/*jslint indent: 2, nomen: true, maxlen: 100, sloppy: true, vars: true, white: true, plusplus: true */
/*global require */

////////////////////////////////////////////////////////////////////////////////
/// @brief query explain actions
///
/// @file
///
/// DISCLAIMER
///
/// Copyright 2012 triagens GmbH, Cologne, Germany
///
/// Licensed under the Apache License, Version 2.0 (the "License");
/// you may not use this file except in compliance with the License.
/// You may obtain a copy of the License at
///
///     http://www.apache.org/licenses/LICENSE-2.0
///
/// Unless required by applicable law or agreed to in writing, software
/// distributed under the License is distributed on an "AS IS" BASIS,
/// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
/// See the License for the specific language governing permissions and
/// limitations under the License.
///
/// Copyright holder is triAGENS GmbH, Cologne, Germany
///
/// @author Jan Steemann
/// @author Copyright 2012, triAGENS GmbH, Cologne, Germany
////////////////////////////////////////////////////////////////////////////////

var actions = require("org/arangodb/actions");

var ArangoError = require("org/arangodb").ArangoError; 
var ERRORS = require("internal").errors;
var EXPLAIN = require("internal").AQL_EXPLAIN;

// -----------------------------------------------------------------------------
// --SECTION--                                                  global variables
// -----------------------------------------------------------------------------

////////////////////////////////////////////////////////////////////////////////
/// @addtogroup ArangoAPI
/// @{
////////////////////////////////////////////////////////////////////////////////

////////////////////////////////////////////////////////////////////////////////
/// @brief explain a query and return information about it
///
/// @RESTHEADER{POST /_api/explain,explains a query}
///
/// @RESTBODYPARAM{body,json,required}
/// The query string needs to be passed in the attribute `query` of a JSON
/// object as the body of the POST request. If the query references any bind 
/// variables, these must also be passed in the attribute `bindVars`.
///
/// @RESTDESCRIPTION
///
/// To explain how an AQL query would be executed on the server, the query string 
/// can be sent to the server via an HTTP POST request. The server will then validate
/// the query and create an execution plan for it, but will not execute it.
///
/// The execution plan that is returned by the server can be used to estimate the
/// probable performance of an AQL query. Though the actual performance will depend
/// on many different factors, the execution plan normally can give some good hint
/// on the amount of work the server needs to do in order to actually run the query. 
///
/// The top-level statements will appear in the result in the same order in which
/// they have been used in the original query. Each result element has at most the 
/// following attributes:
/// - `id`: the row number of the top-level statement, starting at 1
/// - `type`: the type of the top-level statement (e.g. `for`, `return` ...)
/// - `loopLevel`: the nesting level of the top-level statement, starting at 1
/// Depending on the type of top-level statement, there might be other attributes
/// providing additional information, for example, if and which indexed will be
/// used.
/// Many top-level statements will provide an `expression` attribute that
/// contains data about the expression they operate on. This is true for `FOR`,
/// `FILTER`, `SORT`, `COLLECT`, and `RETURN` statements. The 
/// `expression` attribute has the following sub-attributes:
/// - `type`: the type of the expression. Some possible values are:
///   - `collection`: an iteration over documents from a collection. The 
///     `value` attribute will then contain the collection name. The `extra`
///     attribute will contain information about if and which index is used when
///     accessing the documents from the collection. If no index is used, the 
///     `accessType` sub-attribute of the `extra` attribute will have the
///     value `all`, otherwise it will be `index`.
///   - `list`: a list of dynamic values. The `value` attribute will contain the
///     list elements.
///   - `const list`: a list of constant values. The `value` attribute will contain the
///     list elements.
///   - `reference`: a reference to another variable. The `value` attribute
///     will contain the name of the variable that is referenced.
///
/// Please note that the structure of the explain result data might change in future
/// versions of ArangoDB without further notice and without maintaining backwards
/// compatibility. 
///
/// @RESTRETURNCODES
///
/// @RESTRETURNCODE{200}
/// If the query is valid, the server will respond with `HTTP 200` and
/// return a list of the individual query execution steps in the `"plan"`
/// attribute of the response.
///
/// @RESTRETURNCODE{400}
/// The server will respond with `HTTP 400` in case of a malformed request,
/// or if the query contains a parse error. The body of the response will
/// contain the error details embedded in a JSON object.
/// Omitting bind variables if the query references any will result also result
/// in an `HTTP 400` error.
///
/// @RESTRETURNCODE{404}
/// The server will respond with `HTTP 404` in case a non-existing collection is
/// accessed in the query.
///
/// @EXAMPLES
///
/// Valid query:
///
/// @EXAMPLE_ARANGOSH_RUN{RestExplainValid}
///     var url = "/_api/explain";
///     var cn = "products";
///     db._drop(cn);
///     db._create(cn, { waitForSync: true });
///     body = '{ ' +
///       '"query" : "FOR p IN products FILTER p.id == @id LIMIT 2 RETURN p.name", ' +
///       '"bindVars": { "id" : 3 } ' +
///     '}';
///
///     var response = logCurlRequest('POST', url, body);
///
///     assert(response.code === 200);
///
///     logJsonResponse(response);
/// @END_EXAMPLE_ARANGOSH_RUN
///
/// Invalid query:
///
/// @EXAMPLE_ARANGOSH_RUN{RestExplainInvalid}
///     var url = "/_api/explain";
///     var cn = "products";
///     db._drop(cn);
///     db._create(cn, { waitForSync: true });
///     body = '{ "query" : "FOR p IN products FILTER p.id == @id LIMIT 2 RETURN p.n" }';
///
///     var response = logCurlRequest('POST', url, body);
///
///     assert(response.code === 400);
///
///     logJsonResponse(response);
/// @END_EXAMPLE_ARANGOSH_RUN
///
/// The data returned in the `plan` attribute of the result contains one
/// element per AQL top-level statement (i.e. `FOR`, `RETURN`, 
/// `FILTER` etc.). If the query optimiser removed some unnecessary statements,
/// the result might also contain less elements than there were top-level
/// statements in the AQL query.
/// The following example shows a query with a non-sensible filter condition that
/// the optimiser has removed so that there are less top-level statements:
///
/// @EXAMPLE_ARANGOSH_RUN{RestExplainEmpty}
///     var url = "/_api/explain";
///     var cn = "products";
///     db._drop(cn);
///     db._create(cn, { waitForSync: true });
///     body = '{ "query" : "FOR i IN [ 1, 2, 3 ] FILTER 1 == 2 RETURN i" }';
///
///     var response = logCurlRequest('POST', url, body);
///
///     assert(response.code === 200);
///
///     logJsonResponse(response);
/// @END_EXAMPLE_ARANGOSH_RUN
////////////////////////////////////////////////////////////////////////////////

function post_api_explain (req, res) {
  if (req.suffix.length !== 0) {
    actions.resultNotFound(req, 
                           res, 
                           ERRORS.errors.ERROR_HTTP_NOT_FOUND.code, 
                           ERRORS.errors.ERROR_HTTP_NOT_FOUND.message);
    return;
  }

  var json = actions.getJsonBody(req, res);

  if (json === undefined) {
    return;
  }

  var result = EXPLAIN(json.query, json.bindVars);

  if (result instanceof Error) {
    actions.resultException(req, res, result, undefined, false);
    return;
  }

  result = { "plan" : result };
  actions.resultOk(req, res, actions.HTTP_OK, result);
}

// -----------------------------------------------------------------------------
// --SECTION--                                                       initialiser
// -----------------------------------------------------------------------------

////////////////////////////////////////////////////////////////////////////////
/// @brief explain gateway 
////////////////////////////////////////////////////////////////////////////////

actions.defineHttp({
  url : "_api/explain",
  context : "api",

  callback : function (req, res) {
    try {
      switch (req.requestType) {
        case actions.POST: 
          post_api_explain(req, res); 
          break;

        default:
          actions.resultUnsupported(req, res);
      }
    }
    catch (err) {
      actions.resultException(req, res, err, undefined, false);
    }
  }
});

////////////////////////////////////////////////////////////////////////////////
/// @}
////////////////////////////////////////////////////////////////////////////////

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