= Prepared Statements and Bound Variables

Sequel has support for prepared statements and bound variables.  No matter which
database you are using, the Sequel prepared statement/bound variable API remains
the same.  There is native support for prepared statements/bound variables on
the following adapters:

* ibmdb (prepared statements only)
* jdbc
* mysql (server prepared statements using literalized connection variables)
* mysql2 (full support on 0.4+, otherwise server prepared statements using literalized connection variables)
* oracle (requires type specifiers for nil/NULL values)
* postgres (when using the pg driver)
* sqlite
* tinytds

Support on other adapters is emulated.

You can use the prepared_statements model plugin to automatically use prepared
statements for some common model actions such as saving or deleting a model
instance, or looking up a model based on a primary key.

== Placeholders

Generally, when using prepared statements (and certainly when using bound
variables), you need to put placeholders in your SQL to indicate where you
want your bound arguments to appear.  Database support and syntax vary
significantly for placeholders (e.g. :name, $1, ?).  Sequel abstracts all of
that and allows you to specify placeholders by using the :$name format for
placeholders, e.g.:

  ds = DB[:items].where(name: :$n)

You can use these placeholders in most places where you can use the value
directly.  For example, if you want to use placeholders while also using
raw SQL, you can do:

  ds = DB["SELECT * FROM items WHERE name = ?", :$n]

== Bound Variables

Using bound variables for this query is simple:

  ds.call(:select, n: 'Jim')

This will do the equivalent of selecting records that have the name 'Jim'. It
returns all records, and can take a block that is passed to <tt>Dataset#all</tt>.

Deleting or returning the first record works similarly:

  ds.call(:first, n: 'Jim') # First record with name 'Jim'
  ds.call(:delete, n: 'Jim') # Delete records with name 'Jim'

For inserting/updating records, you should also specify a value hash, which
may itself contain placeholders:

  # Insert record with 'Jim', note that the previous filter is ignored
  ds.call(:insert, {n: 'Jim'}, name: :$n)
  # Change name to 'Bob' for all records with name of 'Jim'
  ds.call(:update, {n: 'Jim', new_n: 'Bob'}, name: :$new_n)

== Prepared Statements

Prepared statement support is similar to bound variable support, but you
use <tt>Dataset#prepare</tt> with a name, and <tt>Dataset#call</tt> or <tt>Database#call</tt> later with the values:

  ds = DB[:items].where(name: :$n)
  ps = ds.prepare(:select, :select_by_name)

  ps.call(n: 'Jim')
  DB.call(:select_by_name, n: 'Jim') # same

The <tt>Dataset#prepare</tt> method returns a prepared statement, and also stores a
copy of the prepared statement in the database for later use.  For insert
and update queries, the hash to insert/update is passed to +prepare+:

  ps1 = DB[:items].prepare(:insert, :insert_with_name, name: :$n)

  ps1.call(n: 'Jim')
  DB.call(:insert_with_name, n: 'Jim') # same

  ds = DB[:items].where(name: :$n)
  ps2 = ds.prepare(:update, :update_name, name: :$new_n)

  ps2.call(n: 'Jim', new_n: 'Bob')
  DB.call(:update_name, n: 'Jim', new_n: 'Bob') # same

== Implementation Issues

Currently, creating a prepared statement uses Object#extend, which can hurt
performance.  For high performance applications, it's recommended to create
all of your prepared statements upon application initialization, and not
to create prepared statements dynamically at runtime.

== Database support

=== PostgreSQL

If you are using the postgres-pr driver, PostgreSQL uses the
default emulated support.  If you are using ruby-pg, there is native support
for both prepared statements and bound variables.  Prepared statements are
always server side.

=== SQLite

SQLite supports both prepared statements and bound variables.

=== MySQL/Mysql2

The MySQL and Mysql2 <0.4 ruby drivers do not support bound variables, so the bound
variable methods are emulated.  It uses server side prepared statements.

Mysql2 0.4+ supports both prepared statements and bound variables.

=== JDBC

JDBC supports both prepared statements and bound variables.  Whether these
are server side or client side depends on the JDBC driver.  For PostgreSQL
over JDBC, you can add the prepareThreshold=N parameter to the connection
string, which will use a server side prepared statement after N calls to
the prepared statement.

=== TinyTDS

Uses the sp_executesql stored procedure with bound variables, since
Microsoft SQL Server doesn't support true prepared statements.

=== IBM_DB

DB2 supports both prepared statements and bound variables.

=== Oracle

Oracle supports both prepared statements and bound variables.  Prepared
statements (OCI8::Cursor objects) are cached per connection.  If you
ever plan to use a nil/NULL value as a bound variable/prepared statement
value, you must specify the type in the placeholder using a __* suffix.
You can use any of the schema types that Sequel supports, such as
:$name__string or :$num__integer.  Using blobs as bound variables is
not currently supported.

=== All Others

Support is emulated.
