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Add an extra header byte to TOAST-pointer datums to represent their size

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explicitly.  This means a TOAST pointer takes 18 bytes instead of 17 --- still
smaller than in 8.2 --- which seems a good tradeoff to ensure we won't have
painted ourselves into a corner if we want to support multiple types of TOAST
pointer later on.  Per discussion with Greg Stark.
This commit is contained in:
Tom Lane
2007-09-30 19:54:58 +00:00
parent 3ff0018c2e
commit 27b8922221
4 changed files with 58 additions and 37 deletions
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10
doc/src/sgml/storage.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/storage.sgml,v 1.19 2007/09/21 21:25:42 tgl Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/storage.sgml,v 1.20 2007/09/30 19:54:57 tgl Exp $ -->
<chapter id="storage">
@ -233,8 +233,8 @@ header, and the remaining bits give the total datum size (including length
byte) in bytes. As a special case, if the remaining bits are all zero
(which would be impossible for a self-inclusive length), the value is a
pointer to out-of-line data stored in a separate TOAST table. (The size of
a TOAST pointer is known a priori, so it doesn't need to be represented in
the header.) Values with single-byte headers aren't aligned on any particular
a TOAST pointer is given in the second byte of the datum.)
Values with single-byte headers aren't aligned on any particular
boundary, either. Lastly, when the highest-order or lowest-order bit is
clear but the adjacent bit is set, the content of the datum has been
compressed and must be decompressed before use. In this case the remaining
@ -274,8 +274,8 @@ retrieval of the values. A pointer datum representing an out-of-line
<acronym>TOAST</> table in which to look and the OID of the specific value
(its <structfield>chunk_id</>). For convenience, pointer datums also store the
logical datum size (original uncompressed data length) and actual stored size
(different if compression was applied). Allowing for the varlena header byte,
the total size of a <acronym>TOAST</> pointer datum is therefore 17 bytes
(different if compression was applied). Allowing for the varlena header bytes,
the total size of a <acronym>TOAST</> pointer datum is therefore 18 bytes
regardless of the actual size of the represented value.
</para>