mirror of https://github.com/postgres/postgres
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/pull/35/head
Peter Eisentraut
7 years ago
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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> |
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