PL/PGSQL: Dynamic Record Introspection

Hi all,

I needed introspection capabilities for record types to write more generic
trigger procedures in PL/PGSQL.

With the following patch it's possible to
- extract all field names of a record into an array
- extract field count of a record
- address a single field of a record with a variable
containing the field name (additional to the usual record.fieldname
notation where the fieldname is hardcoded).

The syntax is
- record%NFIELDS gives the number of fields in the record
- record%FIELDNAMES gives the array of the field names
- record%scalarvariable extracts the field whose name
is equal to the contents of scalarvariable

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The patch is nonintrusive in the sense that it only adds things
with one exception:
In function exec_eval_datum(), file pl_exec.c, line 3557 (after the patch)
I chose to convert the record field values to TEXT if the caller does
not require a certain type (expectedtypeid == InvalidOid).
Additionally, I cast the value from the record field type if
the destination Datum is of different type.

As far as I can see, this does no harm because in most cases
the expectedtypeid is correctly set. I just wanted to avoid that
if it is not set, the returned datum is of a more restrictive type
than TEXT.

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The patch is against a HEAD checkout from 07/12/05
The output comes from difforig.

Test code for the patch can be extracted from an example I put into
plpgsql.sgml

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Here is a summary of things that get patched by the file:
- add three new special parsing functions to pl_comp.c
(plpgsql_parse_wordpercentword, plpgsql_parse_wordnfields,
plpgsql_parse_wordfieldnames).
- modify PLpgSQL_recfield in plpgsql.h to either hold
a conventional field name (record.fieldname) or a dno
for the variable (record%variable).
- add two PLPGSQL_DTYPEs for the two new % notations
- modify "case PLPGSQL_DTYPE_RECFIELD:"
in exec_eval_datum() and exec_assign_value()
to deal with index strings from a variable
- add "case PLPGSQL_DTYPE_RECFIELDNAMES"
and "case PLPGSQL_DTYPE_NRECFIELD"
to exec_eval_datum() to evaluate %FIELDNAMES
and %NFIELDS expressions.
- update plpgsql.sgml in the docs directory

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Please notify me if I can be of further assistance.

Regards
Titus