72.2. TOAST
This section provides an overview of TOAST (The Oversized-Attribute Storage Technique).
Postgres Pro 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 TOASTed 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 TOASTed 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 39.13.1 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 (230 - 1 bytes). When both bits are zero, the value is an ordinary un-TOASTed 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.
The compression technique used for either in-line or out-of-line compressed data can be selected for each column by setting the COMPRESSION column option in CREATE TABLE or ALTER TABLE. The default for columns with no explicit setting is to consult the default_toast_compression parameter at the time data is inserted.
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 when a tuple to be stored on disk is too large to be stored as-is. Further details appear in Section 72.2.1. 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 72.2.2.
72.2.1. 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 TOASTed 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 TOASTed 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 TOASTed 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), physical stored size (different if compression was applied), and the compression method used, if any. 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. 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 TOASTed 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-TOASTed comparison table, in which all the HTML pages were cut down to 7 kB to fit.
72.2.2. 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 Postgres Pro 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.
72.2.3. Pluggable TOAST
TOAST is a part of Postgres Pro core. TOAST is not extensible, has the same strategy for all data types and is not effective for structured data (JSON) or data that requires a special workflow (bytea).
Experimental pluggable TOAST provides an open API allowing you to develop and plug in custom TOAST implementations for table columns and data types in addition to the default one. These implementations are called TOASTers. A TOASTer can process TOASTed data using the knowledge about the internal data structure and workflow.
TOASTers can use any storage, advanced compression, encryption and other functionalities internally, without being aware of the table access method that calls the TOASTer.
The pluggable TOAST consists of the core, that is, the TOAST API, and custom TOASTers plugged in to Postgres Pro with this API.
This API does not possess any logic regarding the TOAST functionality, it is just a wraparound that allows registering, calling and dropping plugged TOASTers.
To enable using custom TOAST implementations, the TOAST API defines a new Custom type of TOAST pointer in addition to External and Extended (corresponding to EXTERNAL and EXTENDED TOAST-able column storing strategies).
Currently the pluggable TOAST has several limitations:
Custom TOASTers must only be assigned to a column with the
EXTERNALstoring strategy.Currently the pluggable TOAST does not support logical replication. A custom TOAST pointer is skipped by the logical replication engine and is not replicated. This is because support of custom pointers requires heavy modifications in the Postgres Pro core (the logical replication engine) and even architecture changes as now custom pointers allow updating TOASTed values, while default pointers do not.
Custom TOASTers are not restored from dumps and require manual actions for restoring. The TOASTer extension is not restored from dump, thus the data processed with it are not available. Another point is that the same TOASTer extension has different IDs in different systems, so restoring a TOASTed value from dump would require determining the TOASTer used for processing data.
When a custom TOASTer is assigned to a table column, in all the transactions after the TOASTer assignment, data being stored in the TOAST tables is processed with the TOASTer assigned to the column.
When a custom TOASTer is assigned to a table column, old data values are left as-is and are not re-TOASTed although while being updated, an old data value is TOASTed with the assigned TOASTer because of the workflow of the default TOAST mechanics.
If the TOASTer is somehow dropped, the data TOASTed with it will not be accessible with any standard techniques and is considered to be lost. To protect TOASTed values, the API creates some dependencies in the system (
PG_DEPENDtable):When the TOASTer is added to a system, a TOASTer-extension dependency is created.
When the TOASTer is assigned to a column, a TOASTer-relation dependency is created.
When the TOASTer is unassigned from a column, the TOASTer-relation dependency is deleted.
When the TOASTer is dropped from a system, the TOASTer-extension dependency is deleted.
Custom TOASTers are added as extensions implementing some mandatory functions (check README.toastapi file in /contrib/toastapi for more details). Pluggable TOAST provides a set of SQL functions to control custom TOASTers. These functions are called with SQL SELECT command:
add_toaster(→toaster_nametext,toaster_handler_functext)integerPlug in a new TOASTer (added before as an extension). This function also creates a dependency in
PG_DEPENDfor the TOASTer OID to its extension OID, which is used to protect the TOASTer from being dropped when dropping the TOASTer extension.Arguments:
toaster_name— TOASTer name to be stored inPG_TOASTER. Must be unique.toaster_handler_func— TOASTer handler function name (automatically added as aregprocwhen the TOASTer extension is installed).
Returns OID assigned to the new TOASTer.
set_toaster(→toaster_nametext,tab_nametext,col_nametext)integerAssign a TOASTer to a table column. This function also creates a dependency in
PG_DEPENDfor the TOASTer OID to the relation OID. This dependency is used to protect TOASTed data by restricting dropping the TOASTer that is assigned to a column.Arguments:
toaster_name— name of the TOASTer assigned to a table column.tab_name— table name.col_name— column name.
Returns OID of the TOASTer assigned to the column.
reset_toaster(→tab_nametext,col_nametext)integerReset the TOAST mechanism to a default one for a table column. This function drops the TOASTer-relation dependency, which allows dropping the TOASTer later, when all depencencies are dropped.
Arguments:
tab_name— table name.col_name— table column name.
Returns always 0.
drop_toaster(toaster_nametext)Drop a TOASTer from the system. Only unused TOASTers, which do not have any data TOASTed with them can be dropped. Dropping is protected by the dependencies created by
add_toasterandset_toasterfunctions. If a dependency still exists, the TOASTer would not be dropped. If the TOASTer was dropped successfully, the extension with its implementation can be dropped by theDROP EXTENSIONcommand.Arguments:
toaster_name— name of the TOASTer to be dropped.
Returns the TOASTer OID on success or 0 if the TOASTer is used.
get_toaster(→tab_nametext,col_nametext)integerGet the OID of the TOASTer assigned to a table column.
Arguments:
tab_name— table name.col_name— table column name.
Returns the OID of the TOASTer assigned to a column or 0 if none.
The following example illustrates the use of the pluggable TOAST API:
CREATE EXTENSION toastapi;
CREATE EXTENSION bytea_toaster;
SELECT add_toaster('bytea_toaster');
CREATE TABLE test_bytea_append (id int, a bytea);
ALTER TABLE test_bytea_append ALTER a SET STORAGE external;
SELECT set_toaster('bytea_toaster', 'test_bytea_append', 'a');
test_set_toaster
------------------
(1 row)
...
SELECT get_toaster('test_bytea_append', 'a') AS bytea_toaster_oid;
get_toaster
-------------
16348
(1 row)
SELECT pgpro_toast.reset_toaster('test_bytea_append','a');
reset_toaster
---------------
0
(1 row)
DROP TABLE test_bytea_append;
SELECT pgpro_toast.drop_toaster('bytea_toaster');
drop_toaster
--------------
16348
(1 row)
DROP EXTENSION bytea_toaster;