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Additional docs for toast_tuple_target changes

This commit is contained in:
Simon Riggs 2017-11-27 09:51:51 +00:00
parent 59af8d4384
commit 117469006b
1 changed files with 6 additions and 1 deletions
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7
doc/src/sgml/storage.sgml
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@ -429,7 +429,7 @@ when a row value to be stored in a table is wider than
<symbol>TOAST_TUPLE_THRESHOLD</symbol> bytes (normally 2 kB).
The <acronym>TOAST</acronym> code will compress and/or move
field values out-of-line until the row value is shorter than
<symbol>TOAST_TUPLE_TARGET</symbol> bytes (also normally 2 kB)
<symbol>TOAST_TUPLE_TARGET</symbol> bytes (also normally 2 kB, adjustable)
or no more gains can be had. During an UPDATE
operation, values of unchanged fields are normally preserved as-is; so an
UPDATE of a row with out-of-line values incurs no <acronym>TOAST</acronym> costs if
@ -483,6 +483,11 @@ of that data type, but the strategy for a given table column can be altered
with <link linkend="sql-altertable"><command>ALTER TABLE ... SET STORAGE</command></link>.
</para>
<para>
<symbol>TOAST_TUPLE_TARGET</symbol> can be adjusted for each table using
<link linkend="sql-altertable"><command>ALTER TABLE ... SET (toast_tuple_target = N)</command></link>
</para>
<para>
This scheme has a number of advantages compared to a more straightforward
approach such as allowing row values to span pages. Assuming that queries are