System ColumnsΒΆ

On every user table CrateDB implements several implicitly defined system columns. Their names are reserved and cannot be used as user-defined column names. All system columns are prefixed with an underscore, consist of lowercase letters and might contain underscores in between.

_version
CrateDB uses an internal versioning for every row, the version number is increased on every write.

Note

Using the _version column for Optimistic Concurrency Control has been deprecated in favour of using the _seq_no and _primary_term. See Optimistic Concurrency Control for usage details.

_seq_no
The CrateDB primary shards will increment a sequence number for every insert, update and delete operation executed against a row. The current sequence number of a row is exposed under this column. This column can be used in conjunction with the _primary_term column for Optimistic Concurrency Control, see Optimistic Concurrency Control for usage details.
_primary_term
The CrateDB master will assign each primary shard a term value. This value is incremented with every primary shard assignment. Used in conjunction with _seq_no we can obtain a total order of operations across shards and Optimistic Concurrency Control.
_score

This internal system column is available on all documents retrieved by a SELECT query. It is representing the scoring ratio of the document related to the used query filter and makes most sense on fulltext searches.

The scoring ratio is always related to the highest score determined by a search, thus scores are not directly comparable across searches.

If the query does not include a fulltext search the value is 1.0f in most cases.

_id

_id is an internal system column that is available on each indexed document and can be retrieved by a SELECT query from doc schema tables.

The value is a unique identifier for each row in a table and is a compound string representation of all primary key values of that row. If no primary keys are defined the id is randomly generated. If no dedicated routing column is defined the _id value is used for distributing the records on the shards.

_docid
_docid exposes the internal id a document has within a Lucene segment. Although the id is unique within a segment, it is not unique across segments or shards and can change the value in case segments are merged.