When is good to close (release) the SQLConnection and SqlConnection in Vert.x? - vert.x

Question

Taking a piece from Vert.x website example:
private Future<Void> prepareDatabase() {
Promise<Void> promise = Promise.promise();
dbClient = JDBCClient.createShared(vertx, new JsonObject() //(1)
.put("url", "jdbc:hsqldb:file:db/wiki") //(2)
.put("driver_class", "org.hsqldb.jdbcDriver") //(3)
.put("max_pool_size", 30)); //(4)
dbClient.getConnection(ar -> { //(5)
if (ar.failed()) {
LOGGER.error("Could not open a database connection", ar.cause());
promise.fail(ar.cause()); //(6)
} else {
SQLConnection connection = ar.result(); //(7)
connection.execute(SQL_CREATE_PAGES_TABLE, create -> {
connection.close(); //(8)
if (create.failed()) {
LOGGER.error("Database preparation error", create.cause());
promise.fail(create.cause());
} else {
promise.complete(); //(9)
}
});
}
});
return promise.future();
}
In (8), the connection is closed at the very beginning of the handler. What if we execute a query and then iterate the result in the handler:
private fun jdbcQuery(sql: String, params: JsonArray): Future<ResultSet> {
val promise: Promise<ResultSet> = Promise.promise()
getJDBCClient().getConnection { ar ->
if (ar.succeeded()) {
val connection = ar.result()
connection.queryWithParams(sql, params) { res ->
connection.close() //(10) release the connection
if (res.succeeded()) {
val result = res.result()
promise.complete(result)
} else {
promise.fail(res.cause())
}
}
} else {
promise.fail(ar.cause())
}
}
return promise.future()
}
I can fetch the data inside if (res.succeeded()).
My question is: why we can close and release the connection before iterating to fetch data? How does it work?

Answer 1

The queryWithParams API fetches the entire response from the DB when it is executed. Results are not fetched lazily. For this reason, it is safe to close the connection at the beginning of your response handler callback, because by that time the entire result set has already been received by the client. Results are only fetched lazily when you use the queryStream API. If you were using that API, you would want to wait to close the connection until all the results were received.