69.2. System Catalog Initial Data

69.2.1. 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 => 'TemplateDbOid',
  descr => 'database\'s default template',
  datname => 'template1', encoding => 'ENCODING', datcollate => 'LC_COLLATE',
  datctype => 'LC_CTYPE', datistemplate => 't', datallowconn => 't',
  datconnlimit => '-1', datlastsysoid => '0', datfrozenxid => '0',
  datminmxid => '1', dattablespace => 'pg_default', 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 keys 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 Section 69.2.2 below, while array_type_oid is described in Section 69.2.4. 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 Section 4.1.2.2); 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 Section 69.2.3 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.

69.2.2. 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 12000 at the start of a bootstrap run. Thus objects created by regular SQL commands during the later phases of bootstrap, such as objects created while running the information_schema.sql script, receive OIDs of 12000 or above.

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 bootstrap. These automatically-assigned OIDs are not considered stable, and may change from one installation to another.

69.2.3. 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 such a column, all entries must use the symbolic format except when writing 0 for InvalidOid. (If the column is declared regproc, you can optionally write - instead of 0.) 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.

• In addition to the generic lookup mechanisms, there is a special convention that PGNSP is replaced by the OID of the pg_catalog schema, and PGUID is replaced by the OID of the bootstrap superuser role. These usages are somewhat historical but so far there hasn't been a need to generalize them.

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.

69.2.4. 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.

69.2.5. 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:

For further examples of scripts used for bulk editing, see convert_oid2name.pl and remove_pg_type_oid_symbols.pl attached to this message: https://www.postgresql.org/message-id/CAJVSVGVX8gXnPm+Xa=DxR7kFYprcQ1tNcCT5D0O3ShfnM6jehA@mail.gmail.com