70.2. System Catalog Initial Data

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:

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.

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:

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.

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.

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