Apache Cassandra 0.8 Documentation
CQL Command Reference¶
USE¶
Synopsis
USE <KEYSPACE>;
Description
A USE statement consists of the USE keyword, followed by a valid keyspace name. Its purpose is to assign the per-connection, current working keyspace. All subsequent keyspace-specific actions will be performed in the context of the supplied value.
SELECT¶
Synopsis
SELECT [FIRST N] [REVERSED] <SELECT EXPR> FROM <COLUMN FAMILY> [USING <CONSISTENCY>]
[WHERE <CLAUSE>] [LIMIT N];
Description
A SELECT is used to read one or more records from a Cassandra column family. It returns a result-set of rows, where each row consists of a key and a collection of columns corresponding to the query.
Specifying Columns¶
SELECT [FIRST N] [REVERSED] name1, name2, name3 FROM ...
SELECT [FIRST N] [REVERSED] name1..nameN FROM ...
The SELECT expression determines which columns will appear in the results and takes the form of either a comma separated list of names, or a range. The range notation consists of a start and end column name separated by two periods (..). The set of columns returned for a range is start and end inclusive.
The FIRST option accepts an integer argument and can be used to apply a limit to the number of columns returned per row. When this limit is left unset it defaults to 10,000 columns.
The REVERSED option causes the sort order of the results to be reversed.
It is worth noting that unlike the projection in a SQL SELECT, there is no guarantee that the results will contain all of the columns specified. This is because Cassandra is schema-less and there are no guarantees that a given column exists.
Specifying the Column Family¶
SELECT ... FROM <COLUMN FAMILY> ...
The FROM clause is used to specify the Cassandra column family applicable to a SELECT query.
Setting Consistency Level¶
SELECT ... [USING <CONSISTENCY>] ...
Following the column family clause is an optional consistency level specification.
Filtering Rows¶
SELECT ... WHERE KEY = keyname AND name1 = value1
SELECT ... WHERE KEY >= startkey and KEY =< endkey AND name1 = value1
SELECT ... WHERE KEY IN ('<key>', '<key>', '<key>', ...)
The WHERE clause provides for filtering the rows that appear in results. The clause can filter on a key name, or range of keys, and in the case of indexed columns, on column values. Key filters are specified using the KEY keyword, a relational operator, (one of =, >, >=, <, and <=), and a term value. When terms appear on both sides of a relational operator it is assumed the filter applies to an indexed column. With column index filters, the term on the left of the operator is the name, the term on the right is the value to filter on.
Note
The greater-than and less-than operators (> and <) result in key ranges that are inclusive of the terms. There is no supported notion of strictly greater-than or less-than; these operators are merely supported as aliases to >= and <=.
Limiting the Result Set¶
SELECT ... WHERE <CLAUSE> [LIMIT N] ...
Limiting the number of rows returned can be achieved by adding the LIMIT option to a SELECT expression. LIMIT defaults to 10,000 when left unset.
INSERT¶
Synopsis
INSERT INTO (KEY, , , ...) VALUES (, , , ...) [USING <CONSISTENCY>];
Description
An INSERT is used to write one or more columns to a record in a Cassandra column family. No results are returned.
INSERT works exactly like UPDATE, so for information about Column Family and Consistency Level arguments please refer to the UPDATE section.
UPDATE¶
Synopsis
UPDATE <COLUMN FAMILY> [USING <CONSISTENCY>]
SET name1 = value1, name2 = value2 WHERE KEY = keyname;
Description
An UPDATE is used to write one or more columns to a record in a Cassandra column family. No results are returned.
Column Family¶
UPDATE <COLUMN FAMILY> ...
Statements begin with the UPDATE keyword followed by a Cassandra column family name.
Consistency Level¶
UPDATE ... [USING <CONSISTENCY>] ...
Following the column family identifier is an optional consistency level specification.
Specifying Columns and Rows¶
UPDATE ... SET name1 = value1, name2 = value2 WHERE KEY = keyname;
UPDATE ... SET name1 = value1, name2 = value2 WHERE KEY IN ('<key>', '<key>', ...)
Rows are created or updated by supplying column names and values in term assignment format. Multiple columns can be set by separating the name/value pairs using commas. Each update statement requires exactly one key to be specified using a WHERE clause and the KEY keyword.
DELETE¶
Synopsis
DELETE [COLUMNS] FROM <COLUMN FAMILY> [USING <CONSISTENCY>] WHERE KEY = keyname1
DELETE [COLUMNS] FROM <COLUMN FAMILY> [USING <CONSISTENCY>] WHERE KEY IN (keyname1, keyname2);
Description
A DELETE is used to perform the removal of one or more columns from one or more rows.
Specifying Columns¶
DELETE [COLUMNS] ...
Following the DELETE keyword is an optional comma-delimited list of column name terms. When no column names are specified, the remove applies to the entire row(s) matched by the WHERE clause
Column Family¶
DELETE ... FROM <COLUMN FAMILY> ...
The column family name follows the list of column names.
Consistency Level¶
DELETE ... [USING <CONSISTENCY>] ...
Following the column family identifier is an optional consistency level specification.
Specifying Rows¶
DELETE ... WHERE KEY = keyname1
DELETE ... WHERE KEY IN (keyname1, keyname2)
The WHERE clause is used to determine which row(s) a DELETE applies to. The first form allows the specification of a single keyname using the KEY keyword and the = operator. The second form allows a list of keyname terms to be specified using the IN notation and a parenthesized list of comma-delimited keyname terms.
BATCH¶
Synopsis
BEGIN BATCH [USING CONSISTENCY ]
INSERT or UPDATE or DELETE statements separated by semicolon or "end of line"
APPLY BATCH
Description
A single consistency level is used for the entire batch, it appears after the BEGIN BATCH statement, and uses the standard consistency level specification. Batch defaults to CONSISTENCY.ONE when left unspecified.
Note
While there are no isolation guarantees, UPDATE queries are atomic within a give record.
Example
BEGIN BATCH USING CONSISTENCY QUORUM
INSERT INTO users (KEY, password, name) VALUES ('user2', 'ch@ngem3b', 'second user')
UPDATE users SET password = 'ps22dhds' WHERE KEY = 'user2'
INSERT INTO users (KEY, password) VALUES ('user3', 'ch@ngem3c')
DELETE name FROM users WHERE key = 'user2'
INSERT INTO users (KEY, password, name) VALUES ('user4', 'ch@ngem3c', 'Andrew')
APPLY BATCH
TRUNCATE¶
Synopsis
TRUNCATE <COLUMN FAMILY>
Description
Accepts a single argument for the column family name, and permanently removes all data from said column family.
CREATE KEYSPACE¶
Synopsis
CREATE KEYSPACE <NAME> WITH AND strategy_class = <STRATEGY>
AND strategy_options.<OPTION> = <VALUE> [AND strategy_options. <OPTION> = <VALUE>];
Description
The CREATE KEYSPACE statement creates a new top-level namespace (aka “keyspace”). Valid names are any string constructed of alphanumeric characters and underscores, but must begin with a letter. Properties such as replication strategy and count are specified during creation using the following accepted keyword arguments:
| Keyword | Required | Description |
|---|---|---|
| strategy_options | no | Most strategies require additional arguments which can be supplied by appending the option name to the strategy_options keyword, separated by a colon (:). For example, a strategy option of ‘DC’ with a value of ‘1’ would be specified as strategy_options:DC1 = 1; replication_factor for SimpleStrategy could be strategy_options:replication_factor=3. |
CREATE COLUMNFAMILY¶
Synopsis
CREATE COLUMNFAMILY <COLUMN FAMILY> (KEY <type> PRIMARY KEY [, name1 type, name2 type, ...]);
CREATE COLUMNFAMILY <COLUMN FAMILY> (KEY <type> PRIMARY KEY [, name1 type, name2 type, ...])
[WITH keyword1 = arg1 [AND keyword2 = arg2 [AND ...]]];
Description
CREATE COLUMNFAMILY statements create new column family namespaces under the current keyspace. Valid column family names are strings of alphanumeric characters and underscores, which begin with a letter.
Specifying Key Type¶
CREATE ... (KEY <type> PRIMARY KEY) ...
When creating a new column family, you must specify key type. The list of possible key types is identical to column comparators/validators, (see Specifying Column Type). It’s important to note that the key type must be compatible with the partitioner in use, for example OrderPreservingPartitioner and CollatingOrderPreservingPartitioner both require UTF-8 keys.
Specifying Column Type (optional)¶
CREATE ... (KEY <type> PRIMARY KEY, name1 type, name2 type) ...
It is possible to assign columns a type during column family creation. Columns configured with a type are validated accordingly when a write occurs. Column types are specified as a parenthesized, comma-separated list of column term and type pairs. The list of recognized types are:
| Type | Description |
|---|---|
| bytea | Arbitrary bytes (no validation) |
| ascii | ASCII character string |
| text | UTF8 encoded string |
| varchar | UTF8 encoded string |
| uuid | Type 1, or type 4 UUID |
| varint | Arbitrary-precision integer |
| int | 8-byte long (same as bigint) |
| bigint | 8-byte long |
Note
In addition to the recognized types listed above, it is also possible to supply a string containing the name of a class (a sub-class of AbstractType), either fully qualified, or relative to the org.apache.cassandra.db.marshal package.
Column Family Options (optional)¶
CREATE COLUMNFAMILY ... WITH keyword1 = arg1 AND keyword2 = arg2;
A number of optional keyword arguments can be supplied to control the configuration of a new column family.
| Option | Default Value | Description |
|---|---|---|
| comparator | Text | Determines sorting and validation of column names. |
| comment | None | A free-form, human-readable comment. |
| key_cache_size | 200000 | Number of keys per SSTable whose locations are kept in memory. |
| row_cache_size | 0 | Number of rows whose entire contents to cache in memory. |
| read_repair_chance | 1.0 (always on) | The probability with which read repairs should be invoked on non-quorum reads. |
| gc_grace_seconds | 864000 (10 days) | Time to wait before garbage collecting tombstones (deletion markers). |
| default_validation | Text | Determines validation of column values. |
| min_compaction_threshold | 4 | Minimum number of SSTables needed to start a minor compaction. |
| max_compaction_threshold | 32 | Maximum number of SSTables allowed before a minor compaction is forced. |
| row_cache_save_period_in_seconds | 0 | Number of seconds between saving row caches. |
| key_cache_save_period_in_seconds | 14400 | Number of seconds between saving key caches. |
| memtable_flush_after_mins | 60 | Maximum time to leave a dirty table unflushed. |
| memtable_throughput_in_mb | Dynamic | Maximum size of the memtable before it is flushed. |
| memtable_operations_in_millions | Dynamic | Number of operations in millions before the memtable is flushed. |
| replication_on_write | n/a |
CREATE INDEX¶
Synopsis
CREATE INDEX [index_name] ON <column_family> (column_name);
Description
A CREATE INDEX statement is used to create a new, automatic secondary index for the named column.
DROP¶
Synopsis
DROP <KEYSPACE|COLUMNFAMILY> namespace;
Description
DROP statements result in the immediate, irreversible removal of keyspace and column family namespaces.
Common Idioms¶
Specifying Consistency¶
... USING <CONSISTENCY> ...
Consistency level specifications are made up the keyword USING, followed by a consistency level identifier. Valid consistency levels are as follows:
- CONSISTENCY ZERO
- CONSISTENCY ONE (default)
- CONSISTENCY QUORUM
- CONSISTENCY ALL
- CONSISTENCY EACH_QUORUM
- CONSISTENCY LOCAL_QUORUM
Term specification¶
Terms are used in statements to specify things such as keyspaces, column families, indexes, column names and values, and keyword arguments. The rules governing term specification are as follows:
- Any single quoted string literal (example: ‘apple’).
- Unquoted alpha-numeric strings that begin with a letter (example: carrot).
- Unquoted numeric literals (example: 100).
- UUID strings in hyphen-delimited hex notation (example: 1438fc5c-4ff6-11e0-b97f-0026c650d722).
Terms which do not conform to these rules result in an exception.
How column name/value terms are interpreted is determined by the configured type.
| Option | Term |
|---|---|
| ascii | Any string which can be decoded using ASCII charset |
| text/varchar | Any string which can be decoded using UTF8 charset |
| uuid | Standard UUID string format (hyphen-delimited hex notation) |
| uuid | The string now, to represent a type-1 (time-based) UUID with a date-time component based on the current time |
| uuid | Numeric value representing milliseconds since epoch |
| uuid | An iso8601 timestamp |
| int | Integer value capable of fitting in 8 bytes (same as bigint) |
| bigint | Integer value capable of fitting in 8 bytes |
| varint | Integer value of arbitrary size |
| bytea | Hex-encoded strings (converted directly to the corresponding bytes) |