# System Catalog Declarations and Initial Contents

PostgreSQL uses many different system catalogs to keep track of the existence and properties of database objects, such as tables and functions. Physically there is no difference between a system catalog and a plain user table, but the backend C code knows the structure and properties of each catalog, and can manipulate it directly at a low level. Thus, for example, it is inadvisable to attempt to alter the structure of a catalog on-the-fly; that would break assumptions built into the C code about how rows of the catalog are laid out. But the structure of the catalogs can change between major versions.

The structures of the catalogs are declared in specially formatted C header files in the `src/include/catalog/` directory of the source tree. For each catalog there is a header file named after the catalog (e.g., `pg_class.h` for pg_class), which defines the set of columns the catalog has, as well as some other basic properties such as its OID.

Many of the catalogs have initial data that must be loaded into them during the bootstrap phase of initdb, to bring the system up to a point where it is capable of executing SQL commands. (For example, `pg_class.h` must contain an entry for itself, as well as one for each other system catalog and index.) This initial data is kept in editable form in data files that are also stored in the `src/include/catalog/` directory. For example, `pg_proc.dat` describes all the initial rows that must be inserted into the pg_proc catalog.

To create the catalog files and load this initial data into them, a backend running in bootstrap mode reads a BKI (Backend Interface) file containing commands and initial data. The `postgres.bki` file used in this mode is prepared from the aforementioned header and data files, while building a PostgreSQL distribution, by a Perl script named `genbki.pl`. Although it's specific to a particular PostgreSQL release, `postgres.bki` is platform-independent and is installed in the `share` subdirectory of the installation tree.

`genbki.pl` also produces a derived header file for each catalog, for example `pg_class_d.h` for the pg_class catalog. This file contains automatically-generated macro definitions, and may contain other macros, enum declarations, and so on that can be useful for client C code that reads a particular catalog.

Most PostgreSQL developers don't need to be directly concerned with the BKI file, but almost any nontrivial feature addition in the backend will require modifying the catalog header files and/or initial data files. The rest of this chapter gives some information about that, and for completeness describes the BKI file format.

## System Catalog Declaration Rules

The key part of a catalog header file is a C structure definition describing the layout of each row of the catalog. This begins with a `CATALOG` macro, which so far as the C compiler is concerned is just shorthand for `typedef struct FormData_`catalogname``. Each field in the struct gives rise to a catalog column. Fields can be annotated using the BKI property macros described in `genbki.h`, for example to define a default value for a field or mark it as nullable or not nullable. The `CATALOG` line can also be annotated, with some other BKI property macros described in `genbki.h`, to define other properties of the catalog as a whole, such as whether it is a shared relation.

The system catalog cache code (and most catalog-munging code in general) assumes that the fixed-length portions of all system catalog tuples are in fact present, because it maps this C struct declaration onto them. Thus, all variable-length fields and nullable fields must be placed at the end, and they cannot be accessed as struct fields. For example, if you tried to set pg_type.typrelid to be NULL, it would fail when some piece of code tried to reference `typetup->typrelid` (or worse, `typetup->typelem`, because that follows typrelid). This would result in random errors or even segmentation violations.

As a partial guard against this type of error, variable-length or nullable fields should not be made directly visible to the C compiler. This is accomplished by wrapping them in `#ifdef CATALOG_VARLEN` ... `#endif` (where `CATALOG_VARLEN` is a symbol that is never defined). This prevents C code from carelessly trying to access fields that might not be there or might be at some other offset. As an independent guard against creating incorrect rows, we require all columns that should be non-nullable to be marked so in pg_attribute. The bootstrap code will automatically mark catalog columns as `NOT NULL` if they are fixed-width and are not preceded by any nullable or variable-width column. Where this rule is inadequate, you can force correct marking by using `BKI_FORCE_NOT_NULL` and `BKI_FORCE_NULL` annotations as needed.

Frontend code should not include any `pg_xxx.h` catalog header file, as these files may contain C code that won't compile outside the backend. (Typically, that happens because these files also contain declarations for functions in `src/backend/catalog/` files.) Instead, frontend code may include the corresponding generated `pg_xxx_d.h` header, which will contain OID `#define`s and any other data that might be of use on the client side. If you want macros or other code in a catalog header to be visible to frontend code, write `#ifdef EXPOSE_TO_CLIENT_CODE` ... `#endif` around that section to instruct `genbki.pl` to copy that section to the `pg_xxx_d.h` header.

A few of the catalogs are so fundamental that they can't even be created by the BKI `create` command that's used for most catalogs, because that command needs to write information into these catalogs to describe the new catalog. These are called bootstrap catalogs, and defining one takes a lot of extra work: you have to manually prepare appropriate entries for them in the pre-loaded contents of pg_class and pg_type, and those entries will need to be updated for subsequent changes to the catalog's structure. (Bootstrap catalogs also need pre-loaded entries in pg_attribute, but fortunately `genbki.pl` handles that chore nowadays.) Avoid making new catalogs be bootstrap catalogs if at all possible.

## System Catalog Initial Data

Each catalog that has any manually-created initial data (some do not) has a corresponding `.dat` file that contains its initial data in an editable format.

### Data File Format

Each `.dat` file contains Perl data structure literals that are simply eval'd to produce an in-memory data structure consisting of an array of hash references, one per catalog row. A slightly modified excerpt from `pg_database.dat` will demonstrate the key features:

    [

    # A comment could appear here.
    { oid => '1', oid_symbol => 'Template1DbOid',
      descr => 'database\'s default template',
      datname => 'template1', encoding => 'ENCODING',
      datlocprovider => 'LOCALE_PROVIDER', datistemplate => 't',
      datallowconn => 't', dathasloginevt => 'f', datconnlimit => '-1', datfrozenxid => '0',
      datminmxid => '1', dattablespace => 'pg_default', datcollate => 'LC_COLLATE',
      datctype => 'LC_CTYPE', datlocale => 'DATLOCALE', datacl => '_null_' },

    ]

Points to note:

- The overall file layout is: open square bracket, one or more sets of curly braces each of which represents a catalog row, close square bracket. Write a comma after each closing curly brace.

- Within each catalog row, write comma-separated `key` `=>` `value` pairs. The allowed `key`s are the names of the catalog's columns, plus the metadata keys `oid`, `oid_symbol`, `array_type_oid`, and `descr`. (The use of `oid` and `oid_symbol` is described in system-catalog-oid-assignment below, while `array_type_oid` is described in system-catalog-auto-array-types. `descr` supplies a description string for the object, which will be inserted into pg_description or pg_shdescription as appropriate.) While the metadata keys are optional, the catalog's defined columns must all be provided, except when the catalog's `.h` file specifies a default value for the column. (In the example above, the datdba field has been omitted because `pg_database.h` supplies a suitable default value for it.)

- All values must be single-quoted. Escape single quotes used within a value with a backslash. Backslashes meant as data can, but need not, be doubled; this follows Perl's rules for simple quoted literals. Note that backslashes appearing as data will be treated as escapes by the bootstrap scanner, according to the same rules as for escape string constants (see sql-syntax-strings-escape); for example `\t` converts to a tab character. If you actually want a backslash in the final value, you will need to write four of them: Perl strips two, leaving `\\` for the bootstrap scanner to see.

- Null values are represented by `_null_`. (Note that there is no way to create a value that is just that string.)

- Comments are preceded by `#`, and must be on their own lines.

- Field values that are OIDs of other catalog entries should be represented by symbolic names rather than actual numeric OIDs. (In the example above, dattablespace contains such a reference.) This is described in system-catalog-oid-references below.

- Since hashes are unordered data structures, field order and line layout aren't semantically significant. However, to maintain a consistent appearance, we set a few rules that are applied by the formatting script `reformat_dat_file.pl`:

  - Within each pair of curly braces, the metadata fields `oid`, `oid_symbol`, `array_type_oid`, and `descr` (if present) come first, in that order, then the catalog's own fields appear in their defined order.

  - Newlines are inserted between fields as needed to limit line length to 80 characters, if possible. A newline is also inserted between the metadata fields and the regular fields.

  - If the catalog's `.h` file specifies a default value for a column, and a data entry has that same value, `reformat_dat_file.pl` will omit it from the data file. This keeps the data representation compact.

  - `reformat_dat_file.pl` preserves blank lines and comment lines as-is.

  It's recommended to run `reformat_dat_file.pl` before submitting catalog data patches. For convenience, you can simply change to `src/include/catalog/` and run `make reformat-dat-files`.

- If you want to add a new method of making the data representation smaller, you must implement it in `reformat_dat_file.pl` and also teach `Catalog::ParseData()` how to expand the data back into the full representation.

### OID Assignment

A catalog row appearing in the initial data can be given a manually-assigned OID by writing an `oid => `nnnn`` metadata field. Furthermore, if an OID is assigned, a C macro for that OID can be created by writing an `oid_symbol => `name`` metadata field.

Pre-loaded catalog rows must have preassigned OIDs if there are OID references to them in other pre-loaded rows. A preassigned OID is also needed if the row's OID must be referenced from C code. If neither case applies, the `oid` metadata field can be omitted, in which case the bootstrap code assigns an OID automatically. In practice we usually preassign OIDs for all or none of the pre-loaded rows in a given catalog, even if only some of them are actually cross-referenced.

Writing the actual numeric value of any OID in C code is considered very bad form; always use a macro, instead. Direct references to pg_proc OIDs are common enough that there's a special mechanism to create the necessary macros automatically; see `src/backend/utils/Gen_fmgrtab.pl`. Similarly — but, for historical reasons, not done the same way — there's an automatic method for creating macros for pg_type OIDs. `oid_symbol` entries are therefore not necessary in those two catalogs. Likewise, macros for the pg_class OIDs of system catalogs and indexes are set up automatically. For all other system catalogs, you have to manually specify any macros you need via `oid_symbol` entries.

To find an available OID for a new pre-loaded row, run the script `src/include/catalog/unused_oids`. It prints inclusive ranges of unused OIDs (e.g., the output line `45-900` means OIDs 45 through 900 have not been allocated yet). Currently, OIDs 1–9999 are reserved for manual assignment; the `unused_oids` script simply looks through the catalog headers and `.dat` files to see which ones do not appear. You can also use the `duplicate_oids` script to check for mistakes. (`genbki.pl` will assign OIDs for any rows that didn't get one hand-assigned to them, and it will also detect duplicate OIDs at compile time.)

When choosing OIDs for a patch that is not expected to be committed immediately, best practice is to use a group of more-or-less consecutive OIDs starting with some random choice in the range 8000—9999. This minimizes the risk of OID collisions with other patches being developed concurrently. To keep the 8000—9999 range free for development purposes, after a patch has been committed to the master git repository its OIDs should be renumbered into available space below that range. Typically, this will be done near the end of each development cycle, moving all OIDs consumed by patches committed in that cycle at the same time. The script `renumber_oids.pl` can be used for this purpose. If an uncommitted patch is found to have OID conflicts with some recently-committed patch, `renumber_oids.pl` may also be useful for recovering from that situation.

Because of this convention of possibly renumbering OIDs assigned by patches, the OIDs assigned by a patch should not be considered stable until the patch has been included in an official release. We do not change manually-assigned object OIDs once released, however, as that would create assorted compatibility problems.

If `genbki.pl` needs to assign an OID to a catalog entry that does not have a manually-assigned OID, it will use a value in the range 10000—11999. The server's OID counter is set to 10000 at the start of a bootstrap run, so that any objects created on-the-fly during bootstrap processing also receive OIDs in this range. (The usual OID assignment mechanism takes care of preventing any conflicts.)

Objects with OIDs below FirstUnpinnedObjectId (12000) are considered pinned, preventing them from being deleted. (There are a small number of exceptions, which are hard-wired into `IsPinnedObject()`.) initdb forces the OID counter up to FirstUnpinnedObjectId as soon as it's ready to create unpinned objects. Thus objects created during the later phases of initdb, such as objects created while running the `information_schema.sql` script, will not be pinned, while all objects known to `genbki.pl` will be.

OIDs assigned during normal database operation are constrained to be 16384 or higher. This ensures that the range 10000—16383 is free for OIDs assigned automatically by `genbki.pl` or during initdb. These automatically-assigned OIDs are not considered stable, and may change from one installation to another.

### OID Reference Lookup

In principle, cross-references from one initial catalog row to another could be written just by writing the preassigned OID of the referenced row in the referencing field. However, that is against project policy, because it is error-prone, hard to read, and subject to breakage if a newly-assigned OID is renumbered. Therefore `genbki.pl` provides mechanisms to write symbolic references instead. The rules are as follows:

- Use of symbolic references is enabled in a particular catalog column by attaching `BKI_LOOKUP(`lookuprule`)` to the column's definition, where `lookuprule` is the name of the referenced catalog, e.g., `pg_proc`. `BKI_LOOKUP` can be attached to columns of type `Oid`, `regproc`, `oidvector`, or `Oid[]`; in the latter two cases it implies performing a lookup on each element of the array.

- It's also permissible to attach `BKI_LOOKUP(encoding)` to integer columns to reference character set encodings, which are not currently represented as catalog OIDs, but have a set of values known to `genbki.pl`.

- In some catalog columns, it's allowed for entries to be zero instead of a valid reference. If this is allowed, write `BKI_LOOKUP_OPT` instead of `BKI_LOOKUP`. Then you can write `0` for an entry. (If the column is declared `regproc`, you can optionally write `-` instead of `0`.) Except for this special case, all entries in a `BKI_LOOKUP` column must be symbolic references. `genbki.pl` will warn about unrecognized names.

- Most kinds of catalog objects are simply referenced by their names. Note that type names must exactly match the referenced pg_type entry's typname; you do not get to use any aliases such as `integer` for `int4`.

- A function can be represented by its proname, if that is unique among the `pg_proc.dat` entries (this works like regproc input). Otherwise, write it as `proname(argtypename,argtypename,...)`, like regprocedure. The argument type names must be spelled exactly as they are in the `pg_proc.dat` entry's proargtypes field. Do not insert any spaces.

- Operators are represented by `oprname(lefttype,righttype)`, writing the type names exactly as they appear in the `pg_operator.dat` entry's oprleft and oprright fields. (Write `0` for the omitted operand of a unary operator.)

- The names of opclasses and opfamilies are only unique within an access method, so they are represented by `access_method_name``/``object_name`.

- In none of these cases is there any provision for schema-qualification; all objects created during bootstrap are expected to be in the `pg_catalog` schema.

`genbki.pl` resolves all symbolic references while it runs, and puts simple numeric OIDs into the emitted BKI file. There is therefore no need for the bootstrap backend to deal with symbolic references.

It's desirable to mark OID reference columns with `BKI_LOOKUP` or `BKI_LOOKUP_OPT` even if the catalog has no initial data that requires lookup. This allows `genbki.pl` to record the foreign key relationships that exist in the system catalogs. That information is used in the regression tests to check for incorrect entries. See also the macros `DECLARE_FOREIGN_KEY`, `DECLARE_FOREIGN_KEY_OPT`, `DECLARE_ARRAY_FOREIGN_KEY`, and `DECLARE_ARRAY_FOREIGN_KEY_OPT`, which are used to declare foreign key relationships that are too complex for `BKI_LOOKUP` (typically, multi-column foreign keys).

### Automatic Creation of Array Types

Most scalar data types should have a corresponding array type (that is, a standard varlena array type whose element type is the scalar type, and which is referenced by the typarray field of the scalar type's pg_type entry). `genbki.pl` is able to generate the pg_type entry for the array type automatically in most cases.

To use this facility, just write an `array_type_oid => `nnnn`` metadata field in the scalar type's pg_type entry, specifying the OID to use for the array type. You may then omit the typarray field, since it will be filled automatically with that OID.

The generated array type's name is the scalar type's name with an underscore prepended. The array entry's other fields are filled from `BKI_ARRAY_DEFAULT(`value`)` annotations in `pg_type.h`, or if there isn't one, copied from the scalar type. (There's also a special case for typalign.) Then the typelem and typarray fields of the two entries are set to cross-reference each other.

### Recipes for Editing Data Files

Here are some suggestions about the easiest ways to perform common tasks when updating catalog data files.

### Add a new column with a default to a catalog:

Add the column to the header file with a `BKI_DEFAULT(`value`)` annotation. The data file need only be adjusted by adding the field in existing rows where a non-default value is needed.

### Add a default value to an existing column that doesn't have one:

Add a `BKI_DEFAULT` annotation to the header file, then run `make reformat-dat-files` to remove now-redundant field entries.

### Remove a column, whether it has a default or not:

Remove the column from the header, then run `make reformat-dat-files` to remove now-useless field entries.

### Change or remove an existing default value:

You cannot simply change the header file, since that will cause the current data to be interpreted incorrectly. First run `make expand-dat-files` to rewrite the data files with all default values inserted explicitly, then change or remove the `BKI_DEFAULT` annotation, then run `make reformat-dat-files` to remove superfluous fields again.

### Ad-hoc bulk editing:

`reformat_dat_file.pl` can be adapted to perform many kinds of bulk changes. Look for its block comments showing where one-off code can be inserted. In the following example, we are going to consolidate two Boolean fields in pg_proc into a char field:

1. Add the new column, with a default, to `pg_proc.h`:

       +    /* see PROKIND_ categories below */
       +    char        prokind BKI_DEFAULT(f);

2. Create a new script based on `reformat_dat_file.pl` to insert appropriate values on-the-fly:

       -           # At this point we have the full row in memory as a hash
       -           # and can do any operations we want. As written, it only
       -           # removes default values, but this script can be adapted to
       -           # do one-off bulk-editing.
       +           # One-off change to migrate to prokind
       +           # Default has already been filled in by now, so change to other
       +           # values as appropriate
       +           if ($values{proisagg} eq 't')
       +           {
       +               $values{prokind} = 'a';
       +           }
       +           elsif ($values{proiswindow} eq 't')
       +           {
       +               $values{prokind} = 'w';
       +           }

3. Run the new script:

       $ cd src/include/catalog
       $ perl  rewrite_dat_with_prokind.pl  pg_proc.dat

   At this point `pg_proc.dat` has all three columns, prokind, proisagg, and proiswindow, though they will appear only in rows where they have non-default values.

4. Remove the old columns from `pg_proc.h`:

       -    /* is it an aggregate? */
       -    bool        proisagg BKI_DEFAULT(f);
       -
       -    /* is it a window function? */
       -    bool        proiswindow BKI_DEFAULT(f);

5. Finally, run `make reformat-dat-files` to remove the useless old entries from `pg_proc.dat`.

For further examples of scripts used for bulk editing, see `convert_oid2name.pl` and `remove_pg_type_oid_symbols.pl` attached to this message:

## BKI File Format

This section describes how the PostgreSQL backend interprets BKI files. This description will be easier to understand if the `postgres.bki` file is at hand as an example.

BKI input consists of a sequence of commands. Commands are made up of a number of tokens, depending on the syntax of the command. Tokens are usually separated by whitespace, but need not be if there is no ambiguity. There is no special command separator; the next token that syntactically cannot belong to the preceding command starts a new one. (Usually you would put a new command on a new line, for clarity.) Tokens can be certain key words, special characters (parentheses, commas, etc.), identifiers, numbers, or single-quoted strings. Everything is case sensitive.

Lines starting with `#` are ignored.

## BKI Commands

`create` `tablename` `tableoid` `bootstrap` `shared_relation` `rowtype_oid` `oid` (`name1` = `type1` `FORCE NOT NULL` | `FORCE NULL` , `name2` = `type2` `FORCE NOT NULL` | `FORCE NULL` , ...)

- Create a table named `tablename`, and having the OID `tableoid`, with the columns given in parentheses.

  The following column types are supported directly by `bootstrap.c`: `bool`, `bytea`, `char` (1 byte), `name`, `int2`, `int4`, `regproc`, `regclass`, `regtype`, `text`, `oid`, `tid`, `xid`, `cid`, `int2vector`, `oidvector`, `_int4` (array), `_text` (array), `_oid` (array), `_char` (array), `_aclitem` (array). Although it is possible to create tables containing columns of other types, this cannot be done until after pg_type has been created and filled with appropriate entries. (That effectively means that only these column types can be used in bootstrap catalogs, but non-bootstrap catalogs can contain any built-in type.)

  When `bootstrap` is specified, the table will only be created on disk; nothing is entered into pg_class, pg_attribute, etc., for it. Thus the table will not be accessible by ordinary SQL operations until such entries are made the hard way (with `insert` commands). This option is used for creating pg_class etc. themselves.

  The table is created as shared if `shared_relation` is specified. The table's row type OID (pg_type OID) can optionally be specified via the `rowtype_oid` clause; if not specified, an OID is automatically generated for it. (The `rowtype_oid` clause is useless if `bootstrap` is specified, but it can be provided anyway for documentation.)

`open` `tablename`

- Open the table named `tablename` for insertion of data. Any currently open table is closed.

`close` `tablename`

- Close the open table. The name of the table must be given as a cross-check.

`insert` `(` `oid_value` `value1` `value2` ... `)`

- Insert a new row into the open table using `value1`, `value2`, etc., for its column values.

  NULL values can be specified using the special key word `_null_`. Values that do not look like identifiers or digit strings must be single-quoted. (To include a single quote in a value, write it twice. Escape-string-style backslash escapes are allowed in the string, too.)

`declare` `unique` `index` `indexname` `indexoid` `on` `tablename` `using` `amname` `(` `opclass1` `name1` , ... `)`

- Create an index named `indexname`, having OID `indexoid`, on the table named `tablename`, using the `amname` access method. The fields to index are called `name1`, `name2` etc., and the operator classes to use are `opclass1`, `opclass2` etc., respectively. The index file is created and appropriate catalog entries are made for it, but the index contents are not initialized by this command.

`declare toast` `toasttableoid` `toastindexoid` `on` `tablename`

- Create a TOAST table for the table named `tablename`. The TOAST table is assigned OID `toasttableoid` and its index is assigned OID `toastindexoid`. As with `declare index`, filling of the index is postponed.

`build indices`

- Fill in the indices that have previously been declared.

## Structure of the Bootstrap BKI File

The `open` command cannot be used until the tables it uses exist and have entries for the table that is to be opened. (These minimum tables are pg_class, pg_attribute, pg_proc, and pg_type.) To allow those tables themselves to be filled, `create` with the `bootstrap` option implicitly opens the created table for data insertion.

Also, the `declare index` and `declare toast` commands cannot be used until the system catalogs they need have been created and filled in.

Thus, the structure of the `postgres.bki` file has to be:

1. `create bootstrap` one of the critical tables

2. `insert` data describing at least the critical tables

3. `close`

4. Repeat for the other critical tables.

5. `create` (without `bootstrap`) a noncritical table

6. `open`

7. `insert` desired data

8. `close`

9. Repeat for the other noncritical tables.

10. Define indexes and toast tables.

11. `build indices`

There are doubtless other, undocumented ordering dependencies.

## BKI Example

The following sequence of commands will create the table `test_table` with OID 420, having three columns `oid`, `cola` and `colb` of type `oid`, `int4` and `text`, respectively, and insert two rows into the table:

    create test_table 420 (oid = oid, cola = int4, colb = text)
    open test_table
    insert ( 421 1 'value 1' )
    insert ( 422 2 _null_ )
    close test_table