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Decouple the values of TOAST_TUPLE_THRESHOLD and TOAST_MAX_CHUNK_SIZE.

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Add the latter to the values checked in pg_control, since it can't be changed
without invalidating toast table content.  This commit in itself shouldn't
change any behavior, but it lays some necessary groundwork for experimentation
with these toast-control numbers.

Note: while TOAST_TUPLE_THRESHOLD can now be changed without initdb, some
thought still needs to be given to needs_toast_table() in toasting.c before
unleashing random changes.
This commit is contained in:
Tom Lane
2007-04-03 04:14:26 +00:00
parent 4fea0ca8f3
commit b3005276eb
8 changed files with 113 additions and 47 deletions
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15
doc/src/sgml/storage.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/storage.sgml,v 1.15 2007/03/02 00:48:44 tgl Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/storage.sgml,v 1.16 2007/04/03 04:14:26 tgl Exp $ -->
<chapter id="storage">
@ -240,8 +240,9 @@ of the LZ family of compression techniques. See
<para>
Out-of-line values are divided (after compression if used) into chunks of at
most <literal>TOAST_MAX_CHUNK_SIZE</> bytes (this value is a little less than
<literal>BLCKSZ/4</>, or about 2000 bytes by default). Each chunk is stored
most <symbol>TOAST_MAX_CHUNK_SIZE</> bytes (by default this value is chosen
so that four chunk rows will fit on a page, making it about 2000 bytes).
Each chunk is stored
as a separate row in the <acronym>TOAST</> table for the owning table. Every
<acronym>TOAST</> table has the columns <structfield>chunk_id</> (an OID
identifying the particular <acronym>TOAST</>ed value),
@ -260,10 +261,12 @@ regardless of the actual size of the represented value.
<para>
The <acronym>TOAST</> code is triggered only
when a row value to be stored in a table is wider than <literal>BLCKSZ/4</>
bytes (normally 2 kB). The <acronym>TOAST</> code will compress and/or move
when a row value to be stored in a table is wider than
<symbol>TOAST_TUPLE_THRESHOLD</> bytes (normally 2 kB).
The <acronym>TOAST</> code will compress and/or move
field values out-of-line until the row value is shorter than
<literal>BLCKSZ/4</> bytes or no more gains can be had. During an UPDATE
<symbol>TOAST_TUPLE_TARGET</> bytes (also normally 2 kB)
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</> costs if
none of the out-of-line values change.