Re^2: Dynamic SQL

because the query parameters varry too :

if ($x>10) { $query="(select Name from Customers where CustId='$custid
+')" } else { $query="(select Name from Sales where SalesId='$salesid'
+ and CustId='$custid')" }
[download]

so when preparing and executing the query with the first query:

$sth->execute(?)
[download]

with the second:

$sth->execute(?,?)
[download]

so I can't have a single execute covering both occasions

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Re^3: Dynamic SQL by bitingduck (Deacon) on Apr 06, 2015 at 15:57 UTC
Did you notice that my version has two separate statement handles that are being executed? You can do the same thing with yours. Prepared statements with placeholders will likely be faster than constructing queries on the fly and executing them, and certainly much safer. How many possible queries can you need to construct that you can't afford to use prepared statements and placeholders?	[reply]
Re^4: Dynamic SQL by erix (Prior) on Apr 06, 2015 at 16:02 UTC
prepared statements with placeholders will likely be faster than constructing queries on the fly I ~~don't think~~ rather doubt this is generally true. It would be nice to see it demonstrated.	[reply]
Re^5: Dynamic SQL (prepare) by tye (Sage) on Apr 07, 2015 at 03:47 UTC
prepared statements with placeholders will likely be faster than constructing queries on the fly I rather doubt this is generally true. It would be nice to see it demonstrated. Running a ton of nearly identical queries can get a rather slight performance improvement from only having to prepare the statement once. But this is not particularly significant in the most extreme case and is of no significance at all if your queries are not rather trivial. On the other hand, in all 3 of my most recent jobs I've had to basically disable the "prepare" step as the poor query optimization that is done in the absence of the actual values is often horrible and can result in queries taking several orders of magnitude longer. So I've heard that newer versions of databases that I haven't used recently end up doing query planning twice, once at "prepare" time and then again once the values are known. I don't know how much that impacts the oft-touted "efficiency" of separate prepare/execute, but it surely means it has become even more insignificant. But the documentation says that it is faster so everybody repeats that, usually with much more emphasis than is warranted (and completely ignoring the much, much worse performance problems that can result). - tye	[reply]
Re^5: Dynamic SQL by bitingduck (Deacon) on Apr 06, 2015 at 17:16 UTC
If each prepared statement is used only once then, no, it won't be true. If each prepared statement is used a number of times then it will save the parsing time of the SQL queries each time a prepared statement is reused. The OP has given very limited examples and no sense of the number of possible unique (modulo the values that would go in the placeholders) queries	[reply]
Re^6: Dynamic SQL by erix (Prior) on Apr 06, 2015 at 17:21 UTC
Re^7: Dynamic SQL by Laurent_R (Canon) on Apr 06, 2015 at 17:47 UTC
Re^3: Dynamic SQL by Anonymous Monk on Apr 06, 2015 at 18:22 UTC
`my ( $query, @args ); if ( $x > 10 ) { $query = 'select Name from Customers where CustID = ?'; @args = ( $custid ); } else { $query = 'select Name from Sales where SalesId = ? and CustID = ?' +; @args = ( $salesid, $custid ); } my $sth = $dbh->prepare( $query ); $sth->execute( @args );` [download]	[reply] [d/l]