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><H1
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><A
NAME="STORAGE-TOAST"
>56.2. TOAST</A
></H1
><P
>This section provides an overview of <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
> (The
Oversized-Attribute Storage Technique).</P
><P
><SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> uses a fixed page size (commonly
8 kB), and does not allow tuples to span multiple pages. Therefore, it is
not possible to store very large field values directly. To overcome
this limitation, large field values are compressed and/or broken up into
multiple physical rows. This happens transparently to the user, with only
small impact on most of the backend code. The technique is affectionately
known as <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
> (or <SPAN
CLASS="QUOTE"
>"the best thing since sliced bread"</SPAN
>).</P
><P
>Only certain data types support <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
> — there is no need to
impose the overhead on data types that cannot produce large field values.
To support <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
>, a data type must have a variable-length
(<I
CLASS="FIRSTTERM"
>varlena</I
>) representation, in which the first 32-bit word of any
stored value contains the total length of the value in bytes (including
itself). <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
> does not constrain the rest of the representation.
All the C-level functions supporting a <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
>-able data type must
be careful to handle <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
>ed input values. (This is normally done
by invoking <CODE
CLASS="FUNCTION"
>PG_DETOAST_DATUM</CODE
> before doing anything with an input
value, but in some cases more efficient approaches are possible.)</P
><P
><ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
> usurps two bits of the varlena length word (the high-order
bits on big-endian machines, the low-order bits on little-endian machines),
thereby limiting the logical size of any value of a <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
>-able
data type to 1 GB (2<SUP
>30</SUP
> - 1 bytes). When both bits are zero,
the value is an ordinary un-<ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
>ed value of the data type, and
the remaining bits of the length word give the total datum size (including
length word) in bytes. When the highest-order or lowest-order bit is set,
the value has only a single-byte header instead of the normal four-byte
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 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
bits of the length word give the total size of the compressed datum, not the
original data. Note that compression is also possible for out-of-line data
but the varlena header does not tell whether it has occurred —
the content of the TOAST pointer tells that, instead.</P
><P
>If any of the columns of a table are <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
>-able, the table will
have an associated <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
> table, whose OID is stored in the table's
<TT
CLASS="STRUCTNAME"
>pg_class</TT
>.<TT
CLASS="STRUCTFIELD"
>reltoastrelid</TT
> entry. Out-of-line
<ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
>ed values are kept in the <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
> table, as
described in more detail below.</P
><P
>The compression technique used is a fairly simple and very fast member
of the LZ family of compression techniques. See
<TT
CLASS="FILENAME"
>src/backend/utils/adt/pg_lzcompress.c</TT
> for the details.</P
><P
>Out-of-line values are divided (after compression if used) into chunks of at
most <TT
CLASS="SYMBOL"
>TOAST_MAX_CHUNK_SIZE</TT
> 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
CLASS="ACRONYM"
>TOAST</ACRONYM
> table for the owning table. Every
<ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
> table has the columns <TT
CLASS="STRUCTFIELD"
>chunk_id</TT
> (an OID
identifying the particular <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
>ed value),
<TT
CLASS="STRUCTFIELD"
>chunk_seq</TT
> (a sequence number for the chunk within its value),
and <TT
CLASS="STRUCTFIELD"
>chunk_data</TT
> (the actual data of the chunk). A unique index
on <TT
CLASS="STRUCTFIELD"
>chunk_id</TT
> and <TT
CLASS="STRUCTFIELD"
>chunk_seq</TT
> provides fast
retrieval of the values. A pointer datum representing an out-of-line
<ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
>ed value therefore needs to store the OID of the
<ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
> table in which to look and the OID of the specific value
(its <TT
CLASS="STRUCTFIELD"
>chunk_id</TT
>). 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 bytes,
the total size of a <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
> pointer datum is therefore 18 bytes
regardless of the actual size of the represented value.</P
><P
>The <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
> code is triggered only
when a row value to be stored in a table is wider than
<TT
CLASS="SYMBOL"
>TOAST_TUPLE_THRESHOLD</TT
> bytes (normally 2 kB).
The <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
> code will compress and/or move
field values out-of-line until the row value is shorter than
<TT
CLASS="SYMBOL"
>TOAST_TUPLE_TARGET</TT
> 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
CLASS="ACRONYM"
>TOAST</ACRONYM
> costs if
none of the out-of-line values change.</P
><P
>The <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
> code recognizes four different strategies for storing
<ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
>-able columns:
<P
></P
></P><UL
><LI
><P
> <TT
CLASS="LITERAL"
>PLAIN</TT
> prevents either compression or
out-of-line storage; furthermore it disables use of single-byte headers
for varlena types.
This is the only possible strategy for
columns of non-<ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
>-able data types.
</P
></LI
><LI
><P
> <TT
CLASS="LITERAL"
>EXTENDED</TT
> allows both compression and out-of-line
storage. This is the default for most <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
>-able data types.
Compression will be attempted first, then out-of-line storage if
the row is still too big.
</P
></LI
><LI
><P
> <TT
CLASS="LITERAL"
>EXTERNAL</TT
> allows out-of-line storage but not
compression. Use of <TT
CLASS="LITERAL"
>EXTERNAL</TT
> will
make substring operations on wide <TT
CLASS="TYPE"
>text</TT
> and
<TT
CLASS="TYPE"
>bytea</TT
> columns faster (at the penalty of increased storage
space) because these operations are optimized to fetch only the
required parts of the out-of-line value when it is not compressed.
</P
></LI
><LI
><P
> <TT
CLASS="LITERAL"
>MAIN</TT
> allows compression but not out-of-line
storage. (Actually, out-of-line storage will still be performed
for such columns, but only as a last resort when there is no other
way to make the row small enough to fit on a page.)
</P
></LI
></UL
><P>
Each <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
>-able data type specifies a default strategy for columns
of that data type, but the strategy for a given table column can be altered
with <TT
CLASS="COMMAND"
>ALTER TABLE SET STORAGE</TT
>.</P
><P
>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
usually qualified by comparisons against relatively small key values, most of
the work of the executor will be done using the main row entry. The big values
of <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
>ed attributes will only be pulled out (if selected at all)
at the time the result set is sent to the client. Thus, the main table is much
smaller and more of its rows fit in the shared buffer cache than would be the
case without any out-of-line storage. Sort sets shrink also, and sorts will
more often be done entirely in memory. A little test showed that a table
containing typical HTML pages and their URLs was stored in about half of the
raw data size including the <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
> table, and that the main table
contained only about 10% of the entire data (the URLs and some small HTML
pages). There was no run time difference compared to an un-<ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
>ed
comparison table, in which all the HTML pages were cut down to 7 kB to fit.</P
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