Summary of Greenplum Features

This section provides a high-level overview of the system requirements and feature set of Greenplum Database. It contains the following topics:

• Greenplum SQL Standard Conformance
• Greenplum and PostgreSQL Compatibility

Greenplum SQL Standard Conformance

The SQL language was first formally standardized in 1986 by the American National Standards Institute (ANSI) as SQL 1986. Subsequent versions of the SQL standard have been released by ANSI and as International Organization for Standardization (ISO) standards: SQL 1989, SQL 1992, SQL 1999, SQL 2003, SQL 2006, and finally SQL 2008, which is the current SQL standard. The official name of the standard is ISO/IEC 9075-14:2008. In general, each new version adds more features, although occasionally features are deprecated or removed.

It is important to note that there are no commercial database systems that are fully compliant with the SQL standard. Greenplum Database is almost fully compliant with the SQL 1992 standard, with most of the features from SQL 1999. Several features from SQL 2003 have also been implemented (most notably the SQL OLAP features).

This section addresses the important conformance issues of Greenplum Database as they relate to the SQL standards. For a feature-by-feature list of Greenplum’s support of the latest SQL standard, see SQL 2008 Optional Feature Compliance.

Core SQL Conformance

In the process of building a parallel, shared-nothing database system and query optimizer, certain common SQL constructs are not currently implemented in Greenplum Database. The following SQL constructs are not supported:

1. Some set returning subqueries in EXISTS or NOT EXISTS clauses that Greenplum’s parallel optimizer cannot rewrite into joins.
2. Backwards scrolling cursors, including the use of FETCH PRIOR, FETCH FIRST, FETCH ABSOLUTE, and FETCH RELATIVE.
3. In CREATE TABLE statements (on hash-distributed tables): a UNIQUE or PRIMARY KEY clause must include all of (or a superset of) the distribution key columns. Because of this restriction, only one UNIQUE clause or PRIMARY KEY clause is allowed in a CREATE TABLE statement. UNIQUE or PRIMARY KEY clauses are not allowed on randomly-distributed tables.

4. CREATE UNIQUE INDEX statements that do not contain all of (or a superset of) the distribution key columns. CREATE UNIQUE INDEX is not allowed on randomly-distributed tables.

   Note that UNIQUE INDEXES (but not UNIQUE CONSTRAINTS) are enforced on a part basis within a partitioned table. They guarantee the uniqueness of the key within each part or sub-part.

5. VOLATILE or STABLE functions cannot execute on the segments, and so are generally limited to being passed literal values as the arguments to their parameters.

6. Triggers are not supported since they typically rely on the use of VOLATILE functions.
7. Referential integrity constraints (foreign keys) are not enforced in Greenplum Database. Users can declare foreign keys and this information is kept in the system catalog, however.
8. Sequence manipulation functions CURRVAL and LASTVAL.

SQL 1992 Conformance

The following features of SQL 1992 are not supported in Greenplum Database:

1. NATIONAL CHARACTER (NCHAR) and NATIONAL CHARACTER VARYING (NVARCHAR). Users can declare the NCHAR and NVARCHAR types, however they are just synonyms for CHAR and VARCHAR in Greenplum Database.
2. CREATE ASSERTION statement.
3. INTERVAL literals are supported in Greenplum Database, but do not conform to the standard.
4. GET DIAGNOSTICS statement.
5. GLOBAL TEMPORARY TABLEs and LOCAL TEMPORARY TABLEs. Greenplum TEMPORARY TABLEs do not conform to the SQL standard, but many commercial database systems have implemented temporary tables in the same way. Greenplum temporary tables are the same as VOLATILE TABLEs in Teradata.
6. UNIQUE predicate.
7. MATCH PARTIAL for referential integrity checks (most likely will not be implemented in Greenplum Database).

SQL 1999 Conformance

The following features of SQL 1999 are not supported in Greenplum Database:

1. Large Object data types: BLOB, CLOB, NCLOB. However, the BYTEA and TEXT columns can store very large amounts of data in Greenplum Database (hundreds of megabytes).
2. MODULE (SQL client modules).

3. CREATE PROCEDURE (SQL/PSM). This can be worked around in Greenplum Database by creating a FUNCTION that returns void, and invoking the function as follows:

   SELECT myfunc(args);

4. The PostgreSQL/Greenplum function definition language (PL/PGSQL) is a subset of Oracle’s PL/SQL, rather than being compatible with the SQL/PSM function definition language. Greenplum Database also supports function definitions written in Python, Perl, Java, and R.

5. BIT and BIT VARYING data types (intentionally omitted). These were deprecated in SQL 2003, and replaced in SQL 2008.
6. Greenplum supports identifiers up to 63 characters long. The SQL standard requires support for identifiers up to 128 characters long.
7. Prepared transactions (PREPARE TRANSACTION, COMMIT PREPARED, ROLLBACK PREPARED). This also means Greenplum does not support XA Transactions (2 phase commit coordination of database transactions with external transactions).
8. CHARACTER SET option on the definition of CHAR() or VARCHAR() columns.
9. Specification of CHARACTERS or OCTETS (BYTES) on the length of a CHAR() or VARCHAR() column. For example, VARCHAR(15 CHARACTERS) or VARCHAR(15 OCTETS) or VARCHAR(15 BYTES).
10. CREATE DISTINCT TYPE statement. CREATE DOMAIN can be used as a work-around in Greenplum.
11. The explicit table construct.

SQL 2003 Conformance

The following features of SQL 2003 are not supported in Greenplum Database:

1. MERGE statements.
2. IDENTITY columns and the associated GENERATED ALWAYS/GENERATED BY DEFAULT clause. The SERIAL or BIGSERIAL data types are very similar to INT or BIGINT GENERATED BY DEFAULT AS IDENTITY.
3. MULTISET modifiers on data types.
4. ROW data type.
5. Greenplum Database syntax for using sequences is non-standard. For example, nextval(‘seq’) is used in Greenplum instead of the standard NEXT VALUE FOR seq.
6. GENERATED ALWAYS AS columns. Views can be used as a work-around.
7. The sample clause (TABLESAMPLE) on SELECT statements. The random() function can be used as a work-around to get random samples from tables.
8. The partitioned join tables construct (PARTITION BY in a join).
9. Greenplum array data types are almost SQL standard compliant with some exceptions. Generally customers should not encounter any problems using them.

SQL 2008 Conformance

The following features of SQL 2008 are not supported in Greenplum Database:

1. BINARY and VARBINARY data types. BYTEA can be used in place of VARBINARY in Greenplum Database.

2. The ORDER BY clause is ignored in views and subqueries unless a LIMIT clause is also used. This is intentional, as the Greenplum optimizer cannot determine when it is safe to avoid the sort, causing an unexpected performance impact for such ORDER BY clauses. To work around, you can specify a really large LIMIT. For example:

   SELECT * FROM mytable ORDER BY 1 LIMIT 9999999999

3. The row subquery construct is not supported.

4. TRUNCATE TABLE does not accept the CONTINUE IDENTITY and RESTART IDENTITY clauses.

Greenplum and PostgreSQL Compatibility

Greenplum Database is based on PostgreSQL 9.4. To support the distributed nature and typical workload of a Greenplum Database system, some SQL commands have been added or modified, and there are a few PostgreSQL features that are not supported. Greenplum has also added features not found in PostgreSQL, such as physical data distribution, parallel query optimization, external tables, resource queues, and enhanced table partitioning. For full SQL syntax and references, see the SQL Commands.

Note: Greenplum Database does not support the PostgreSQL large object facility for streaming user data that is stored in large-object structures.

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