arangodb Facet计算/聚合缓慢吗？

Question

I wonder why the following facet calculation takes so slow:

FOR q IN LRQ  
    COLLECT profile = q.LongRunningQuery.Profile INTO profiles 
RETURN { "Profile" : profile, "Count" : LENGTH(profiles)} 

It takes about 30 seconds, although only 5.000 documents are in the db, and only 30 different facets are in the result.

The field LongRunningQuery.Profile is indexed with a hash index, and with a skiplist index. (I also tried with different combinations of them).

Has anybody a hint for me what might go wrong? Is it possible that the queries do not benefit from the indexes? (the 5.000 records are about 1 GB of size, so I assume the hash index will not be used, maybe a fulltable scan instead?)

Interestingly, the following alternative only lasts 2 seconds:

FOR q IN SKIPLIST(LRQ, { "LongRunningQuery.Profile": [ [ '>',  ''  ] ] })[*].LongRunningQuery.Profile
    COLLECT profile = q INTO profiles
RETURN { "Profile" : profile, "Count" : LENGTH(profiles) } 

But it still needs 2 seconds - for such a small amount of records. Here it looks like the skiplist index is used, but it is maybe not the perfect index variant.

---

Update 2014-11-27:

arangosh [_system]> stmt._query
    FOR q IN LRQ COLLECT profile = q.LongRunningQuery.Profile INTO profiles RETURN {
     "Profile" : profile, "Count" : LENGTH(profiles)}

arangosh [_system]> db.LRQ.ensureHashIndex("LongRunningQuery.Profile");
{
  "id" : "LRQ/296017913256",
  "type" : "hash",
  "unique" : false,
  "fields" : [
    "LongRunningQuery.Profile"
  ],
  "isNewlyCreated" : false,
  "error" : false,
  "code" : 200
}

The query took about 32 seconds and returned 31 short results.

Execution plan:

    {
        "plan": {
            "nodes": [
                {
                    "type": "SingletonNode",
                    "dependencies": [],
                    "id": 1,
                    "estimatedCost": 1,
                    "estimatedNrItems": 1
                },
                {
                    "type": "EnumerateCollectionNode",
                    "dependencies": [
                        1
                    ],
                    "id": 2,
                    "estimatedCost": 5311,
                    "estimatedNrItems": 5310,
                    "database": "_system",
                    "collection": "LRQ",
                    "outVariable": {
                        "id": 0,
                        "name": "q"
                    }
                },
                {
                    "type": "CalculationNode",
                    "dependencies": [
                        2
                    ],
                    "id": 3,
                    "estimatedCost": 10621,
                    "estimatedNrItems": 5310,
                    "expression": {
                        "type": "attribute access",
                        "name": "Profile",
                        "subNodes": [
                            {
                                "type": "attribute access",
                                "name": "LongRunningQuery",
                                "subNodes": [
                                    {
                                        "type": "reference",
                                        "name": "q",
                                        "id": 0
                                    }
                                ]
                            }
                        ]
                    },
                    "outVariable": {
                        "id": 3,
                        "name": "3"
                    },
                    "canThrow": false
                },
                {
                    "type": "SortNode",
                    "dependencies": [
                        3
                    ],
                    "id": 4,
                    "estimatedCost": 56166.713176593075,
                    "estimatedNrItems": 5310,
                    "elements": [
                        {
                            "inVariable": {
                                "id": 3,
                                "name": "3"
                            },
                            "ascending": true
                        }
                    ],
                    "stable": true
                },
                {
                    "type": "AggregateNode",
                    "dependencies": [
                        4
                    ],
                    "id": 5,
                    "estimatedCost": 61476.713176593075,
                    "estimatedNrItems": 5310,
                    "aggregates": [
                        {
                            "outVariable": {
                                "id": 1,
                                "name": "profile"
                            },
                            "inVariable": {
                                "id": 3,
                                "name": "3"
                            }
                        }
                    ],
                    "outVariable": {
                        "id": 2,
                        "name": "profiles"
                    }
                },
                {
                    "type": "CalculationNode",
                    "dependencies": [
                        5
                    ],
                    "id": 6,
                    "estimatedCost": 66786.71317659307,
                    "estimatedNrItems": 5310,
                    "expression": {
                        "type": "array",
                        "subNodes": [
                            {
                                "type": "array element",
                                "name": "Profile",
                                "subNodes": [
                                    {
                                        "type": "reference",
                                        "name": "profile",
                                        "id": 1
                                    }
                                ]
                            },
                            {
                                "type": "array element",
                                "name": "Count",
                                "subNodes": [
                                    {
                                        "type": "function call",
                                        "name": "LENGTH",
                                        "subNodes": [
                                            {
                                                "type": "list",
                                                "subNodes": [
                                                    {
                                                        "type": "reference",
                                                        "name": "profiles",
                                                        "id": 2
                                                    }
                                                ]
                                            }
                                        ]
                                    }
                                ]
                            }
                        ]
                    },
                    "outVariable": {
                        "id": 4,
                        "name": "4"
                    },
                    "canThrow": false
                },
                {
                    "type": "ReturnNode",
                    "dependencies": [
                        6
                    ],
                    "id": 7,
                    "estimatedCost": 72096.71317659307,
                    "estimatedNrItems": 5310,
                    "inVariable": {
                        "id": 4,
                        "name": "4"
                    }
                }
            ],
            "rules": [],
            "collections": [
                {
                    "name": "LRQ",
                    "type": "read"
                }
            ],
            "variables": [
                {
                    "id": 0,
                    "name": "q"
                },
                {
                    "id": 1,
                    "name": "profile"
                },
                {
                    "id": 4,
                    "name": "4"
                },
                {
                    "id": 2,
                    "name": "profiles"
                },
                {
                    "id": 3,
                    "name": "3"
                }
            ],
            "estimatedCost": 72096.71317659307,
            "estimatedNrItems": 5310
        },
        "warnings": []
    }

Update 2014-12-05:

Here are additional measures: Understood, thanks. Here's the output:

Execution of AQL_EXECUTE('FOR q IN LRQ FILTER q.LongRunningQuery.Profile == "Admin" LIMIT 1 RETURN q.LongRunningQuery.Profile', {}, { profile : true }).profile --> { "initializing" : 0, "parsing" : 0, "optimizing ast" : 15.364980936050415, "instanciating plan" : 0, "optimizing plan" : 0, "executing" : 0 }

Execution of AQL_EXECUTE('FOR q IN LRQ COLLECT profile = q.LongRunningQuery.Profile INTO profiles RETURN { "Profile" : profile, "Count" : LENGTH(profiles)}', {}, { profile : true }).profile --> { "initializing" : 0, "parsing" : 0, "optimizing ast" : 0, "instanciating plan" : 0, "optimizing plan" : 0, "executing" : 77.88313102722168 }

Update 19.12.2014:

Since 2.3.2 the execution plan for the query arangosh [_system]> stmt2 = db._createStatement('FOR q IN LRQ COLLECT profile = q.LongRunningQuery.Profile INTO profiles RETURN { "Profile" : profile, "Count" : LENGTH(profiles)} ')

looks like this:

arangosh [_system]> stmt2.explain()
{
  "plan" : {
    "nodes" : [
      {
        "type" : "SingletonNode",
        "dependencies" : [ ],
        "id" : 1,
        "estimatedCost" : 1,
        "estimatedNrItems" : 1
      },
      {
        "type" : "IndexRangeNode",
        "dependencies" : [
          1
        ],
        "id" : 8,
        "estimatedCost" : 5311,
        "estimatedNrItems" : 5310,
        "database" : "_system",
        "collection" : "LRQ",
        "outVariable" : {
          "id" : 0,
          "name" : "q"
        },
        "ranges" : [
          [ ]
        ],
        "index" : {
          "type" : "skiplist",
          "id" : "530975525379",
          "unique" : false,
          "fields" : [
            "LongRunningQuery.Profile"
          ]
        },
        "reverse" : false
      },
      {
        "type" : "CalculationNode",
        "dependencies" : [
          8
        ],
        "id" : 3,
        "estimatedCost" : 10621,
        "estimatedNrItems" : 5310,
        "expression" : {
          "type" : "attribute access",
          "name" : "Profile",
          "subNodes" : [
            {
              "type" : "attribute access",
              "name" : "LongRunningQuery",
              "subNodes" : [
                {
                  "type" : "reference",
                  "name" : "q",
                  "id" : 0
                }
              ]
            }
          ]
        },
        "outVariable" : {
          "id" : 3,
          "name" : "3"
        },
        "canThrow" : false
      },
      {
        "type" : "AggregateNode",
        "dependencies" : [
          3
        ],
        "id" : 5,
        "estimatedCost" : 15931,
        "estimatedNrItems" : 5310,
        "aggregates" : [
          {
            "outVariable" : {
              "id" : 1,
              "name" : "profile"
            },
            "inVariable" : {
              "id" : 3,
              "name" : "3"
            }
          }
        ],
        "outVariable" : {
          "id" : 2,
          "name" : "profiles"
        }
      },
      {
        "type" : "CalculationNode",
        "dependencies" : [
          5
        ],
        "id" : 6,
        "estimatedCost" : 21241,
        "estimatedNrItems" : 5310,
        "expression" : {
          "type" : "array",
          "subNodes" : [
            {
              "type" : "array element",
              "name" : "Profile",
              "subNodes" : [
                {
                  "type" : "reference",
                  "name" : "profile",
                  "id" : 1
                }
              ]
            },
            {
              "type" : "array element",
              "name" : "Count",
              "subNodes" : [
                {
                  "type" : "function call",
                  "name" : "LENGTH",
                  "subNodes" : [
                    {
                      "type" : "list",
                      "subNodes" : [
                        {
                          "type" : "reference",
                          "name" : "profiles",
                          "id" : 2
                        }
                      ]
                    }
                  ]
                }
              ]
            }
          ]
        },
        "outVariable" : {
          "id" : 4,
          "name" : "4"
        },
        "canThrow" : false
      },
      {
        "type" : "ReturnNode",
        "dependencies" : [
          6
        ],
        "id" : 7,
        "estimatedCost" : 26551,
        "estimatedNrItems" : 5310,
        "inVariable" : {
          "id" : 4,
          "name" : "4"
        }
      }
    ],
    "rules" : [
      "use-index-for-sort"
    ],
    "collections" : [
      {
        "name" : "LRQ",
        "type" : "read"
      }
    ],
    "variables" : [
      {
        "id" : 0,
        "name" : "q"
      },
      {
        "id" : 1,
        "name" : "profile"
      },
      {
        "id" : 4,
        "name" : "4"
      },
      {
        "id" : 2,
        "name" : "profiles"
      },
      {
        "id" : 3,
        "name" : "3"
      }
    ],
    "estimatedCost" : 26551,
    "estimatedNrItems" : 5310
  },
  "warnings" : [ ],
  "stats" : {
    "rulesExecuted" : 25,
    "rulesSkipped" : 0,
    "plansCreated" : 1
  }
}

Answer 1

hm, looking at the explain there is a sortnode, while your query doesn't provide a sort? the collect probably keeps the optimizer from employing your index (you would have an IndexRangeNode instead of a EnumerateCollectionNode then)

If you pass the options parameter of the query (4th parameter of db._query()) { profile : true } it will output the time used by the phases; can you re-run your query with that, and show us the reply?

Answer 2

The COLLECT statement requires sorted input. Therefore, a SORT statement will be added to the execution plan automatically, even if the original query string does not contain an explicit SORT statement.

This is why a SortNode appeared in the plan. The SortNode will be optimized away if there is a skiplist index on the sort attribute (in this case LongRunningQuery.Profile ). So adding a skiplist index on the attribute will speed it up as the (expensive) sort step can be spared.

If you have set up such index and run the query, it should be faster than when there is only a hash index. In fact, the original query should have ignored the hash index.

If you have set up the skiplist index and explain the query, you should also see that there is no SortNode anymore.

Starting with ArangoDB 2.4 (currently in devel stage), there is a more efficient syntax addition for just counting facets:

FOR q IN LRQ  
  COLLECT profile = q.LongRunningQuery.Profile WITH COUNT INTO numProfiles
  RETURN { "Profile" : profile, "Count" : numProfiles } 

This should speed up the query even more.