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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>68.2. TOAST</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="storage-file-layout.html" title="68.1. Database File Layout" /><link rel="next" href="storage-fsm.html" title="68.3. Free Space Map" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">68.2. TOAST</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="storage-file-layout.html" title="68.1. Database File Layout">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="storage.html" title="Chapter 68. Database Physical Storage">Up</a></td><th width="60%" align="center">Chapter 68. Database Physical Storage</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="storage-fsm.html" title="68.3. Free Space Map">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="STORAGE-TOAST"><div class="titlepage"><div><div><h2 class="title" style="clear: both">68.2. TOAST</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="storage-toast.html#STORAGE-TOAST-ONDISK">68.2.1. Out-of-line, on-disk TOAST storage</a></span></dt><dt><span class="sect2"><a href="storage-toast.html#STORAGE-TOAST-INMEMORY">68.2.2. Out-of-line, in-memory TOAST storage</a></span></dt></dl></div><a id="id-1.10.20.4.2" class="indexterm"></a><a id="id-1.10.20.4.3" class="indexterm"></a><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">“<span class="quote">the best thing since sliced bread</span>”</span>).
The <acronym class="acronym">TOAST</acronym> infrastructure is also used to improve handling of
large data values in-memory.
</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
(<em class="firstterm">varlena</em>) representation, in which, ordinarily, the first
four-byte 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 data type's representation. The special representations collectively
called <em class="firstterm"><acronym class="acronym">TOAST</acronym>ed values</em> work by modifying or
reinterpreting this initial length word. Therefore, the C-level functions
supporting a <acronym class="acronym">TOAST</acronym>-able data type must be careful about how they
handle potentially <acronym class="acronym">TOAST</acronym>ed input values: an input might not
actually consist of a four-byte length word and contents until after it's
been <em class="firstterm">detoasted</em>. (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.
See <a class="xref" href="xtypes.html#XTYPES-TOAST" title="38.12.1. TOAST Considerations">Section 38.12.1</a> for more detail.)
</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 of that byte give the total datum size
(including length byte) in bytes. This alternative supports space-efficient
storage of values shorter than 127 bytes, while still allowing the data type
to grow to 1 GB at need. Values with single-byte headers aren't aligned on
any particular boundary, whereas values with four-byte headers are aligned on
at least a four-byte boundary; this omission of alignment padding provides
additional space savings that is significant compared to short values.
As a special case, if the remaining bits of a single-byte header are all
zero (which would be impossible for a self-inclusive length), the value is
a pointer to out-of-line data, with several possible alternatives as
described below. The type and size of such a <em class="firstterm">TOAST pointer</em>
are determined by a code stored in the second byte of the datum.
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 four-byte
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 <acronym class="acronym">TOAST</acronym> pointer tells that, instead.
</p><p>
As mentioned, there are multiple types of <acronym class="acronym">TOAST</acronym> pointer datums.
The oldest and most common type is a pointer to out-of-line data stored in
a <em class="firstterm"><acronym class="acronym">TOAST</acronym> table</em> that is separate from, but
associated with, the table containing the <acronym class="acronym">TOAST</acronym> pointer datum
itself. These <em class="firstterm">on-disk</em> pointer datums are created by the
<acronym class="acronym">TOAST</acronym> management code (in <code class="filename">access/heap/tuptoaster.c</code>)
when a tuple to be stored on disk is too large to be stored as-is.
Further details appear in <a class="xref" href="storage-toast.html#STORAGE-TOAST-ONDISK" title="68.2.1. Out-of-line, on-disk TOAST storage">Section 68.2.1</a>.
Alternatively, a <acronym class="acronym">TOAST</acronym> pointer datum can contain a pointer to
out-of-line data that appears elsewhere in memory. Such datums are
necessarily short-lived, and will never appear on-disk, but they are very
useful for avoiding copying and redundant processing of large data values.
Further details appear in <a class="xref" href="storage-toast.html#STORAGE-TOAST-INMEMORY" title="68.2.2. Out-of-line, in-memory TOAST storage">Section 68.2.2</a>.
</p><p>
The compression technique used for either in-line or out-of-line compressed
data is a fairly simple and very fast member
of the LZ family of compression techniques. See
<code class="filename">src/common/pg_lzcompress.c</code> for the details.
</p><div class="sect2" id="STORAGE-TOAST-ONDISK"><div class="titlepage"><div><div><h3 class="title">68.2.1. Out-of-line, on-disk TOAST storage</h3></div></div></div><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
<code class="structname">pg_class</code>.<code class="structfield">reltoastrelid</code> entry. On-disk
<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>
Out-of-line values are divided (after compression if used) into chunks of at
most <code class="symbol">TOAST_MAX_CHUNK_SIZE</code> 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
belonging to the owning table. Every
<acronym class="acronym">TOAST</acronym> table has the columns <code class="structfield">chunk_id</code> (an OID
identifying the particular <acronym class="acronym">TOAST</acronym>ed value),
<code class="structfield">chunk_seq</code> (a sequence number for the chunk within its value),
and <code class="structfield">chunk_data</code> (the actual data of the chunk). A unique index
on <code class="structfield">chunk_id</code> and <code class="structfield">chunk_seq</code> provides fast
retrieval of the values. A pointer datum representing an out-of-line on-disk
<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 <code class="structfield">chunk_id</code>). For convenience, pointer datums also store the
logical datum size (original uncompressed data length) and physical stored size
(different if compression was applied). Allowing for the varlena header bytes,
the total size of an on-disk <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> management code is triggered only
when a row value to be stored in a table is wider than
<code class="symbol">TOAST_TUPLE_THRESHOLD</code> 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
<code class="symbol">TOAST_TUPLE_TARGET</code> 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 class="acronym">TOAST</acronym> costs if
none of the out-of-line values change.
</p><p>
The <acronym class="acronym">TOAST</acronym> management code recognizes four different strategies
for storing <acronym class="acronym">TOAST</acronym>-able columns on disk:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
<code class="literal">PLAIN</code> 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 class="listitem"><p>
<code class="literal">EXTENDED</code> 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 class="listitem"><p>
<code class="literal">EXTERNAL</code> allows out-of-line storage but not
compression. Use of <code class="literal">EXTERNAL</code> will
make substring operations on wide <code class="type">text</code> and
<code class="type">bytea</code> 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 class="listitem"><p>
<code class="literal">MAIN</code> 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></div><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 <a class="link" href="sql-altertable.html" title="ALTER TABLE"><code class="command">ALTER TABLE ... SET STORAGE</code></a>.
</p><p>
<code class="symbol">TOAST_TUPLE_TARGET</code> can be adjusted for each table using
<a class="link" href="sql-altertable.html" title="ALTER TABLE"><code class="command">ALTER TABLE ... SET (toast_tuple_target = N)</code></a>
</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></div><div class="sect2" id="STORAGE-TOAST-INMEMORY"><div class="titlepage"><div><div><h3 class="title">68.2.2. Out-of-line, in-memory TOAST storage</h3></div></div></div><p>
<acronym class="acronym">TOAST</acronym> pointers can point to data that is not on disk, but is
elsewhere in the memory of the current server process. Such pointers
obviously cannot be long-lived, but they are nonetheless useful. There
are currently two sub-cases:
pointers to <em class="firstterm">indirect</em> data and
pointers to <em class="firstterm">expanded</em> data.
</p><p>
Indirect <acronym class="acronym">TOAST</acronym> pointers simply point at a non-indirect varlena
value stored somewhere in memory. This case was originally created merely
as a proof of concept, but it is currently used during logical decoding to
avoid possibly having to create physical tuples exceeding 1 GB (as pulling
all out-of-line field values into the tuple might do). The case is of
limited use since the creator of the pointer datum is entirely responsible
that the referenced data survives for as long as the pointer could exist,
and there is no infrastructure to help with this.
</p><p>
Expanded <acronym class="acronym">TOAST</acronym> pointers are useful for complex data types
whose on-disk representation is not especially suited for computational
purposes. As an example, the standard varlena representation of a
<span class="productname">PostgreSQL</span> array includes dimensionality information, a
nulls bitmap if there are any null elements, then the values of all the
elements in order. When the element type itself is variable-length, the
only way to find the <em class="replaceable"><code>N</code></em>'th element is to scan through all the
preceding elements. This representation is appropriate for on-disk storage
because of its compactness, but for computations with the array it's much
nicer to have an <span class="quote">“<span class="quote">expanded</span>”</span> or <span class="quote">“<span class="quote">deconstructed</span>”</span>
representation in which all the element starting locations have been
identified. The <acronym class="acronym">TOAST</acronym> pointer mechanism supports this need by
allowing a pass-by-reference Datum to point to either a standard varlena
value (the on-disk representation) or a <acronym class="acronym">TOAST</acronym> pointer that
points to an expanded representation somewhere in memory. The details of
this expanded representation are up to the data type, though it must have
a standard header and meet the other API requirements given
in <code class="filename">src/include/utils/expandeddatum.h</code>. C-level functions
working with the data type can choose to handle either representation.
Functions that do not know about the expanded representation, but simply
apply <code class="function">PG_DETOAST_DATUM</code> to their inputs, will automatically
receive the traditional varlena representation; so support for an expanded
representation can be introduced incrementally, one function at a time.
</p><p>
<acronym class="acronym">TOAST</acronym> pointers to expanded values are further broken down
into <em class="firstterm">read-write</em> and <em class="firstterm">read-only</em> pointers.
The pointed-to representation is the same either way, but a function that
receives a read-write pointer is allowed to modify the referenced value
in-place, whereas one that receives a read-only pointer must not; it must
first create a copy if it wants to make a modified version of the value.
This distinction and some associated conventions make it possible to avoid
unnecessary copying of expanded values during query execution.
</p><p>
For all types of in-memory <acronym class="acronym">TOAST</acronym> pointer, the <acronym class="acronym">TOAST</acronym>
management code ensures that no such pointer datum can accidentally get
stored on disk. In-memory <acronym class="acronym">TOAST</acronym> pointers are automatically
expanded to normal in-line varlena values before storage — and then
possibly converted to on-disk <acronym class="acronym">TOAST</acronym> pointers, if the containing
tuple would otherwise be too big.
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