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# 4.6. Tag search and reverse resolution of reference relationships

As you know, Groonga supports to store array in column which refers other table. In fact, you can do tag search by using array data which refers other table.

Tag search is very fast because Groonga use inverted index as data structure.

## 4.6.1. Tag search

Let’s consider to create a search engine for an web site to share movies. Each movie may be associated with multiple keywords which represents the content of movie.

Let’s create tables for movie information, then search the movies.

First, create the `Video` table which stores movie information. the `Video` table has two columns. the `title` column stores title of the movie. the `tags` column stores multiple tag information in reference Tag table.

Next, create the `Tag` table which stores tag information. the `Tag` table has one column. The tag string is stored as primary key, then `index_tags` stores indexes for tags column of Video table.

Execution example:

    table_create --name Video --flags TABLE_HASH_KEY --key_type UInt32
    # [[0,1337566253.89858,0.000355720520019531],true]
    table_create --name Tag --flags TABLE_HASH_KEY --key_type ShortText
    # [[0,1337566253.89858,0.000355720520019531],true]
    column_create --table Video --name title --flags COLUMN_SCALAR --type ShortText
    # [[0,1337566253.89858,0.000355720520019531],true]
    column_create --table Video --name tags --flags COLUMN_VECTOR --type Tag
    # [[0,1337566253.89858,0.000355720520019531],true]
    column_create --table Tag --name index_tags --flags COLUMN_INDEX --type Video --source tags
    # [[0,1337566253.89858,0.000355720520019531],true]
    load --table Video
    [
    {"_key":1,"title":"Soccer 2010","tags":["Sports","Soccer"]},
    {"_key":2,"title":"Zenigata Kinjirou","tags":["Variety","Money"]},
    {"_key":3,"title":"groonga Demo","tags":["IT","Server","groonga"]},
    {"_key":4,"title":"Moero!! Ultra Baseball","tags":["Sports","Baseball"]},
    {"_key":5,"title":"Hex Gone!","tags":["Variety","Quiz"]},
    {"_key":6,"title":"Pikonyan 1","tags":["Animation","Pikonyan"]},
    {"_key":7,"title":"Draw 8 Month","tags":["Animation","Raccoon"]},
    {"_key":8,"title":"K.O.","tags":["Animation","Music"]}
    ]
    # [[0,1337566253.89858,0.000355720520019531],8]

After creating indexed column, you can do full-text search very fast. The indexed column is also automatically updated when stored data is refreshed.

List up the movies that specific keywords are given.

Execution example:

    select --table Video --query tags:@Variety --output_columns _key,title
    # [
    #   [
    #     0,
    #     1337566253.89858,
    #     0.000355720520019531
    #   ],
    #   [
    #     [
    #       [
    #         2
    #       ],
    #       [
    #         [
    #           "_key",
    #           "UInt32"
    #         ],
    #         [
    #           "title",
    #           "ShortText"
    #         ]
    #       ],
    #       [
    #         2,
    #         "Zenigata Kinjirou"
    #       ],
    #       [
    #         5,
    #         "Hex Gone!"
    #       ]
    #     ]
    #   ]
    # ]
    select --table Video --query tags:@Sports --output_columns _key,title
    # [
    #   [
    #     0,
    #     1337566253.89858,
    #     0.000355720520019531
    #   ],
    #   [
    #     [
    #       [
    #         2
    #       ],
    #       [
    #         [
    #           "_key",
    #           "UInt32"
    #         ],
    #         [
    #           "title",
    #           "ShortText"
    #         ]
    #       ],
    #       [
    #         1,
    #         "Soccer 2010"
    #       ],
    #       [
    #         4,
    #         "Moero!! Ultra Baseball"
    #       ]
    #     ]
    #   ]
    # ]
    select --table Video --query tags:@Animation --output_columns _key,title
    # [
    #   [
    #     0,
    #     1337566253.89858,
    #     0.000355720520019531
    #   ],
    #   [
    #     [
    #       [
    #         3
    #       ],
    #       [
    #         [
    #           "_key",
    #           "UInt32"
    #         ],
    #         [
    #           "title",
    #           "ShortText"
    #         ]
    #       ],
    #       [
    #         6,
    #         "Pikonyan 1"
    #       ],
    #       [
    #         7,
    #         "Draw 8 Month"
    #       ],
    #       [
    #         8,
    #         "K.O."
    #       ]
    #     ]
    #   ]
    # ]

You can search by tags such as “Variety”, “Sports” and “Animation”.

## 4.6.2. Reverse resolution of reference relationships

Groonga supports indexes for reverse resolution among tables. Tag search is one of concrete examples.

For example, you can search friendships by reverse resolution in social networking site.

Following example shows how to create `User` table which stores user information, `username` column which stores user name, `friends` column which stores list of user’s friends in array, `index_friends` column as indexed column.

Execution example:

    table_create --name User --flags TABLE_HASH_KEY --key_type ShortText
    # [[0,1337566253.89858,0.000355720520019531],true]
    column_create --table User --name username --flags COLUMN_SCALAR --type ShortText
    # [[0,1337566253.89858,0.000355720520019531],true]
    column_create --table User --name friends --flags COLUMN_VECTOR --type User
    # [[0,1337566253.89858,0.000355720520019531],true]
    column_create --table User --name index_friends --flags COLUMN_INDEX --type User --source friends
    # [[0,1337566253.89858,0.000355720520019531],true]
    load --table User
    [
    {"_key":"ken","username":"健作","friends":["taro","jiro","tomo","moritapo"]}
    {"_key":"moritapo","username":"森田","friends":["ken","tomo"]}
    {"_key":"taro","username":"ぐるんが太郎","friends":["jiro","tomo"]}
    {"_key":"jiro","username":"ぐるんが次郎","friends":["taro","tomo"]}
    {"_key":"tomo","username":"トモちゃん","friends":["ken","hana"]}
    {"_key":"hana","username":"花子","friends":["ken","taro","jiro","moritapo","tomo"]}
    ]
    # [[0,1337566253.89858,0.000355720520019531],6]

Let’s show list of users who contains specified user in friend list.

Execution example:

    select --table User --query friends:@tomo --output_columns _key,username
    # [
    #   [
    #     0,
    #     1337566253.89858,
    #     0.000355720520019531
    #   ],
    #   [
    #     [
    #       [
    #         5
    #       ],
    #       [
    #         [
    #           "_key",
    #           "ShortText"
    #         ],
    #         [
    #           "username",
    #           "ShortText"
    #         ]
    #       ],
    #       [
    #         "ken",
    #         "健作"
    #       ],
    #       [
    #         "taro",
    #         "ぐるんが太郎"
    #       ],
    #       [
    #         "jiro",
    #         "ぐるんが次郎"
    #       ],
    #       [
    #         "moritapo",
    #         "森田"
    #       ],
    #       [
    #         "hana",
    #         "花子"
    #       ]
    #     ]
    #   ]
    # ]
    select --table User --query friends:@jiro --output_columns _key,username
    # [
    #   [
    #     0,
    #     1337566253.89858,
    #     0.000355720520019531
    #   ],
    #   [
    #     [
    #       [
    #         3
    #       ],
    #       [
    #         [
    #           "_key",
    #           "ShortText"
    #         ],
    #         [
    #           "username",
    #           "ShortText"
    #         ]
    #       ],
    #       [
    #         "ken",
    #         "健作"
    #       ],
    #       [
    #         "taro",
    #         "ぐるんが太郎"
    #       ],
    #       [
    #         "hana",
    #         "花子"
    #       ]
    #     ]
    #   ]
    # ]

Then drilldown the count which shows user is listed as friend.

Execution example:

    select --table User --limit 0 --drilldown friends
    # [
    #   [
    #     0,
    #     1337566253.89858,
    #     0.000355720520019531
    #   ],
    #   [
    #     [
    #       [
    #         6
    #       ],
    #       [
    #         [
    #           "_id",
    #           "UInt32"
    #         ],
    #         [
    #           "_key",
    #           "ShortText"
    #         ],
    #         [
    #           "friends",
    #           "User"
    #         ],
    #         [
    #           "index_friends",
    #           "UInt32"
    #         ],
    #         [
    #           "username",
    #           "ShortText"
    #         ]
    #       ]
    #     ],
    #     [
    #       [
    #         6
    #       ],
    #       [
    #         [
    #           "_key",
    #           "ShortText"
    #         ],
    #         [
    #           "_nsubrecs",
    #           "Int32"
    #         ]
    #       ],
    #       [
    #         "taro",
    #         3
    #       ],
    #       [
    #         "jiro",
    #         3
    #       ],
    #       [
    #         "tomo",
    #         5
    #       ],
    #       [
    #         "moritapo",
    #         2
    #       ],
    #       [
    #         "ken",
    #         3
    #       ],
    #       [
    #         "hana",
    #         1
    #       ]
    #     ]
    #   ]
    # ]

As you can see, it shows the results which follows reverse resolution of reference relationship.

## 4.6.3. Geo location search with index

Groonga supports to add indexes to the column which stores geo location information. Groonga is very fast because it use such indexes against the column which contains geo location information to search enormous number of records.

Execution example:

    table_create --name GeoSite --flags TABLE_HASH_KEY --key_type ShortText
    # [[0,1337566253.89858,0.000355720520019531],true]
    column_create --table GeoSite --name location --type WGS84GeoPoint
    # [[0,1337566253.89858,0.000355720520019531],true]
    table_create --name GeoIndex --flags TABLE_PAT_KEY --key_type WGS84GeoPoint
    # [[0,1337566253.89858,0.000355720520019531],true]
    column_create --table GeoIndex --name index_point --type GeoSite --flags COLUMN_INDEX --source location
    # [[0,1337566253.89858,0.000355720520019531],true]
    load --table GeoSite
    [
     {"_key":"http://example.org/","location":"128452975x503157902"},
     {"_key":"http://example.net/","location":"128487316x502920929"}
    ]
    # [[0,1337566253.89858,0.000355720520019531],2]
    select --table GeoSite --filter 'geo_in_circle(location, "128515259x503187188", 5000)' --output_columns _key,location
    # [
    #   [
    #     0,
    #     1337566253.89858,
    #     0.000355720520019531
    #   ],
    #   [
    #     [
    #       [
    #         1
    #       ],
    #       [
    #         [
    #           "_key",
    #           "ShortText"
    #         ],
    #         [
    #           "location",
    #           "WGS84GeoPoint"
    #         ]
    #       ],
    #       [
    #         "http://example.org/",
    #         "128452975x503157902"
    #       ]
    #     ]
    #   ]
    # ]

These indexes are also used when sorting the records with geo location search.

Execution example:

    select --table GeoSite --filter 'geo_in_circle(location, "128515259x503187188", 50000)' --output_columns _key,location,_score --sort_keys '-geo_distance(location, "128515259x503187188")' --scorer '_score = geo_distance(location, "128515259x503187188")'
    # [
    #   [
    #     0,
    #     1337566253.89858,
    #     0.000355720520019531
    #   ],
    #   [
    #     [
    #       [
    #         2
    #       ],
    #       [
    #         [
    #           "_key",
    #           "ShortText"
    #         ],
    #         [
    #           "location",
    #           "WGS84GeoPoint"
    #         ],
    #         [
    #           "_score",
    #           "Int32"
    #         ]
    #       ],
    #       [
    #         "http://example.org/",
    #         "128452975x503157902",
    #         2054
    #       ],
    #       [
    #         "http://example.net/",
    #         "128487316x502920929",
    #         6720
    #       ]
    #     ]
    #   ]
    # ]