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# 7.19. Alias

Added in version 5.1.2.

You can refer a table and column by multiple names by using alias feature.

## 7.19.1. Summary

The alias feature is useful for the following cases:

> - You want to rename a table but you can’t change some Groonga clients that uses the current table name.

> - You want to change column type without downtime.

In the former case, some Groonga clients can use the current table name after you rename a table. Because the alias feature maps the current table name to the renamed new table name.

In the latter case, all Groonga clients access the column by aliased name such as `aliased_column`. `aliased_column` refers `current_column`. You create a new column `new_column` with new type and copy data from `current_column` by column_copy. You change `aliased_column` to refer `new_column` from `current_column`. Now, all Groonga clients access `new_column` by `aliased_column` without stopping search requests.

## 7.19.2. Usage

You manage alias to real name mapping by a normal table and a normal column.

You can use any table type except TABLE_NO_KEY for the table. TABLE_HASH_KEY is recommended because exact key match search is only used for the alias feature. TABLE_HASH_KEY is the fastest table type for exact key match search.

The column must be Scalar column and type is `ShortText`. You can also use `Text` and `LongText` types but they are meaningless. Because the max table/column name size is 4KiB. `ShortText` can store 4KiB data.

Here are example definitions of table and column for managing aliases:

Execution example:

    table_create Aliases TABLE_HASH_KEY ShortText
    # [[0,1337566253.89858,0.000355720520019531],true]
    column_create Aliases real_name COLUMN_SCALAR ShortText
    # [[0,1337566253.89858,0.000355720520019531],true]

You need to register the table and column by Configuration. The alias feature uses `alias.column` configuration item. You can register the table and column by the following config_set:

Execution example:

    config_set alias.column Aliases.real_name
    # [[0,1337566253.89858,0.000355720520019531],true]

Here are schema and data to show how to use alias:

Execution example:

    table_create Users TABLE_HASH_KEY ShortText
    # [[0,1337566253.89858,0.000355720520019531],true]
    column_create Users age COLUMN_SCALAR UInt8
    # [[0,1337566253.89858,0.000355720520019531],true]
    load --table Users
    [
    {"_key": "alice", "age": 14},
    {"_key": "bob",   "age": 29}
    ]
    # [[0,1337566253.89858,0.000355720520019531],2]

You can use `Users.age` in select:

Execution example:

    select Users --filter 'age < 20'
    # [
    #   [
    #     0,
    #     1337566253.89858,
    #     0.000355720520019531
    #   ],
    #   [
    #     [
    #       [
    #         1
    #       ],
    #       [
    #         [
    #           "_id",
    #           "UInt32"
    #         ],
    #         [
    #           "_key",
    #           "ShortText"
    #         ],
    #         [
    #           "age",
    #           "UInt8"
    #         ]
    #       ],
    #       [
    #         1,
    #         "alice",
    #         14
    #       ]
    #     ]
    #   ]
    # ]

You can’t use `Users.age` when you rename `Users.age` to `Users.years` by column_rename:

Execution example:

    column_rename Users age years
    # [[0,1337566253.89858,0.000355720520019531],true]
    select Users --filter 'age < 20'
    # [
    #   [
    #     -63,
    #     1337566253.89858,
    #     0.000355720520019531,
    #     "Syntax error: <age| |< 20>: [expr][parse] unknown identifier: <age>",
    #     [
    #       [
    #         "yy_syntax_error",
    #         "grn_ecmascript.lemon",
    #         2929
    #       ]
    #     ]
    #   ]
    # ]

But you can use `Users.age` by registering `Users.age` to `Users.years` mapping to `Aliases`.

Execution example:

    load --table Aliases
    [
    {"_key": "Users.age", "real_name": "Users.years"}
    ]
    # [[0,1337566253.89858,0.000355720520019531],1]
    select Users --filter 'age < 20'
    # [
    #   [
    #     0,
    #     1337566253.89858,
    #     0.000355720520019531
    #   ],
    #   [
    #     [
    #       [
    #         1
    #       ],
    #       [
    #         [
    #           "_id",
    #           "UInt32"
    #         ],
    #         [
    #           "_key",
    #           "ShortText"
    #         ],
    #         [
    #           "years",
    #           "UInt8"
    #         ]
    #       ],
    #       [
    #         1,
    #         "alice",
    #         14
    #       ]
    #     ]
    #   ]
    # ]

Now, you can use `Users.age` as alias of `Users.years`.

## 7.19.3. How to resolve alias

This section describes how to resolve alias.

Groonga uses the alias feature when nonexistent object name (table name, column name, command name, function name and so on) is referred. It means that you can’t override existing object (table, column, command, function and so on) by the alias feature.

For example, alias isn’t resolved in the following example because `Users.years` exists:

Execution example:

    column_rename Users years years_old
    # [[0,1337566253.89858,0.000355720520019531],true]
    select Users --filter 'age < 20'
    # [
    #   [
    #     0,
    #     1337566253.89858,
    #     0.000355720520019531
    #   ],
    #   [
    #     [
    #       [
    #         1
    #       ],
    #       [
    #         [
    #           "_id",
    #           "UInt32"
    #         ],
    #         [
    #           "_key",
    #           "ShortText"
    #         ],
    #         [
    #           "years_old",
    #           "UInt8"
    #         ]
    #       ],
    #       [
    #         1,
    #         "alice",
    #         14
    #       ]
    #     ]
    #   ]
    # ]

Alias is resolved recursively. If you rename `Users.years` to `Users.years_old` and you refer `Users.age`, Groonga replaces `Users.age` with `Users.years` and then `Users.years` with `Users.years_old`. Because `Aliases` table has the following records:

`_key` | `real_name`
--- | ---
`Users.age` | `Users.years`
`Users.years` | `Users.years_old`

Here is an example to `Users.age` is resolved recursively:

Execution example:

    column_rename Users years years_old
    # [[0,1337566253.89858,0.000355720520019531],true]
    select Users --filter 'age < 20'
    # [
    #   [
    #     0,
    #     1337566253.89858,
    #     0.000355720520019531
    #   ],
    #   [
    #     [
    #       [
    #         1
    #       ],
    #       [
    #         [
    #           "_id",
    #           "UInt32"
    #         ],
    #         [
    #           "_key",
    #           "ShortText"
    #         ],
    #         [
    #           "years_old",
    #           "UInt8"
    #         ]
    #       ],
    #       [
    #         1,
    #         "alice",
    #         14
    #       ]
    #     ]
    #   ]
    # ]

## 7.19.4. See also

> - Configuration

> - config_set

> - table_create

> - column_create

> - select