PostgreSQL wire protocol

    If a node is started with postgres support enabled it will bind to port 5432 by default. To use a custom port, set the corresponding Ports in the .

    However, even though connecting PostgreSQL tools and client libraries is supported, the actual SQL statements have to be supported by CrateDB’s SQL dialect. A notable difference is that CrateDB doesn’t support transactions, which is why clients should generally enable autocommit.

    Note

    In order to use setFetchSize in JDBC it is possible to set autocommit to false.

    The client will utilize the fetchSize on SELECT statements and only load up to fetchSize rows into memory.

    See the PostgreSQL JDBC Query docs for more information.

    Write operations will still behave as if autocommit was enabled and commit or rollback calls are ignored.

    CrateDB emulates PostgreSQL server version 10.5.

    SSL Support

    SSL can be configured using .

    Authentication methods can be configured using Host-Based Authentication (HBA).

    ParameterStatus

    After the authentication has succeeded the server has the possibility to send multiple ParameterStatus messages to the client. These are used to communicate information like server_version (emulates PostgreSQL 9.5) or server_encoding.

    CrateDB also sends a message containing the crate_version parameter. This contains the current CrateDB version number.

    This information is useful for clients to detect that they’re connecting to CrateDB instead of a PostgreSQL instance.

    Database selection

    Since CrateDB uses schemas instead of databases, the database parameter sets the default schema name for future queries. If no schema is specified, the schema doc will be used as default. Additionally, the only supported charset is UTF8.

    Query Modes

    Simple query

    The protocol mode is fully implemented.

    The PostgreSQL extended query protocol mode is implemented with the following limitations:

    • The ParameterDescription message works for the most common use cases except for DDL statements.

    • To optimize the execution of bulk operations the execution of statements is delayed until the Sync message is received

    Copy operations

    CrateDB does not support the COPY sub-protocol.

    Function call

    The sub-protocol is not supported since it’s a legacy feature.

    Operations can be cancelled using the KILL statement, hence the message is unsupported. Consequently, the server won’t send a BackendKeyData message during connection initialization.

    For improved compatibility, the pg_catalog schema is implemented containing following tables:

    Some clients require the pg_catalog.pg_type in order to be able to stream arrays or other non-primitive types.

    For compatibility reasons, there is a trimmed down pg_type table available in CrateDB:

    Note

    This is just a snapshot of the table. Check table to get information for all supported columns.

    Object identifiers are used internally by PostgreSQL for various system tables. The oid type is currently mapped to the integer data type.

    The oid type might have the following type aliases:

    Note

    Currently, casting a string or integer literal to the regproc type wouldn’t result in a lookup. Instead, casting the string literal to the regproc type results in an object of the regproc type that has a name that corresponds to the string literal and the oid hash of the literal as oid. Casting an integer literal to the regproc type results in an object of the regproc type that has a name that corresponds to the string representation of the literal and the literal value as oid.

    For compatibility with JDBC the SHOW TRANSACTION ISOLATION LEVEL statement is implemented:

    CrateDB will silently ignore the COMMIT, , and START TRANSACTION statements and all respective parameters.

    pgjdbc JDBC drivers version 9.4.1209 and above are compatible.

    Limitations

    • reflection methods like conn.getMetaData().getTables(...) won’t work since the required tables are unavailable in CrateDB.

      As a workaround it’s possible to use SHOW TABLES or query the information_schema tables manually using SELECT statements.

    • OBJECT and GEO_SHAPE columns can be streamed as JSON but require version 9.4.1210 or newer.

    • Multidimensional arrays will be streamed as JSON encoded string to avoid a protocol limitation where all sub-arrays are required to have the same length.

    • The behavior of PreparedStatement.executeBatch in error cases depends on in which stage an error occurs: A BatchUpdateException is thrown if no processing has been done yet, whereas single operations failing after the processing started are indicated by an EXECUTE_FAILED (-3) return value.

    • Transaction limitations as described above.

    Connection failover and load balancing is supported as described here: PostgreSQL JDBC connection failover.

    Note

    It is not recommended to use the targetServerType parameter since CrateDB has no concept of master-replica nodes.

    The PostgreSQL Wire Protocol makes it easy to use many PostgreSQL compatible tools and libraries directly with CrateDB. However, many of these tools assume that they are talking to PostgreSQL specifically, and thus rely on SQL extensions and idioms that are unique to PostgreSQL. Because of this, some tools or libraries may not work with other SQL databases such as CrateDB.

    CrateDB’s SQL query engine enables real-time search & aggregations for online analytic processing (OLAP) and business intelligence (BI) with the benefit of the ability to scale horizontally. The use-cases of CrateDB are different than those of PostgreSQL, as CrateDB’s specialized storage schema and query execution engine address different requirements (see ).

    The listed features below cover the main differences in implementation and dialect between CrateDB and PostgreSQL. A detailed comparison between CrateDB’s SQL dialect and standard SQL is defined in SQL compatibility.

    COPY

    CrateDB does not support the distinct sub-protocol that is used to serve COPY operations and provides another implementation for transferring bulk data using the and COPY TO statements.

    The definition of structured values by using JSON types, composite types or HSTORE are not supported. CrateDB alternatively allows the definition of nested documents (of type ) that store fieldscontaining any CrateDB supported data type, including nested object types.

    CrateDB accepts the Type conversion syntax for conversion of one data type to another.

    See also

    CrateDB value expressions

    Arrays

    Declaration of arrays

    While multidimensional arrays in PostgreSQL must have matching extends for each dimension, CrateDB allows different length nested arrays as this example shows:

    Accessing arrays

    Fetching arbitrary rectangular slices of an array using in the array subscript is not supported.

    See also

    PostgreSQL Arrays

    Text search functions and operators

    The and operators provided by PostgreSQL for (see PostgreSQL Fulltext Search) are not compatible with those provided by CrateDB.

    If you are missing features, functions or dialect improvements and have a great use case for it, let us know on . We’re always improving and extending CrateDB and we love to hear feedback.