On 3/13/2017 3:41 PM, David Storrs wrote:
On Mon, Mar 13, 2017 at 2:04 PM, Ryan Culpepper <ry...@ccs.neu.edu
<mailto:ry...@ccs.neu.edu>> wrote:
If you are using `prepare` just for speed, it might help to know that
most base connections have an internal statement cache that maps SQL
strings to prepared statement objects. The cache is only used inside
of transactions, though, to avoid issues with concurrent schema changes.
You can check on statement caching by setting the #:debug? argument
when connecting. Aside from lots of other noise, queries will print
out whether they are using the statement cache. Here's an example:
> (define c (dsn-connect 'pg #:debug? #t))
....
> (start-transaction c)
....
** in managed transaction
> (query c "select 1")
....
** caching statement
....
> (query c "select 1")
** using cached statement
....
Could that replace your use of `prepare`?
Looks like inside a transaction all statements are prepared. Cool.
If so, that's very nearly perfect -- it adds and removes all the
concerns that transactions always add/remove, but offhand I can't
think of a case where doing it in a transaction would be a problem.
There's a difference between "prepared" and "compiled".
Every query is compiled. The compiled version can be - and typically is
in most DBMS - cached for reuse if the same *identical* query string is
presented again. With a normal query, the compiled version is optimized
based on the specific values contained in the statement.
Prepare is different. The idea of "preparation" is to be able to reuse
a generic query with different arguments. A prepared query is compiled
to use variables rather than inline values, so it is not as well
optimized as compilation of a normal query. The savings in preparation
comes mainly from avoiding compiles on subsequent reuse.
The compiled version of a prepared query persists until explicitly
dropped or the connection is closed. It is not cached in the same way
as are normal queries.
I'm not sure exactly what #:debug? is showing you. Ryan knows better
than I do, but if you haven't explicitly called "prepare", then I
suspect it is only the specific syntax of the cached query that can be
reused, and that if you change the query in any way - e.g., by passing
different arguments - then the cached version will not be used.
George
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