# groonga - An open-source fulltext search engine and column store.

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# 8.5. Tables

## 8.5.1. Summary

Table in Groonga manages relation between ID and key. Groonga provides four table types. They are `TABLE_NO_KEY`, `TABLE_HASH_KEY`, `TABLE_PAT_KEY` and `TABLE_DAT_KEY`.

All tables except `TABLE_NO_KEY` provides both fast ID search by key and fast key search by ID. `TABLE_NO_KEY` doesn't support key. `TABLE_NO_KEY` only manages ID. So `TABLE_NO_KEY` doesn't provides ID search and key search.

## 8.5.2. Characteristics

Here is a chracteristic table of all tables in Groonga. (`TABLE_` prefix is omitted in the table.)

| `NO_KEY` | `HASH_KEY` | `PAT_KEY` | `DAT_KEY`
--- | --- | --- | --- | ---
Data structure | Array | Hash table | Patricia trie | Double array trie
ID support | o | o | o | o
Key support | x | o | o | o
Value support | o | o | o | x
Key -> ID speed o: fast x: slow | - | oo | x | o
Update speed o: fast x: slow | ooo | o | o | x
Size o: small x: large | ooo | o | oo | x
Key update | - | x | x | o
Common prefix search | - | x | o | o
Predictive search | - | x | o | o
Range search | - | x | o | o

### 8.5.2.1. `TABLE_NO_KEY`

`TABLE_NO_KEY` is very fast and very small but it doesn't support key. `TABLE_NO_KEY` is a only table that doesn't support key.

You cannot use `TABLE_NO_KEY` for lexicon for fulltext search because lexicon stores tokens as key. `TABLE_NO_KEY` is useful for no key records such as log.

### 8.5.2.2. `TABLE_HASH_KEY`

`TABLE_HASH_KEY` is fast but it doesn't support advanced search functions such as common prefix search and predictive search.

`TABLE_HASH_KEY` is useful for index for exact search such as tag search.

### 8.5.2.3. `TABLE_PAT_KEY`

`TABLE_PAT_KEY` is small and supports advanced search functions.

`TABLE_PAT_KEY` is useful for lexicon for fulltext search and index for range search.

### 8.5.2.4. `TABLE_DAT_KEY`

`TABLE_DAT_KEY` is fast and supports key update but it is large. It is not suitable for storing many records. `TABLE_DAT_KEY` is a only table that supports key update.

`TABLE_DAT_KEY` is used in Groonga database. Groonga database needs to convert object name such as `ShortText`, `TokenBigram` and table names to object ID. And Groonga database needs to rename object name. Those features are implemented by `TABLE_DAT_KEY`. The number of objects is small. So large data size demerit of `TABLE_DAT_KEY` can be ignored.

## 8.5.3. Record ID

Record ID is assigned automatically. You cannot assign record ID.

Record ID of deleted record may be reused.

Valid record ID range is between 1 and 268435455. (1 and 268435455 are valid IDs.)

## 8.5.4. Persistent table and temporary table

Table is persistent table or temporary table.

### 8.5.4.1. Persistent table

Persistent table is named and registered to database. Records in persistent table aren't deleted after closing table or database.

Persistent table can be created by table_create command.

### 8.5.4.2. Temporary table

Temporary table is anonymous. Records in temporary table are deleted after closing table. Temporary table is used to store search result, sort result, group (drilldown) result and so on. `TABLE_HASH_KEY` is used for search result and group result. `TABLE_NO_KEY` is used for sort result.

## 8.5.5. Limitations

The max number of records is 268435455. You cannot add 268435456 or more records in a table.

The max number of a key size is 4096byte. You cannot use 4097byte or larger key. You can use column instead of key for 4097byte or larger size data. `Text` and `LargeText` types supports 4097byte or larger size data.

The max number of total key size is 4GiB. You need to split a table, split a database (sharding) or reduce each key size to handle 4GiB or more larger total key size.

## 8.5.6. See also

- table_create

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