Database.CQL.IO.PrepQuery

Documentation

data PrepQuery k a b

Representation of a prepared QueryString. A prepared query is executed in two stages:

1. The query string is sent to a server without parameters for preparation. The server responds with a QueryId.
2. The prepared query is executed by sending the QueryId and parameters to the server.

Thereby step 1 is only performed when the query has not yet been prepared with the host (coordinator) used for query execution. Thus, prepared queries enhance performance by avoiding the repeated sending and parsing of query strings.

Query preparation is handled transparently by the client. See setPrepareStrategy.

Note

Prepared statements are fully supported but rely on some assumptions beyond the scope of the CQL binary protocol specification (spec):

1. The spec scopes the QueryId to the node the query has been prepared with. The spec does not state anything about the format of the QueryId. However the official Java driver assumes that any given QueryString yields the same QueryId on every node. This client make the same assumption.
2. In case a node does not know a given QueryId an Unprepared error is returned. We assume that it is always safe to transparently re-prepare the corresponding QueryString and to re-execute the original request against the same node.

Besides these assumptions there is also a potential tradeoff in regards to eager vs. lazy query preparation. We understand eager to mean preparation against all current nodes of a cluster and lazy to mean preparation against a single node on demand, i.e. upon receiving an Unprepared error response. Which strategy to choose depends on the scope of query reuse and the size of the cluster. The global default can be changed through the Settings module as well as locally using withPrepareStrategy.

Instances

IsString (PrepQuery k a b)

Defined in Database.CQL.IO.PrepQuery

Methods

fromString :: String -> PrepQuery k a b

prepared :: QueryString k a b -> PrepQuery k a b

queryString :: PrepQuery k a b -> QueryString k a b

data PreparedQueries

new :: IO PreparedQueries

lookupQueryId :: PrepQuery k a b -> PreparedQueries -> STM (Maybe (QueryId k a b))

lookupQueryString :: QueryId k a b -> PreparedQueries -> STM (Maybe (QueryString k a b))

insert :: PrepQuery k a b -> QueryId k a b -> PreparedQueries -> STM ()

delete :: PrepQuery k a b -> PreparedQueries -> STM ()

queryStrings :: PreparedQueries -> STM [Text]