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doc: Add appendix detailing some limits of PostgreSQL
This used to be on the web site but was removed. The documentation is a better place for it anyway. Author: David Rowley <david.rowley@2ndquadrant.com> Reviewed-by: John Naylor <jcnaylor@gmail.com> Discussion: https://www.postgresql.org/message-id/flat/CAKJS1f_dKdejdKB94nKZC9S5NzB-UZRcAKkE84e=JEEecDuotg@mail.gmail.com/
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‎doc/src/sgml/filelist.sgml

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<!ENTITY release-7.4 SYSTEM "release-7.4.sgml">
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<!ENTITY release-old SYSTEM "release-old.sgml">
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<!ENTITY limits SYSTEM "limits.sgml">
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<!ENTITY acronyms SYSTEM "acronyms.sgml">
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<!ENTITY features-supported SYSTEM "features-supported.sgml">

‎doc/src/sgml/limits.sgml

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<!-- doc/src/sgml/limits.sgml -->
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<appendix id="limits">
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<title><productname>PostgreSQL</productname> Limits</title>
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<para>
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<xref linkend="limits-table"/> describes various hard limits of
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<productname>PostgreSQL</productname>. However, practical limits, such as
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performance limitations or available disk space may apply before absolute
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hard limits are reached.
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</para>
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<table id="limits-table">
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<title><productname>PostgreSQL</productname> limitations</title>
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<tgroup cols="3">
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<thead>
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<row>
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<entry>Item</entry>
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<entry>Upper Limit</entry>
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<entry>Comment</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>database size</entry>
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<entry>unlimited</entry>
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<entry></entry>
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</row>
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<row>
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<entry>number of databases</entry>
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<!-- 2^32 - FirstNormalObjectId - 1 -->
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<entry>4,294,950,911</entry>
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<entry></entry>
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</row>
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<row>
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<entry>relations per database</entry>
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<!-- (2^32 - FirstNormalObjectId - 1) / 3 (3 because of the table and the
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two types that are created to go with it) -->
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<entry>1,431,650,303</entry>
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<entry></entry>
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</row>
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<row>
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<entry>relation size</entry>
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<entry>32 TB</entry>
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<entry>with the default <symbol>BLCKSZ</symbol> of 8192 bytes</entry>
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</row>
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<row>
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<entry>rows per table</entry>
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<entry>limited by the number of tuples that can fit onto 4,294,967,295 pages</entry>
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<entry></entry>
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</row>
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<row>
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<entry>columns per table</entry>
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<entry>1600</entry>
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<entry>further limited by tuple size fitting on a single page; see note
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below</entry>
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</row>
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<row>
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<entry>field size</entry>
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<entry>1 GB</entry>
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<entry></entry>
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</row>
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<row>
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<entry>identifier length</entry>
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<entry>63 bytes</entry>
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<entry>can be increased by recompiling <productname>PostgreSQL</productname></entry>
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</row>
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<row>
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<entry>indexes per table</entry>
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<entry>unlimited</entry>
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<entry>constrained by maximum relations per database</entry>
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</row>
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<row>
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<entry>columns per index</entry>
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<entry>32</entry>
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<entry>can be increased by recompiling <productname>PostgreSQL</productname></entry>
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</row>
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<row>
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<entry>partition keys</entry>
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<entry>32</entry>
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<entry>can be increased by recompiling <productname>PostgreSQL</productname></entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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The maximum number of columns for a table is further reduced as the tuple
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being stored must fit in a single 8192-byte heap page. For example,
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excluding the tuple header, a tuple made up of 1600 <type>int</type> columns
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would consume 6400 bytes and could be stored in a heap page, but a tuple of
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1600 <type>bigint</type> columns would consume 12800 bytes and would
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therefore not fit inside a heap page.
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Variable-length fields of
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types such as <type>text</type>, <type>varchar</type>, and <type>char</type>
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can have their values stored out of line in the table's TOAST table when the
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values are large enough to require it. Only an 18-byte pointer must remain
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inside the tuple in the table's heap. For shorter length variable-length
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fields, either a 4-byte or 1-byte field header is used and the value is
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stored inside the heap tuple.
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</para>
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<para>
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Columns that have been dropped from the table also contribute to the maximum
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column limit. Moreover, although the dropped column values for newly
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created tuples are internally marked as null in the tuple's null bitmap, the
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null bitmap also occupies space.
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</para>
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</appendix>

‎doc/src/sgml/postgres.sgml

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&external-projects;
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&sourcerepo;
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&docguide;
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&limits;
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&acronyms;
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</part>

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