Appendix D. SQL Conformance

Appendix D. SQL Conformance
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Table of Contents

D.3. XML Limits and Conformance to SQL/XML

D.3.1. Queries Are Restricted to XPath 1.0
D.3.2. Incidental Limits of the Implementation

+ This section attempts to outline to what extent + PostgreSQL conforms to the current SQL + standard. The following information is not a full statement of + conformance, but it presents the main topics in as much detail as is + both reasonable and useful for users. +

+ The formal name of the SQL standard is ISO/IEC 9075 “Database + Language SQL”. A revised version of the standard is released + from time to time; the most recent update appearing in 2023. + The 2023 version is referred to as ISO/IEC 9075:2023, or simply as SQL:2023. + The versions prior to that were SQL:2016, SQL:2011, SQL:2008, SQL:2006, SQL:2003, + SQL:1999, and SQL-92. Each version + replaces the previous one, so claims of conformance to earlier + versions have no official merit. + PostgreSQL development aims for + conformance with the latest official version of the standard where + such conformance does not contradict traditional features or common + sense. Many of the features required by the SQL + standard are supported, though sometimes with slightly differing + syntax or function. Further moves towards conformance can be + expected over time. +

+ SQL-92 defined three feature sets for + conformance: Entry, Intermediate, and Full. Most database + management systems claiming SQL standard + conformance were conforming at only the Entry level, since the + entire set of features in the Intermediate and Full levels was + either too voluminous or in conflict with legacy behaviors. +

+ Starting with SQL:1999, the SQL standard defines + a large set of individual features rather than the ineffectively + broad three levels found in SQL-92. A large + subset of these features represents the “Core” + features, which every conforming SQL implementation must supply. + The rest of the features are purely optional. +

+ The standard is split into a number of parts, each also known by a shorthand + name: + +

ISO/IEC 9075-1 Framework (SQL/Framework)
ISO/IEC 9075-2 Foundation (SQL/Foundation)
ISO/IEC 9075-3 Call Level Interface (SQL/CLI)
ISO/IEC 9075-4 Persistent Stored Modules (SQL/PSM)
ISO/IEC 9075-9 Management of External Data (SQL/MED)
ISO/IEC 9075-10 Object Language Bindings (SQL/OLB)
ISO/IEC 9075-11 Information and Definition Schemas (SQL/Schemata)
ISO/IEC 9075-13 Routines and Types using the Java Language (SQL/JRT)
ISO/IEC 9075-14 XML-related specifications (SQL/XML)
ISO/IEC 9075-15 Multi-dimensional arrays (SQL/MDA)
ISO/IEC 9075-16 Property Graph Queries (SQL/PGQ)

+ + Note that some part numbers are not (or no longer) used. +

+ The PostgreSQL core covers parts 1, 2, 9, + 11, and 14. Part 3 is covered by the ODBC driver, and part 13 is + covered by the PL/Java plug-in, but exact conformance is currently + not being verified for these components. There are currently no + implementations of parts 4, 10, 15, and 16 + for PostgreSQL. +

+ PostgreSQL supports most of the major features of SQL:2023. Out of + 177 mandatory features required for full Core conformance, + PostgreSQL conforms to at least 170. In addition, there is a long + list of supported optional features. It might be worth noting that at + the time of writing, no current version of any database management + system claims full conformance to Core SQL:2023. +

+ In the following two sections, we provide a list of those features + that PostgreSQL supports, followed by a + list of the features defined in SQL:2023 which + are not yet supported in PostgreSQL. + Both of these lists are approximate: There might be minor details that + are nonconforming for a feature that is listed as supported, and + large parts of an unsupported feature might in fact be implemented. + The main body of the documentation always contains the most accurate + information about what does and does not work. +

Note

+ Feature codes containing a hyphen are subfeatures. Therefore, if a + particular subfeature is not supported, the main feature is listed + as unsupported even if some other subfeatures are supported. +

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Appendix C. SQL Key Words	Home	D.1. Supported Features