68.2. TOAST
This section provides an overview of TOAST (The Oversized-Attribute Storage Technique).
PostgreSQL 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 TOAST (or " the best thing since sliced bread " ). The TOAST infrastructure is also used to improve handling of large data values in-memory.
Only certain data types support
TOAST
- there is no need to
impose the overhead on data types that cannot produce large field values.
To support
TOAST
, a data type must have a variable-length
(
varlena
) representation, in which, ordinarily, the first
four-byte word of any stored value contains the total length of the value in
bytes (including itself).
TOAST
does not constrain the rest
of the data type's representation. The special representations collectively
called
TOAST
ed values
work by modifying or
reinterpreting this initial length word. Therefore, the C-level functions
supporting a
TOAST
-able data type must be careful about how they
handle potentially
TOAST
ed input values: an input might not
actually consist of a four-byte length word and contents until after it's
been
detoasted
. (This is normally done by invoking
PG_DETOAST_DATUM
before doing anything with an input value,
but in some cases more efficient approaches are possible.
See
Section 38.12.1
for more detail.)
TOAST 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 TOAST -able data type to 1 GB (2 30 - 1 bytes). When both bits are zero, the value is an ordinary un- TOAST 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 TOAST pointer 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 TOAST pointer tells that, instead.
As mentioned, there are multiple types of
TOAST
pointer datums.
The oldest and most common type is a pointer to out-of-line data stored in
a
TOAST
table
that is separate from, but
associated with, the table containing the
TOAST
pointer datum
itself. These
on-disk
pointer datums are created by the
TOAST
management code (in
access/heap/tuptoaster.c
)
when a tuple to be stored on disk is too large to be stored as-is.
Further details appear in
Section 68.2.1
.
Alternatively, a
TOAST
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
Section 68.2.2
.
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
src/common/pg_lzcompress.c
for the details.
68.2.1. Out-of-line, on-disk TOAST storage
If any of the columns of a table are
TOAST
-able, the table will
have an associated
TOAST
table, whose OID is stored in the table's
pg_class
.
reltoastrelid
entry. On-disk
TOAST
ed values are kept in the
TOAST
table, as
described in more detail below.
Out-of-line values are divided (after compression if used) into chunks of at
most
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
TOAST
table
belonging to the owning table. Every
TOAST
table has the columns
chunk_id
(an OID
identifying the particular
TOAST
ed value),
chunk_seq
(a sequence number for the chunk within its value),
and
chunk_data
(the actual data of the chunk). A unique index
on
chunk_id
and
chunk_seq
provides fast
retrieval of the values. A pointer datum representing an out-of-line on-disk
TOAST
ed value therefore needs to store the OID of the
TOAST
table in which to look and the OID of the specific value
(its
chunk_id
). 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
TOAST
pointer datum is therefore 18
bytes regardless of the actual size of the represented value.
The
TOAST
management code is triggered only
when a row value to be stored in a table is wider than
TOAST_TUPLE_THRESHOLD
bytes (normally 2 kB).
The
TOAST
code will compress and/or move
field values out-of-line until the row value is shorter than
TOAST_TUPLE_TARGET
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
TOAST
costs if
none of the out-of-line values change.
The TOAST management code recognizes four different strategies for storing TOAST -able columns on disk:
-
PLAINprevents 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- TOAST -able data types. -
EXTENDEDallows both compression and out-of-line storage. This is the default for most TOAST -able data types. Compression will be attempted first, then out-of-line storage if the row is still too big. -
EXTERNALallows out-of-line storage but not compression. Use ofEXTERNALwill make substring operations on widetextandbyteacolumns 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. -
MAINallows 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.)
Each
TOAST
-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
ALTER TABLE ... SET STORAGE
.
TOAST_TUPLE_TARGET
can be adjusted for each table using
ALTER TABLE ... SET (toast_tuple_target = N)
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 TOAST 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 TOAST 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- TOAST ed comparison table, in which all the HTML pages were cut down to 7 kB to fit.
68.2.2. Out-of-line, in-memory TOAST storage
TOAST 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 indirect data and pointers to expanded data.
Indirect TOAST 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.
Expanded
TOAST
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
PostgreSQL
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
N
'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
"
expanded
"
or
"
deconstructed
"
representation in which all the element starting locations have been
identified. The
TOAST
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
TOAST
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
src/include/utils/expandeddatum.h
. 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
PG_DETOAST_DATUM
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.
TOAST pointers to expanded values are further broken down into read-write and read-only 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.
For all types of in-memory TOAST pointer, the TOAST management code ensures that no such pointer datum can accidentally get stored on disk. In-memory TOAST pointers are automatically expanded to normal in-line varlena values before storage - and then possibly converted to on-disk TOAST pointers, if the containing tuple would otherwise be too big.