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7.3 gotchas for applications and client libraries

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: pgsql-hackers(at)postgreSQL(dot)org, pgsql-interfaces(at)postgreSQL(dot)org
Subject: 7.3 gotchas for applications and client libraries
Date: 2002-09-03 01:54:00
Message-ID: 851.1031018040@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-interfaces
Bruce suggested that we need a porting guide to help people look for
application and client-library code that will be broken by the changes
in PG 7.3.  Here is a first cut at documenting the issues.
Comments welcome --- in particular, what have I missed?

			regards, tom lane


Revising client-side code for PG 7.3 system catalogs


Here are some notes about things to look out for in updating client-side
code for PG 7.3.  Almost anything that looks at the system catalogs is
probably going to need work, if you want it to behave reasonably when you
start using 7.3's new features such as schemas and DROP COLUMN.

As an example, consider the task of listing the names and datatypes for
a table named "foo".  In the past you may have done this with a query like

	SELECT a.attname, format_type(a.atttypid, a.atttypmod)
	FROM pg_class c, pg_attribute a
	WHERE c.relname = 'foo'
	  AND a.attnum > 0 AND a.attrelid = c.oid
	ORDER BY a.attnum

(this in fact is exactly what 7.2 psql uses to implement "\d foo").
This query will work perfectly well in 7.2 or 7.1, but it's broken in half
a dozen ways for 7.3.

The biggest problem is that with the addition of schemas, there might be
several tables named "foo" listed in pg_class.  The old query will produce
a list of all of their attributes mixed together.  For example, after
	create schema a;
	create schema b;
	create table a.foo (f1 int, f2 text);
	create table b.foo (f1 text, f2 numeric(10,1));
we'd get:

 attname |  format_type
---------+---------------
 f1      | text
 f1      | integer
 f2      | text
 f2      | numeric(10,1)
(4 rows)

Not good.  We need to decide exactly which foo we want, and restrict the
query to find only that row in pg_class.  There are a couple of ways to
do this, depending on how fancy you want to get.

If you just want to handle an unqualified table name "foo", and find the
same foo that would be found if you said "select * from foo", then one way
to do it is to restrict the query to "visible" rows of pg_class:

	SELECT ...
	FROM ...
	WHERE c.relname = 'foo' AND pg_table_is_visible(c.oid)
	  AND ...

pg_table_is_visible() will only return true for pg_class rows that are in
your current search path and are not hidden by similarly-named tables that
are in earlier schemas of the search path.

An alternative way is to eliminate the explicit join to pg_class, and
instead use the new datatype "regclass" to look up the correct pg_class
OID:

	SELECT ...
	FROM pg_attribute a
	WHERE a.attrelid = 'foo'::regclass
	  AND a.attnum > 0
	ORDER BY a.attnum

The regclass input converter looks up the given string as a table name
(obeying schema visibility rules) and produces an OID constant that you
can compare directly to attrelid.  This is more efficient than doing
the join, but there are a couple of things to note about it.  One is
that if there isn't any "foo" table, you'll get an ERROR message from
the regclass input converter, whereas with the old query you got zero
rows out and no error message.  You might or might not prefer the old
behavior.  Another limitation is that there isn't any way to adapt
this approach to search for a partially-specified table name;
whereas in the original query you could use a LIKE or regex pattern to
match the table name, not only a simple equality test.

Now, what if you'd like to be able to specify a qualified table name
--- that is, show the attributes of "a.foo" or "b.foo" on demand?
It will not work to say
	WHERE c.relname = 'a.foo'
so this is another way in which the original query fails for 7.3.

It turns out that the regclass method will work for this: if you say
	WHERE a.attrelid = 'a.foo'::regclass
then the right things happen.

If you don't want to use regclass then you're going to have to do an
explicit join against pg_namespace to find out which foo you want:

	SELECT a.attname, format_type(a.atttypid, a.atttypmod)
	FROM pg_namespace n, pg_class c, pg_attribute a
	WHERE n.nspname = 'a' AND c.relname = 'foo'
	  AND c.relnamespace = n.oid
	  AND a.attnum > 0 AND a.attrelid = c.oid
	ORDER BY a.attnum

This is somewhat tedious because you have to be prepared to split the
qualified name into its components on the client side.  An advantage
is that once you've done that, you can again consider using LIKE or
regex patterns instead of simple name equality.  This is essentially
what 7.3 psql does to support wildcard patterns like "\dt a*.f*".

Okay, I think we've about beaten the issue of "which foo do you want"
to death.  But what other ways are there for the schema feature to cause
trouble in this apparently now well-fixed-up query?

One way is that the system catalogs themselves live in a schema, and
if that schema isn't frontmost in your search path then your references
to pg_class and so forth might find the wrong tables.  (It's legal now
for ordinary users to create tables named like "pg_xxx", so long as they
don't try to put 'em in the system's schema.)  This is probably not a
big issue for standalone applications, which can assume they know what the
search path is.  But in client-side libraries, psql, and similar code
that has to be able to deal with someone else's choice of search path,
we really ought to make the references to system catalogs be fully
qualified:

	SELECT ...
	FROM pg_catalog.pg_namespace n, pg_catalog.pg_class c,
	     pg_catalog.pg_attribute a
	WHERE ...

(If you weren't using table aliases in your queries before, here is a good
place to start...)

In fact it's worse than that: function names, type names, etc also live in
schemas.  So you really ought to qualify references to built-in functions
and types too:

	SELECT ..., pg_catalog.format_type(...) ...

	WHERE a.attrelid = 'foo'::pg_catalog.regclass ...

The truly paranoid might want to qualify their operator names too,
though I draw the line at this because of the horribly ugly syntax needed:

	WHERE a.attrelid OPERATOR(pg_catalog.=) 'foo'::pg_catalog.regclass

There's another, non-schema-related, gotcha in this apparently simple task
of showing attribute names: in 7.3 you need to exclude dropped attributes
from your display.  ALTER TABLE DROP COLUMN doesn't remove the
pg_attribute entry for the dropped column, it only changes it to have
attisdropped = true.  So you will typically want to add

	WHERE NOT attisdropped

when looking at pg_attribute.

Note however that excluding dropped columns like this means there may be
gaps in the series of attnum values you see.  That doesn't bother this
particular query, but it could easily confuse applications that expect the
attributes to have consecutive attnums 1 to N.  For example, pg_dump makes
an array of attributes and wants to index into the array with attnums.
It proved easier to make pg_dump include dropped attributes in its array
(and filter them out later) than to change the indexing logic.

If you have client-side code that looks in pg_proc, pg_type, or
pg_operator then exactly the same sorts of schema-related issues appear:
the name alone is no longer unique, you have to think about identifying
the function, type, or operator within the schema you want.

That's about all the mileage I can get out of the "show a table's
attributes" example, but there are still more schema-related trouble
items to check for.

One problem to look for is code that scans pg_class (or another system
table, but most commonly pg_class) to make a list of things to operate
on.  For example, various people have built scripts to automatically
reindex every table in a database.  Such code will fail as soon as you
start using schemas, because it will find tables that aren't in your
current schema search path and try to operate on them.  Depending on what
you want to do, you could change the code to emit fully qualified names
of tables it wants to operate on (so it will work no matter which schema
they are in), or you could restrict the pg_class search to find only
visible tables.

If you want to use qualified names, the straightforward way to do it
is to join against pg_namespace to get the schema name:

	SELECT nspname, relname FROM pg_class c, pg_namespace n
	WHERE relnamespace = n.oid AND relname LIKE 'foo%' AND ...

A less obvious way is to use the regclass output converter:

	SELECT c.oid::regclass FROM pg_class c
	WHERE relname LIKE 'foo%' AND ...

This will give you back a table name that is qualified only if it needs to
be (i.e., the table is not visible in your search path), so you can use
it directly in the command you want to give next.  Another interesting
property of the regclass converter is that it will double-quote the name
correctly if necessary --- for example, you'll get "TEST" (with the
quotes) if the table is named TEST.  So you can splice the name directly
into a SQL command without any special pushups and be confident that it
will produce the right results.

BTW, there are similar output converters for type, function, and operator
names, if you need them.

Another thing to look for is code that tries to exclude system tables by
excluding tablenames beginning with "pg_"; typical code is like
	WHERE relname NOT LIKE 'pg\\_%'
or
	WHERE relname !~ '^pg_'
This is not the preferred method anymore: the right way to do this is to
join against pg_namespace and exclude tables that live in schemas whose
names begin with "pg_".

A related point is that temporary tables no longer have names (in the
catalogs) of the form "pg_temp_NNN"; rather they have exactly the name
that the creating user gave them.  They are kept separate from other
tables by placing them in schemas named like "pg_temp_NNN" (where now
NNN identifies an active backend, not a single table).  So if you wanted
your scan to exclude temp tables then you'd definitely better change to
excluding on the basis of schema name not table name.  On the upside,
if you do want your scan to show temp tables then it's much easier than
before.  (BTW, the pg_table_is_visible function is the best way of
distinguishing your own session's temp tables from other people's.  Yours
will be visible, other people's won't.)

Other things that are less likely to concern most applications, but could
break some:

Aggregate functions now have entries in pg_proc; pg_aggregate has lost
most of its columns and now is just an extension of a pg_proc entry.
If you have code that knows the difference between a plain function and
an aggregate function then it will surely need work.

pg_class.relkind has a new possible value, 'c' for a composite type.

pg_type.typtype has two new possible values, 'd' for a domain and 'p' for
a pseudo-type.

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