trick, it's quite useful - even though perhaps in this situation it is not needed. It might be needed if your query was more complex; for example, if in addition to x <= max_x you also had a condition like state = 'MN'. Then the optimizer may choose to use state = 'MN' as access predicate, and both x <= max_x and your functional condition as filter predicates. It is still likely that it will evaluate the inequality first, but you could use Gordon's trick just to make sure.
Oracle does not guarantee the ordering of conditional expressions (no database does). So, mere ordering is not sufficient. The same might be true of using subqueries -- Oracle reserves the right to rearrange expressions if it thinks doing so makes things more efficient. The problem is that the optimizer could be wrong.
This is one of the situations where a case in the where clause can be appropriate.
where (case when x >= max_x then 'outside'
when POINT_IN_SHAPE(...) then 'inside'
end) = 'inside'
The conditions in the case are guaranteed to be evaluated sequentially. Note: this just uses strings 'inside' and 'outside' for clarity.
It depends on how your DB engine will execute this query, what execution plan engine chooses.
Other way to write query is to first filter desired record based on condition in a sub query and after that apply function call on filtered output.
In your case you can try this.
select POINT_IN_SHAPE('shape_name', x, y)
from (select * from points where x <= max_X) data
In Java, a logical OR condition behaves such that if the first condition is true then it does not evaluate the second condition.
For example:
int a = 10;
if (a == 10 || a == 0) {
// Logic
}
Java does not evaluate the second test (a == 0) because the first condition (a == 10) is true.
If we have an Oracle SQL statement like this:
select * from student where city = :city and
(:age is null or age > :age)
How are (age > :age or :age is null) evaluated? If the parameter :age is NULL, then does it evaluate the second condition as well?
The database cost optimizer will consider many factors in structuring the execution of a query. Probably the most important will be the existence of indexes on the columns in question. It will decide the order based on the selectivity of the test and could perform them in different order at different times. Since SQL is a declarative and not procedural language, you cannot generally control the way in which these conditions are evaluated.
There may be some "hints" you can provide to suggest a specific execution order, but you risk adversely affecting performance.
PL/SQL
In PL/SQL, Oracle OR is another example of short circuit evaluation. Oracle PL/SQL Language Fundamentals says (in part)
Short-Circuit Evaluation
When evaluating a logical expression, PL/SQL uses short-circuit evaluation. That is, PL/SQL stops evaluating the expression as soon as it can determine the result. Therefore, you can write expressions that might otherwise cause errors.
SQL
However, in regular SQL, the OR might be evaluated in either order. As pointed out by #JonHeller in his comment the expressions in this question are safe, more caution would be required if dealing with potential division by 0.
Let Oracle decide for you. It will most of the time make a much better decision. In this case, there is even a construct that combines test for null with testing a value.
Replace
:age is null or age > :age
With
age > nvl(:age, age - 1)
Are boolean expressions in SQL WHERE clauses short-circuit evaluated
?
For example:
SELECT *
FROM Table t
WHERE #key IS NULL OR (#key IS NOT NULL AND #key = t.Key)
If #key IS NULL evaluates to true, is #key IS NOT NULL AND #key = t.Key evaluated?
If no, why not?
If yes, is it guaranteed? Is it part of ANSI SQL or is it database specific?
If database specific, SQLServer? Oracle? MySQL?
ANSI SQL Draft 2003 5WD-01-Framework-2003-09.pdf
6.3.3.3 Rule evaluation order
[...]
Where the precedence is not determined by the Formats or by
parentheses, effective evaluation of expressions is generally
performed from left to right. However, it is
implementation-dependent whether expressions are actually evaluated left to right, particularly when operands or operators might
cause conditions to be raised or if the results of the expressions
can be determined without completely evaluating all parts of the
expression.
From the above, short circuiting is not really available.
If you need it, I suggest a Case statement:
Where Case when Expr1 then Expr2 else Expr3 end = desiredResult
Expr1is always evaluated, but only one of Expr2 and Expr3 will be evaluated per row.
I think this is one of the cases where I'd write it as if it didn't short-circuit, for three reasons.
Because for MSSQL, it's not resolved by looking at BOL in the obvious place, so for me, that makes it canonically ambiguous.
because at least then I know my code will work. And more importantly, so will those who come after me, so I'm not setting them up to worry through the same question over and over again.
I write often enough for several DBMS products, and I don't want to have to remember the differences if I can work around them easily.
I don't believe that short circuiting in SQL Server (2005) is guaranteed. SQL Server runs your query through its optimization algorithm that takes into account a lot of things (indexes, statistics, table size, resources, etc) to come up with an effective execution plan. After this evaluation, you can't say for sure that your short circuit logic is guaranteed.
I ran into the same question myself sometime ago and my research really did not give me a definitive answer. You may write a small query to give you a sense of proof that it works but can you be sure that as the load on your database increases, the tables grow to be bigger, and things get optimized and changed in the database, that conclusion will hold. I could not and therefore erred on the side of caution and used CASE in WHERE clause to ensure short circuit.
You have to keep in mind how databases work. Given a parameterized query the db builds an execution plan based on that query without the values for the parameters. This query is used every time the query is run regardless of what the actual supplied values are. Whether the query short-circuits with certain values will not matter to the execution plan.
I typically use this for optional parameters. Is this the same as short circuiting?
SELECT [blah]
FROM Emp
WHERE ((#EmpID = -1) OR (#EmpID = EmpID))
This gives me the option to pass in -1 or whatever to account for optional checking of an attribute. Sometimes this involves joining on multiple tables, or preferably a view.
Very handy, not entirely sure of the extra work that it gives to the db engine.
Just stumbled over this question, and had already found this blog-entry: http://rusanu.com/2009/09/13/on-sql-server-boolean-operator-short-circuit/
The SQL server is free to optimize a query anywhere she sees fit, so in the example given in the blog post, you cannot rely on short-circuiting.
However, a CASE is apparently documented to evaluate in the written order - check the comments of that blog post.
For SQL Server, I think it depends on the version but my experience with SQL Server 2000 is that it still evaluates #key = t.Key even when #key is null. In other words, it does not do efficient short circuiting when evaluating the WHERE clause.
I've seen people recommending a structure like your example as a way of doing a flexible query where the user can enter or not enter various criteria. My observation is that Key is still involved in the query plan when #key is null and if Key is indexed then it does not use the index efficiently.
This sort of flexible query with varying criteria is probably one case where dynamically created SQL is really the best way to go. If #key is null then you simply don't include it in the query at all.
Main characteristic of short circuit evaluation is that it stops evaluating the expression as soon as the result can be determined. That means that rest of expression can be ignored because result will be same regardless it is evaluated or not.
Binary boolean operators are comutative, meaning:
a AND b == b AND a
a OR b == b OR a
a XOR b == b XOR a
so there is no guarantee on order of evaluation. Order of evaluation will be determined by query optimizer.
In languages with objects there can be situations where you can write boolean expressions that can be evaluated only with short circuit evaluation. Your sample code construction is often used in such languages (C#, Delphi, VB). For example:
if(someString == null | someString.Length == 0 )
printf("no text in someString");
This C# example will cause exception if someString == null because it will be fully evaluated. In short circuit evaluation, it will work every time.
SQL operates only on scalar variables (no objects) that cannot be uninitialized, so there is no way to write boolean expression that cannot be evaluated. If you have some NULL value, any comparison will return false.
That means that in SQL you cannot write expression that is differently evaluated depending on using short circuit or full evaluation.
If SQL implementation uses short circuit evaluation, it can only hopefully speed up query execution.
i don't know about short circuting, but i'd write it as an if-else statement
if (#key is null)
begin
SELECT *
FROM Table t
end
else
begin
SELECT *
FROM Table t
WHERE t.Key=#key
end
also, variables should always be on the right side of the equation. this makes it sargable.
http://en.wikipedia.org/wiki/Sargable
Below a quick and dirty test on SQL Server 2008 R2:
SELECT *
FROM table
WHERE 1=0
AND (function call to complex operation)
This returns immediately with no records. Kind of short circuit behavior was present.
Then tried this:
SELECT *
FROM table
WHERE (a field from table) < 0
AND (function call to complex operation)
knowing no record would satisfy this condition:
(a field from table) < 0
This took several seconds, indicating the short circuit behavior was not there any more and the complex operation was being evaluated for every record.
Hope this helps guys.
Here is a demo to prove that MySQL does perform WHERE clause short-circuiting:
http://rextester.com/GVE4880
This runs the following queries:
SELECT myint FROM mytable WHERE myint >= 3 OR myslowfunction('query #1', myint) = 1;
SELECT myint FROM mytable WHERE myslowfunction('query #2', myint) = 1 OR myint >= 3;
The only difference between these is the order of operands in the OR condition.
myslowfunction deliberately sleeps for a second and has the side effect of adding an entry to a log table each time it is run. Here are the results of what is logged when running the above two queries:
myslowfunction called for query #1 with value 1
myslowfunction called for query #1 with value 2
myslowfunction called for query #2 with value 1
myslowfunction called for query #2 with value 2
myslowfunction called for query #2 with value 3
myslowfunction called for query #2 with value 4
The above shows that a slow function is executed more times when it appears on the left side of an OR condition when the other operand isn't always true (due to short-circuiting).
This takes an extra 4 seconds in query analyzer, so from what I can see IF is not even shorted...
SET #ADate = NULL
IF (#ADate IS NOT NULL)
BEGIN
INSERT INTO #ABla VALUES (1)
(SELECT bla from a huge view)
END
It would be nice to have a guaranteed way!
The quick answer is: The "short-circuit" behavior is undocumented implementation.
Here's an excellent article that explains this very topic.
Understanding T-SQL Expression Short-Circuiting
It is but obvious that MS Sql server supports Short circuit theory, to improve the performance by avoiding unnecessary checking,
Supporting Example:
SELECT 'TEST'
WHERE 1 = 'A'
SELECT 'TEST'
WHERE 1 = 1 OR 1 = 'A'
Here, the first example would result into error 'Conversion failed when converting the varchar value 'A' to data type int.'
While the second runs easily as the condition 1 = 1 evaluated to TRUE and thus the second condition doesn't ran at all.
Further more
SELECT 'TEST'
WHERE 1 = 0 OR 1 = 'A'
here the first condition would evaluate to false and hence the DBMS would go for the second condition and again you will get the error of conversion as in above example.
NOTE: I WROTE THE ERRONEOUS CONDITION JUST TO REALIZE WEATHER THE CONDITION IS EXECUTED OR SHORT-CIRCUITED
IF QUERY RESULTS IN ERROR MEANS THE CONDITION EXECUTED, SHORT-CIRCUITED OTHERWISE.
SIMPLE EXPLANATION
Consider,
WHERE 1 = 1 OR 2 = 2
as the first condition is getting evaluated to TRUE, its meaningless to evaluate the second condition because its evaluation in whatever value
would not affect the result at all, so its good opportunity for Sql Server to save Query Execution time by skipping unnecessary condition checking or evaluation.
in case of "OR" if first condition is evaluated to TRUE the entire chain connected by "OR" would considered as evaluated to true without evaluating others.
condition1 OR condition2 OR ..... OR conditionN
if the condition1 is evaluated to true, rest all of the conditions till conditionN would be skipped.
In generalized words at determination of first TRUE, all other conditions linked by OR would be skipped.
Consider the second condition
WHERE 1 = 0 AND 1 = 1
as the first condition is getting evalutated to FALSE its meaningless to evaluate the second condition because its evaluation in whatever value
would not affect the result at all, so again its good opportunity for Sql Server to save Query Execution time by skipping unnecessary condition checking or evaluation.
in case of "AND" if first condition is evaluated to FALSE the entire chain connected with the "AND" would considered as evaluated to FALSE without evaluating others.
condition1 AND condition2 AND ..... conditionN
if the condition1 is evaluated to FALSE, rest all of the conditions till conditionN would be skipped.
In generalized words at determination of first FALSE, all other conditions linked by AND would be skipped.
THEREFOR, A WISE PROGRAMMER SHOULD ALWAYS PROGRAM THE CHAIN OF CONDITIONS IN SUCH A WAY THAT, LESS EXPENSIVE OR MOST ELIMINATING CONDITION GETS EVALUATED FIRST,
OR ARRANGE THE CONDITION IN SUCH A WAY THAT CAN TAKE MAXIMUM BENEFIT OF SHORT CIRCUIT
I want to be able to pass something into an SQL query to determine if I want to select only the ones where a certain column is null. If I was just building a query string instead of using bound variables, I'd do something like:
if ($search_undeleted_only)
{
$sqlString .= " AND deleted_on IS NULL";
}
but I want to use bound queries. Would this be the best way?
my $stmt = $dbh->prepare(...
"AND (? = 0 OR deleted_on IS NULL) ");
$stmt->execute($search_undeleted_only);
Yes; a related trick is if you have X potential filters, some of them optional, is to have the template say " AND ( ?=-1 OR some_field = ? ) ", and create a special function that wraps the execute call and binds all the second ?s. (in this case, -1 is a special value meaning 'ignore this filter').
Update from Paul Tomblin: I edited the answer to include a suggestion from the comments.
So you're relying on short-circuiting semantics of boolean expressions to invoke your IS NULL condition? That seems to work.
One interesting point is that a constant expression like 1 = 0 that did not have parameters should be factored out by the query optimizer. In this case, since the optimizer doesn't know if the expression is a constant true or false until execute time, that means it can't factor it out. It must evaluate the expression for every row.
So one can assume this add a minor cost to the query, relative to what it would cost if you had used a non-parameterized constant expression.
Then combining with OR with the IS NULL expression may also have implications for the optimizer. It might decide it can't benefit from an index on deleted_on, whereas in a simpler expression it would have. This depends on the RDBMS implementation you're using, and the distribution of values in your database.
I think that's a reasonable approach. It follows the normal filter pattern nicely and should give good performance.