Glossary

Table of contents

Terms
- C
- M
- O
- R
- U

Binary operator

See operation.

C

CLUSTERED BY column

See .

Evaluation

See expression.

Expression

Any valid SQL that produces a value (e.g., , comparison operators, and ) through a process known as evaluation.

Contrary to a statement.

See also

Built-ins: Subquery expressions

Scalar functions: Conditional functions and expressions

Function

A token (e.g., replace) that takes zero or more arguments (e.g., three ), performs a specific task, and may return one or more values (e.g., a modified string). Functions that return more than one value are called multi-valued functions.

Functions may be in an SQL statement, like so:

See also

Scalar functions

Table functions

User-defined functions

Metadata gateway

Persists cluster metadata on disk every time the metadata changes. This data is stored persistently across full cluster restarts and recovered after nodes are started again.

See also

Multi-valued function

A function that returns two or more values.

See also

Window functions

N

Nonscalar

Contrary to a .

See also

Geographic types

Operand

See operator.

Operation

See .

Operator

A reserved keyword (e.g., IN) or sequence of symbols (e.g., ) that can be used in an SQL statement to manipulate one or more expressions and return a result (e.g., or ). This process is known as an operation and the expressions can be called operands or arguments.

An operator that takes one operand is known as a unary operator and an operator that takes two is known as a binary operator.

See also

Comparison operators

Partition column

A column used to partition a table. Specified by the .

Also known as a PARTITIONED BY column or partitioned column.

A table may be partitioned by one or more columns:

If a table is partitoned by multiple columns, a new partition is created for every unique combination of row values in those partition columns

See also

Data definition: Partitioned tables

CREATE TABLE: PARTITIONED BY clause

REFRESH: PARTITION clause

COPY TO: PARTITION clause

CREATE SNAPSHOT: PARTITION clause

PARTITIONED BY column

See partition column.

Partitioned column

See .

Regular expression

An expression used to search for patterns in a .

See also

Wikipedia: Regular expression

Querying: Regular expressions

Table functions: regexp_matches

Routing column

Values in this column are used to compute a hash which is then used to route the corresponding row to a specific shard.

Also known as the CLUSTERED BY column.

All rows that have the same routing column row value are stored in the same shard.

The routing of rows to a specific shard is not the same as the routing of shards to a specific node (also known as ).

CREATE TABLE: CLUSTERED clause

S

Scalar

A with a single value (e.g., numbers and ).

Contrary to a nonscalar.

See also

Shard allocation

The process by which CrateDB allocates shards to a specific nodes.

Note

Shard allocation is sometimes referred to as shard routing, which is not to be confused with row routing.

Altering tables: Changing the number of shards

Shard recovery

The process by which CrateDB synchronizes a replica shard from a primary shard.

Shard recovery can happen during node startup, after node failure, when replicating a primary shard, when moving a shard to another node (i.e., when rebalancing the cluster), or during .

A shard that is being recovered cannot be queried until the recovery process is complete.

See also

Cluster settings: Recovery

Shard routing

See shard allocation.

Statement

Any valid SQL that serves as a database instruction (e.g., , INSERT, and ) instead of producing a value.

Contrary to an expression.

See also

Data manipulation

SQL Statements

Subquery