Data Definition

This reference covers CQL specification version 3.3.1

CQL stores data in tables, whose schema defines the layout of said data in the table, and those tables are grouped in keyspaces. A keyspace defines a number of options that applies to all the tables it contains, most prominently of which is the replication strategy used by the keyspace. It is generally encouraged to use one keyspace by application, and thus many cluster may define only one keyspace. CQL stores data in tables, whose schema defines the layout of data in the table, and tables are grouped in keyspaces. A keyspace defines a number of options that apply to all of the tables it contains, most prominently of which is the replication strategy used by the keyspace. We generally encourage you to use one keyspace per application. Many clusters may thus define only one keyspace.

This section describes the statements used to create, modify, and remove keyspaces and tables.

CREATE KEYSPACE

USE

ALTER KEYSPACE

DROP KEYSPACE

CREATE TABLE

ALTER TABLE

DROP TABLE

TRUNCATE

Common Definitions

Keyspace and table names are defined by the following grammar:

keyspace_name: `name`
table_name: [ `keyspace_name` '.' ] `name`
name: `unquoted_name` | `quoted_name`
unquoted_name: re('[a-zA-Z_0-9]{1, 48}')
quoted_name: '"' `unquoted_name` '"'

Both keyspace and table names consist of only alphanumeric characters, cannot be empty and are limited in size to 48 characters (that limit exists mostly to avoid filenames, which may include the keyspace and table name, to go over the limits of certain file systems). By default, keyspace and table names are case insensitive (myTable is equivalent to mytable) but case sensitivity can be forced by using double-quotes ("myTable" is different from mytable).

Further, a table is always part of a keyspace and a table name can be provided fully-qualified by the keyspace it is part of. If is is not fully-qualified, the table is assumed to be in the current keyspace (see USE statement).

Further, valid column names are simply defined as:

column_name: `identifier`

We also define the notion of statement options for use in the following section:

options: `option` ( AND `option` )*
option: `identifier` '=' ( `identifier` | `constant` | `map_literal` )

CREATE KEYSPACE

A keyspace is created using a CREATE KEYSPACE statement:

create_keyspace_statement: CREATE KEYSPACE [ IF NOT EXISTS ] `keyspace_name` WITH `options`

For instance:

CREATE KEYSPACE Excelsior
WITH replication = {'class': 'SimpleStrategy', 'replication_factor' : 3};

CREATE KEYSPACE Excalibur
WITH replication = {'class': 'NetworkTopologyStrategy', 'DC1' : 1, 'DC2' : 3}
AND durable_writes = false;

The supported options are:

name kind mandatory default description
replication map yes   The replication strategy and options to use for the keyspace (see details below).
durable_writes simple no true Whether to use the commit log for updates on this keyspace (disable this option at your own risk!).

The replication property is mandatory and must at least contains the 'class' sub-option which defines the replication strategy class to use. The rest of the sub-options depend on what replication strategy is used. By default, Scylla support the following 'class':

  • 'SimpleStrategy': defines a replication factor for the whole cluster. The only sub-options supported is 'replication_factor' to define that replication factor and is mandatory.
  • 'NetworkTopologyStrategy': A replication strategy that lets you set the replication factor independently for each data-center. The rest of the sub-options are key-value pairs where a key is a data-center name and its value is the associated replication factor.

Attempting to create a keyspace that already exists will return an error unless the IF NOT EXISTS option is used. If it is used, the statement will be a no-op if the keyspace already exists.

USE

The USE statement allows you to change the current keyspace (for the connection on which it is executed). A number of objects in CQL are bound to a keyspace (tables, user-defined types, functions, …) and the current keyspace is the default keyspace used when those objects are referred without a fully-qualified name (that is, without being prefixed a keyspace name). A USE statement simply takes the keyspace to use as current as argument:

use_statement: USE `keyspace_name`

ALTER KEYSPACE

An ALTER KEYSPACE statement lets you modify the options of a keyspace:

alter_keyspace_statement: ALTER KEYSPACE `keyspace_name` WITH `options`

For instance:

ALTER KEYSPACE Excelsior
          WITH replication = {'class': 'SimpleStrategy', 'replication_factor' : 4};

The supported options are the same than for creating a keyspace.

DROP KEYSPACE

Dropping a keyspace can be done using the DROP KEYSPACE statement:

drop_keyspace_statement: DROP KEYSPACE [ IF EXISTS ] `keyspace_name`

For instance:

DROP KEYSPACE Excelsior;

Dropping a keyspace results in the immediate, irreversible removal of that keyspace, including all the tables, UTD and functions in it, and all the data contained in those tables.

If the keyspace does not exist, the statement will return an error, unless IF EXISTS is used in which case the operation is a no-op.

CREATE TABLE

Creating a new table uses the CREATE TABLE statement:

create_table_statement: CREATE TABLE [ IF NOT EXISTS ] `table_name`
                      : '('
                      :     `column_definition`
                      :     ( ',' `column_definition` )*
                      :     [ ',' PRIMARY KEY '(' `primary_key` ')' ]
                      : ')' [ WITH `table_options` ]

column_definition: `column_name` `cql_type` [ STATIC ] [ PRIMARY KEY]

primary_key: `partition_key` [ ',' `clustering_columns` ]

partition_key: `column_name`
             : | '(' `column_name` ( ',' `column_name` )* ')'

clustering_columns: `column_name` ( ',' `column_name` )*

table_options: COMPACT STORAGE [ AND `table_options` ]
                : | CLUSTERING ORDER BY '(' `clustering_order` ')' [ AND `table_options` ]
                : | `options`

clustering_order: `column_name` (ASC | DESC) ( ',' `column_name` (ASC | DESC) )*

For instance:

CREATE TABLE monkeySpecies (
    species text PRIMARY KEY,
    common_name text,
    population varint,
    average_size int
) WITH comment='Important biological records'
   AND read_repair_chance = 1.0;

CREATE TABLE timeline (
    userid uuid,
    posted_month int,
    posted_time uuid,
    body text,
    posted_by text,
    PRIMARY KEY (userid, posted_month, posted_time)
) WITH compaction = { 'class' : 'LeveledCompactionStrategy' };

CREATE TABLE loads (
    machine inet,
    cpu int,
    mtime timeuuid,
    load float,
    PRIMARY KEY ((machine, cpu), mtime)
) WITH CLUSTERING ORDER BY (mtime DESC);

A CQL table has a name and is composed of a set of rows. Creating a table amounts to defining which columns the rows will be composed, which of those columns compose the primary key, as well as optional options for the table.

Attempting to create an already existing table will return an error unless the IF NOT EXISTS directive is used. If it is used, the statement will be a no-op if the table already exists.

Column definitions

Every row in a CQL table has a set of predefined columns defined at the time of the table creation (or added later using an alter statement).

A column_definition is primarily comprised of the name of the column defined and it’s type, which restrict which values are accepted for that column. Additionally, a column definition can have the following modifiers:

STATIC
declares the column as being a static column.
PRIMARY KEY
declares the column as being the sole component of the primary key of the table.

Static columns

Some columns can be declared as STATIC in a table definition. A column that is static will be “shared” by all the rows belonging to the same partition (having the same partition key). For instance:

CREATE TABLE t (
    pk int,
    t int,
    v text,
    s text static,
    PRIMARY KEY (pk, t)
);

INSERT INTO t (pk, t, v, s) VALUES (0, 0, 'val0', 'static0');
INSERT INTO t (pk, t, v, s) VALUES (0, 1, 'val1', 'static1');

SELECT * FROM t;
   pk | t | v      | s
  ----+---+--------+-----------
   0  | 0 | 'val0' | 'static1'
   0  | 1 | 'val1' | 'static1'

As can be seen, the s value is the same (static1) for both of the row in the partition (the partition key in that example being pk, both rows are in that same partition): the 2nd insertion has overridden the value for s.

Static columns have the following restrictions:

  • tables with the COMPACT STORAGE option (see below) cannot use them.
  • a table without clustering columns cannot have static columns (in a table without clustering columns, every partition has only one row, and so every column is inherently static).
  • only non PRIMARY KEY columns can be static.

The Primary key

Within a table, a row is uniquely identified by its PRIMARY KEY, and hence all table must define a PRIMARY KEY (and only one). A PRIMARY KEY definition is composed of one or more of the columns defined in the table. Syntactically, the primary key is defined the keywords PRIMARY KEY followed by comma-separated list of the column names composing it within parenthesis, but if the primary key has only one column, one can alternatively follow that column definition by the PRIMARY KEY keywords. The order of the columns in the primary key definition matter.

A CQL primary key is composed of 2 parts:

  • the partition key part. It is the first component of the primary key definition. It can be a single column or, using additional parenthesis, can be multiple columns. A table always have at least a partition key, the smallest possible table definition is:

    CREATE TABLE t (k text PRIMARY KEY);
    
  • the clustering columns. Those are the columns after the first component of the primary key definition, and the order of those columns define the clustering order.

Some example of primary key definition are:

  • PRIMARY KEY (a): a is the partition key and there is no clustering columns.
  • PRIMARY KEY (a, b, c) : a is the partition key and b and c are the clustering columns.
  • PRIMARY KEY ((a, b), c) : a and b compose the partition key (this is often called a composite partition key) and c is the clustering column.

The partition key

Within a table, CQL defines the notion of a partition. A partition is simply the set of rows that share the same value for their partition key. Note that if the partition key is composed of multiple columns, then rows belong to the same partition only they have the same values for all those partition key column. So for instance, given the following table definition and content:

CREATE TABLE t (
    a int,
    b int,
    c int,
    d int,
    PRIMARY KEY ((a, b), c, d)
);

SELECT * FROM t;
   a | b | c | d
  ---+---+---+---
   0 | 0 | 0 | 0    // row 1
   0 | 0 | 1 | 1    // row 2
   0 | 1 | 2 | 2    // row 3
   0 | 1 | 3 | 3    // row 4
   1 | 1 | 4 | 4    // row 5

row 1 and row 2 are in the same partition, row 3 and row 4 are also in the same partition (but a different one) and row 5 is in yet another partition.

Note that a table always has a partition key, and that if the table has no clustering columns, then every partition of that table is only comprised of a single row (since the primary key uniquely identifies rows and the primary key is equal to the partition key if there is no clustering columns).

The most important property of partition is that all the rows belonging to the same partition are guarantee to be stored on the same set of replica nodes. In other words, the partition key of a table defines which of the rows will be localized together in the cluster, and it is thus important to choose your partition key wisely so that rows that needs to be fetch together are in the same partition (so that querying those rows together require contacting a minimum of nodes).

Please note however that there is a flip-side to this guarantee: as all rows sharing a partition key are guaranteed to be stored on the same set of replica node, a partition key that groups too much data can create a hotspot.

Another useful property of a partition is that when writing data, all the updates belonging to a single partition are done atomically and in isolation, which is not the case across partitions.

The proper choice of the partition key and clustering columns for a table is probably one of the most important aspects of data modeling in Scylla, and it largely impacts which queries can be performed, and how efficient they are.

The clustering columns

The clustering columns of a table defines the clustering order for the partition of that table. For a given partition, all the rows are physically ordered inside Scylla by that clustering order. For instance, given:

CREATE TABLE t (
    a int,
    b int,
    c int,
    PRIMARY KEY (a, b, c)
);

SELECT * FROM t;
   a | b | c
  ---+---+---
   0 | 0 | 4     // row 1
   0 | 1 | 9     // row 2
   0 | 2 | 2     // row 3
   0 | 3 | 3     // row 4

then the rows (which all belong to the same partition) are all stored internally in the order of the values of their b column (the order they are displayed above). So where the partition keys of the table lets you group rows on the same replica set, the clustering columns control how those rows are stored on the replica. That sorting allows the retrieval of a range of rows within a partition (for instance, in the example above, SELECT * FROM t WHERE a = 0 AND b > 1 and b <= 3) is very efficient.

Table options

A CQL table has a number of options that can be set at creation (and, for most of them, altered later). These options are specified after the WITH keyword.

Amongst those options, two important ones cannot be changed after creation and influence which queries can be done against the table: the COMPACT STORAGE option and the CLUSTERING ORDER option. Those, as well as the other options of a table are described in the following sections.

Compact tables

A compact table is one defined with the COMPACT STORAGE option. This option is only maintained for backward compatibility for definitions created before CQL version 3 and shouldn’t be used for new tables. Declaring a table with this option creates limitations for the table which are largely arbitrary (and exists for historical reasons). Amongst these limitations:

  • a compact table cannot use collections nor static columns.
  • if a compact table has at least one clustering column, then it must have exactly one column outside of the primary key ones. This implies that you cannot add or remove columns in particular after creation.
  • a compact table is limited as to the indexes it can create, and no materialized view can be created on it.

Reversing the clustering order

The clustering order of a table is defined by the clustering columns of that table. By default, that ordering is based on natural order of the clustering order, but the CLUSTERING ORDER lets you change that clustering order to use the reverse natural order for some (potentially all) of the columns.

The CLUSTERING ORDER option takes the comma-separated list of the clustering column, each with a ASC (for ascendant, e.g. the natural order) or DESC (for descendant, e.g. the reverse natural order). Note in particular that the default (if the CLUSTERING ORDER option is not used) is strictly equivalent to using the option with all clustering columns using the ASC modifier.

Note that this option is basically a hint for the storage engine to change the order in which it stores the row but it has 3 visible consequences:

# it limits which ORDER BY clause are allowed for selects on that table. You can only
order results by the clustering order or the reverse clustering order. Meaning that if a table has 2 clustering column a and b, and you define WITH CLUSTERING ORDER (a DESC, b ASC), then in queries, you will be allowed to use ORDER BY (a DESC, b ASC) and (reverse clustering order) ORDER BY (a ASC, b DESC) but not ORDER BY (a ASC, b ASC) (nor ORDER BY (a DESC, b DESC)).
# it also changes the default order of results when queried (if no ORDER BY is provided). Results are always returned
in clustering order (within a partition).
# it has a small performance impact on some queries as queries in reverse clustering order are slower than the one in
forward clustering order. In practice, this means that if you plan on querying mostly in the reverse natural order of your columns (which is common with time series for instance where you often want data from the newest to the oldest), it is an optimization to declare a descending clustering order.

Other table options

A table supports the following options:

option kind default description
comment simple none A free-form, human-readable comment.
read_repair_chance simple 0 The probability that extra nodes are queried (e.g. more nodes than required by the consistency level) for the purpose of read repairs.
dclocal_read_repair_chance simple 0.1 The probability that extra nodeas are queried (e.g. more nodes than required by the consistency level) belonging to the same data center as the read coordinator for the purpose of read repairs.
gc_grace_seconds simple 864000 Time to wait before garbage collecting tombstones (deletion markers).
bloom_filter_fp_chance simple 0.00075 The target probability of false positive of the sstable bloom filters. Sstable bloom filters will be sized to provide the provided probability (thus lowering this value impact the size of bloom filters in-memory and on-disk).
default_time_to_live simple 0 The default expiration time (“TTL”) in seconds for a table.
compaction map see below Compaction options.
compression map see below Compression options.
Compaction options

The compaction options must at least define the 'class' sub-option, which defines the compaction strategy class to use. The default supported class are 'SizeTieredCompactionStrategy', 'LeveledCompactionStrategy' and 'DateTieredCompactionStrategy' Custom strategy can be provided by specifying the full class name as a string constant. All default strategies support a number of common options, as well as options specific to the strategy chosen (see the section corresponding to your strategy for details: STCS, LCS and DTCS).

Compression options

The compression options define if and how the sstables of the table are compressed. The following sub-options are available:

Option Default Description
class LZ4Compressor The compression algorithm to use. Default compressors are: LZ4Compressor, SnappyCompressor and DeflateCompressor. Use 'enabled' : false to disable compression. Custom compressor can be provided by specifying the full class name as a “string constant”:#constants.
enabled true Enable/disable sstable compression.
chunk_length_in_kb 4KB On disk SSTables are compressed by block (to allow random reads). This defines the size (in KB) of the block. Bigger values may improve the compression rate, but increases the minimum size of data to be read from disk for a read.
Other considerations:
  • Adding new columns (see ALTER TABLE below) is a constant time operation. There is thus no need to try to anticipate future usage when creating a table.

ALTER TABLE

Altering an existing table uses the ALTER TABLE statement:

alter_table_statement: ALTER TABLE `table_name` `alter_table_instruction`
alter_table_instruction: ADD `column_name` `cql_type` ( ',' `column_name` `cql_type` )*
                       : | DROP `column_name` ( `column_name` )*
                       : | WITH `options`

For instance:

ALTER TABLE addamsFamily ADD gravesite varchar;

ALTER TABLE addamsFamily
       WITH comment = 'A most excellent and useful table'
       AND read_repair_chance = 0.2;

The ALTER TABLE statement can:

  • Add new column(s) to the table (through the ADD instruction). Note that the primary key of a table cannot be changed and thus newly added column will, by extension, never be part of the primary key. Also note that compact tables have restrictions regarding column addition. Note that this is constant (in the amount of data the cluster contains) time operation.
  • Remove column(s) from the table. This drops both the column and all its content, but note that while the column becomes immediately unavailable, its content is only removed lazily during compaction. Please also note the warnings below. Due to lazy removal, the altering itself is a constant (in the amount of data removed or contained in the cluster) time operation.
  • Change some of the table options (through the WITH instruction). The supported options are the same that when creating a table (outside of COMPACT STORAGE and CLUSTERING ORDER that cannot be changed after creation). Note that setting any compaction sub-options has the effect of erasing all previous compaction options, so you need to re-specify all the sub-options if you want to keep them. The same note applies to the set of compression sub-options.

Warning

Dropping a column assumes that the timestamps used for the value of this column are “real” timestamp in microseconds. Using “real” timestamps in microseconds is the default is and is strongly recommended but as Scylla allows the client to provide any timestamp on any table it is theoretically possible to use another convention. Please be aware that if you do so, dropping a column will not work correctly.

Warning

Once a column is dropped, it is allowed to re-add a column with the same name than the dropped one unless the type of the dropped column was a (non-frozen) column (due to an internal technical limitation).

DROP TABLE

Dropping a table uses the DROP TABLE statement:

drop_table_statement ::=  DROP TABLE [ IF EXISTS ] table_name

Dropping a table results in the immediate, irreversible removal of the table, including all data it contains.

If the table does not exist, the statement will return an error, unless IF EXISTS is used in which case the operation is a no-op.

TRUNCATE

A table can be truncated using the TRUNCATE statement:

truncate_statement ::=  TRUNCATE [ TABLE ] table_name

Note that TRUNCATE TABLE foo is allowed for consistency with other DDL statements but tables are the only object that can be truncated currently and so the TABLE keyword can be omitted.

Truncating a table permanently removes all existing data from the table, but without removing the table itself.

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