Lorenzo Alberton

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Extracting META information from a PostgreSQL database using the INFORMATION_SCHEMA views and the system catalogs (pg_class, pg_user, pg_view)

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Extracting META information from PostgreSQL (INFORMATION_SCHEMA)

Following my tutorial on how to extract meta informations from Firebird SQL, I'm now going to show how to retrieve the same informations from PostgreSQL, using the INFORMATION_SCHEMA (available since PostgreSQL 7.4) and with system catalogs (pg_class, pg_user, pg_view, etc).
NB: as you probably know, you can list tables/indices/sequences/views from the command line with the \d{t|i|s|v} command, but here I want to show how to extract these informations using standard SQL queries.

Test data

We need a few sample tables, indices and views to test the following queries, so let's create them. We also create a sample TRIGGER and a function.

-- sample data to test PostgreSQL INFORMATION_SCHEMA
 
-- TABLE TEST
CREATE TABLE TEST (
  TEST_NAME CHAR(30) NOT NULL,
  TEST_ID INTEGER DEFAULT '0' NOT NULL,
  TEST_DATE TIMESTAMP NOT NULL
);
ALTER TABLE TEST ADD CONSTRAINT PK_TEST PRIMARY KEY (TEST_ID);
 
-- TABLE TEST2 with some CONSTRAINTs and an INDEX
CREATE TABLE TEST2 (
  ID INTEGER NOT NULL,
  FIELD1 INTEGER,
  FIELD2 CHAR(15),
  FIELD3 VARCHAR(50),
  FIELD4 INTEGER,
  FIELD5 INTEGER,
  ID2 INTEGER NOT NULL
);
ALTER TABLE TEST2 ADD CONSTRAINT PK_TEST2 PRIMARY KEY (ID2);
ALTER TABLE TEST2 ADD CONSTRAINT TEST2_FIELD1ID_IDX UNIQUE (ID, FIELD1);
ALTER TABLE TEST2 ADD CONSTRAINT TEST2_FIELD4_IDX UNIQUE (FIELD4);
CREATE INDEX TEST2_FIELD5_IDX ON TEST2(FIELD5);
 
-- TABLE NUMBERS
CREATE TABLE NUMBERS (
  NUMBER INTEGER DEFAULT '0' NOT NULL,
  EN CHAR(100) NOT NULL,
  FR CHAR(100) NOT NULL
);
 
-- TABLE NEWTABLE
CREATE TABLE NEWTABLE (
  ID INT DEFAULT 0 NOT NULL,
  SOMENAME VARCHAR (12),
  SOMEDATE TIMESTAMP NOT NULL
);
ALTER TABLE NEWTABLE ADD CONSTRAINT PKINDEX_IDX PRIMARY KEY (ID);
CREATE SEQUENCE NEWTABLE_SEQ INCREMENT 1 START 1;
 
-- VIEW on TEST
CREATE VIEW "testview"(
  TEST_NAME,
  TEST_ID,
  TEST_DATE
) AS
SELECT *
FROM TEST
WHERE TEST_NAME LIKE 't%';
 
-- VIEW on NUMBERS
CREATE VIEW "numbersview"(
  NUMBER,
  TRANS_EN,
  TRANS_FR
) AS
SELECT *
FROM NUMBERS
WHERE NUMBER > 100;
 
-- TRIGGER on NEWTABLE
CREATE FUNCTION add_stamp() RETURNS OPAQUE AS '
    BEGIN
      IF (NEW.somedate IS NULL OR NEW.somedate = 0) THEN
        NEW.somedate := CURRENT_TIMESTAMP;
        RETURN NEW;
      END IF;
    END;
' LANGUAGE 'plpgsql';
 
CREATE TRIGGER ADDCURRENTDATE
BEFORE INSERT OR UPDATE
ON newtable FOR EACH ROW
  EXECUTE PROCEDURE add_stamp();
 
-- TABLEs for testing CONSTRAINTs
CREATE TABLE testconstraints (
  someid integer NOT NULL,
  somename character varying(10) NOT NULL,
  CONSTRAINT testconstraints_id_pk PRIMARY KEY (someid)
);
CREATE TABLE testconstraints2 (
  ext_id integer NOT NULL,
  modified date,
  uniquefield character varying(10) NOT NULL,
  usraction integer NOT NULL,
  CONSTRAINT testconstraints_id_fk FOREIGN KEY (ext_id)
      REFERENCES testconstraints (someid) MATCH SIMPLE
      ON UPDATE CASCADE ON DELETE CASCADE,
  CONSTRAINT unique_2_fields_idx UNIQUE (modified, usraction),
  CONSTRAINT uniquefld_idx UNIQUE (uniquefield)
);

List TABLEs

Here's the query that will return the names of the tables defined in the current database:

SELECT relname
  FROM pg_class
 WHERE relname !~ '^(pg_|sql_)'
   AND relkind = 'r';
 
-- using INFORMATION_SCHEMA:
 
SELECT table_name
  FROM information_schema.tables
 WHERE table_type = 'BASE TABLE'
   AND table_schema NOT IN
       ('pg_catalog', 'information_schema');

List VIEWs

Here's the query that will return the names of the VIEWs defined in the current database:

-- with postgresql 7.2:
 
SELECT viewname
  FROM pg_views
 WHERE viewname !~ '^pg_';
 
-- with postgresql 7.4 and later:
 
SELECT viewname
  FROM pg_views
 WHERE schemaname NOT IN
       ('pg_catalog', 'information_schema')
   AND viewname !~ '^pg_';
 
-- using INFORMATION_SCHEMA:
 
SELECT table_name
  FROM information_schema.tables
 WHERE table_type = 'VIEW'
   AND table_schema NOT IN
       ('pg_catalog', 'information_schema')
   AND table_name !~ '^pg_';
 
-- or
 
SELECT table_name
  FROM information_schema.views
 WHERE table_schema NOT IN ('pg_catalog', 'information_schema')
   AND table_name !~ '^pg_';

List users

SELECT usename
  FROM pg_user;

List table fields

Here's the query that will return the names of the fields of the TEST2 table:

SELECT a.attname
  FROM pg_class c, pg_attribute a, pg_type t
 WHERE c.relname = 'test2'
   AND a.attnum > 0
   AND a.attrelid = c.oid
   AND a.atttypid = t.oid
 
-- with INFORMATION_SCHEMA:
 
SELECT column_name
  FROM information_schema.columns
 WHERE table_name = 'test2';

Detailed table field info

If you want some more info about the field definitions, you can retrieve a larger subset of the fields available in the schema:

SELECT a.attnum AS ordinal_position,
         a.attname AS column_name,
         t.typname AS data_type,
         a.attlen AS character_maximum_length,
         a.atttypmod AS modifier,
         a.attnotnull AS notnull,
         a.atthasdef AS hasdefault
    FROM pg_class c,
         pg_attribute a,
         pg_type t
   WHERE c.relname = 'test2'
     AND a.attnum > 0
     AND a.attrelid = c.oid
     AND a.atttypid = t.oid
ORDER BY a.attnum;
 
-- with INFORMATION_SCHEMA:
 
  SELECT ordinal_position,
         column_name,
         data_type,
         column_default,
         is_nullable,
         character_maximum_length,
         numeric_precision
    FROM information_schema.columns
   WHERE table_name = 'test2'
ORDER BY ordinal_position;

List INDICES

Here's the query that will return the names of the INDICES defined in the TEST2 table. Unfortunately I have no idea how to extract them from the INFORMATION_SCHEMA. If you do, please let me know.
NB: the CONSTRAINTs are not listed

SELECT relname
  FROM pg_class
 WHERE oid IN (
    SELECT indexrelid
      FROM pg_index, pg_class
     WHERE pg_class.relname='test2'
       AND pg_class.oid=pg_index.indrelid
       AND indisunique != 't'
       AND indisprimary != 't';

Detailed INDEX info

If you want to know which table columns are referenced by an index, you can do it in two steps: first you get the table name and field(s) position with this query:

SELECT relname, indkey
  FROM pg_class, pg_index
 WHERE pg_class.oid = pg_index.indexrelid
   AND pg_class.oid IN (
    SELECT indexrelid
      FROM pg_index, pg_class
     WHERE pg_class.relname='test2'
       AND pg_class.oid=pg_index.indrelid
       AND indisunique != 't'
       AND indisprimary != 't'
);

Then, using your favorite language, you explode the indkey (the key separator is a space), and for each key you run this query:

SELECT t.relname, a.attname, a.attnum
     FROM pg_index c
LEFT JOIN pg_class t
       ON c.indrelid  = t.oid
LEFT JOIN pg_attribute a
       ON a.attrelid = t.oid
      AND a.attnum = ANY(indkey)
    WHERE t.relname = 'test2'
      AND a.attnum = 6; -- this is the index key

List CONSTRAINTs

Here's the query that will return the names of the CONSTRAINTs defined in the TEST2 table:

SELECT relname
  FROM pg_class
 WHERE oid IN (
    SELECT indexrelid
      FROM pg_index, pg_class
     WHERE pg_class.relname='test2'
       AND pg_class.oid=pg_index.indrelid
       AND (   indisunique = 't'
            OR indisprimary = 't'
       )
);
 
-- with INFORMATION_SCHEMA:
 
SELECT constraint_name, constraint_type
  FROM information_schema.table_constraints
 WHERE table_name = 'test2';

Detailed CONSTRAINT info

If you want to retrieve detailed info from any constraint (fields, type, rules, referenced table and fields for FOREIGN KEYs, etc.) given its name and table, here's the query to do so:

SELECT c.conname AS constraint_name,
          CASE c.contype
            WHEN 'c' THEN 'CHECK'
            WHEN 'f' THEN 'FOREIGN KEY'
            WHEN 'p' THEN 'PRIMARY KEY'
            WHEN 'u' THEN 'UNIQUE'
          END AS "constraint_type",
          CASE WHEN c.condeferrable = 'f' THEN 0 ELSE 1 END AS is_deferrable,
          CASE WHEN c.condeferred = 'f' THEN 0 ELSE 1 END AS is_deferred,
          t.relname AS table_name,
          array_to_string(c.conkey, ' ') AS constraint_key,
          CASE confupdtype
            WHEN 'a' THEN 'NO ACTION'
            WHEN 'r' THEN 'RESTRICT'
            WHEN 'c' THEN 'CASCADE'
            WHEN 'n' THEN 'SET NULL'
            WHEN 'd' THEN 'SET DEFAULT'
          END AS on_update,
          CASE confdeltype
            WHEN 'a' THEN 'NO ACTION'
            WHEN 'r' THEN 'RESTRICT'
            WHEN 'c' THEN 'CASCADE'
            WHEN 'n' THEN 'SET NULL'
            WHEN 'd' THEN 'SET DEFAULT'
          END AS on_delete,
          CASE confmatchtype
            WHEN 'u' THEN 'UNSPECIFIED'
            WHEN 'f' THEN 'FULL'
            WHEN 'p' THEN 'PARTIAL'
          END AS match_type,
          t2.relname AS references_table,
          array_to_string(c.confkey, ' ') AS fk_constraint_key
     FROM pg_constraint c
LEFT JOIN pg_class t  ON c.conrelid  = t.oid
LEFT JOIN pg_class t2 ON c.confrelid = t2.oid
    WHERE t.relname = 'testconstraints2'
     AND c.conname = 'testconstraints_id_fk';
     
-- with INFORMATION_SCHEMA:
 
   SELECT tc.constraint_name,
          tc.constraint_type,
          tc.table_name,
          kcu.column_name,
	  tc.is_deferrable,
          tc.initially_deferred,
          rc.match_option AS match_type,
          rc.update_rule AS on_update,
          rc.delete_rule AS on_delete,
          ccu.table_name AS references_table,
          ccu.column_name AS references_field
     FROM information_schema.table_constraints tc
LEFT JOIN information_schema.key_column_usage kcu
       ON tc.constraint_catalog = kcu.constraint_catalog
      AND tc.constraint_schema = kcu.constraint_schema
      AND tc.constraint_name = kcu.constraint_name
LEFT JOIN information_schema.referential_constraints rc
       ON tc.constraint_catalog = rc.constraint_catalog
      AND tc.constraint_schema = rc.constraint_schema
      AND tc.constraint_name = rc.constraint_name
LEFT JOIN information_schema.constraint_column_usage ccu
       ON rc.unique_constraint_catalog = ccu.constraint_catalog
      AND rc.unique_constraint_schema = ccu.constraint_schema
      AND rc.unique_constraint_name = ccu.constraint_name
    WHERE tc.table_name = 'testconstraints2'
      AND tc.constraint_name = 'testconstraints_id_fk';

The "constraint_key" and "fk_constraint_key" fields returned by the first query are space-separated strings containing the position of the fields involved (in the FOREIGN KEY constraint and those referenced by it), so you may need to retrieve them with another query on the respective tables. Since the field positions are stored as arrays, you can't (to the best of my knowledge) get all the field names with an unique query (well, you could with a stored procedure).
The second query, the one using the INFORMATION_SCHEMA, is certainly more straightforward, albeit slower.

List sequences

A SEQUENCE is an object that automatically generate sequence numbers. A SEQUENCE is often used to ensure a unique value in a PRIMARY KEY that must uniquely identify the associated row.

SELECT relname
  FROM pg_class
 WHERE relkind = 'S'
   AND relnamespace IN (
        SELECT oid
          FROM pg_namespace
         WHERE nspname NOT LIKE 'pg_%'
           AND nspname != 'information_schema'
);

List TRIGGERs

SELECT trg.tgname AS trigger_name
  FROM pg_trigger trg, pg_class tbl
 WHERE trg.tgrelid = tbl.oid
   AND tbl.relname !~ '^pg_';
-- or
SELECT tgname AS trigger_name
  FROM pg_trigger
 WHERE tgname !~ '^pg_';
 
-- with INFORMATION_SCHEMA:
 
SELECT DISTINCT trigger_name
  FROM information_schema.triggers
 WHERE trigger_schema NOT IN
       ('pg_catalog', 'information_schema');

List only the triggers for a given table:

SELECT trg.tgname AS trigger_name
  FROM pg_trigger trg, pg_class tbl
 WHERE trg.tgrelid = tbl.oid
   AND tbl.relname = 'newtable';
 
-- with INFORMATION_SCHEMA:
 
SELECT DISTINCT trigger_name
  FROM information_schema.triggers
 WHERE event_object_table = 'newtable'
   AND trigger_schema NOT IN
       ('pg_catalog', 'information_schema');

Detailed TRIGGER info

Show more informations about the trigger definitions:

SELECT trg.tgname AS trigger_name,
       tbl.relname AS table_name,
       p.proname AS function_name,
       CASE trg.tgtype & cast(2 as int2)
         WHEN 0 THEN 'AFTER'
         ELSE 'BEFORE'
       END AS trigger_type,
       CASE trg.tgtype & cast(28 as int2)
         WHEN 16 THEN 'UPDATE'
         WHEN  8 THEN 'DELETE'
         WHEN  4 THEN 'INSERT'
         WHEN 20 THEN 'INSERT, UPDATE'
         WHEN 28 THEN 'INSERT, UPDATE, DELETE'
         WHEN 24 THEN 'UPDATE, DELETE'
         WHEN 12 THEN 'INSERT, DELETE'
       END AS trigger_event,
       CASE trg.tgtype & cast(1 as int2)
         WHEN 0 THEN 'STATEMENT'
         ELSE 'ROW'
       END AS action_orientation
  FROM pg_trigger trg,
       pg_class tbl,
       pg_proc p
 WHERE trg.tgrelid = tbl.oid
   AND trg.tgfoid = p.oid
   AND tbl.relname !~ '^pg_';
 
-- with INFORMATION_SCHEMA:
 
SELECT *
  FROM information_schema.triggers
 WHERE trigger_schema NOT IN
       ('pg_catalog', 'information_schema');

List FUNCTIONs

SELECT proname
  FROM pg_proc pr,
       pg_type tp
 WHERE tp.oid = pr.prorettype
   AND pr.proisagg = FALSE
   AND tp.typname <> 'trigger'
   AND pr.pronamespace IN (
       SELECT oid
         FROM pg_namespace
        WHERE nspname NOT LIKE 'pg_%'
          AND nspname != 'information_schema'
);
 
-- with INFORMATION_SCHEMA:
 
SELECT routine_name
  FROM information_schema.routines
 WHERE specific_schema NOT IN
       ('pg_catalog', 'information_schema')
   AND type_udt_name != 'trigger';

Albe Laurenz sent me the following function that is even more informative: for a function name and schema, it selects the position in the argument list, the direction, the name and the data-type of each argument. This procedure requires PostgreSQL 8.1 or later.

CREATE OR REPLACE FUNCTION public.function_args(
  IN funcname character varying,
  IN schema character varying,
  OUT pos integer,
  OUT direction character,
  OUT argname character varying,
  OUT datatype character varying)
RETURNS SETOF RECORD AS $$DECLARE
  rettype character varying;
  argtypes oidvector;
  allargtypes oid[];
  argmodes "char"[];
  argnames text[];
  mini integer;
  maxi integer;
BEGIN
  /* get object ID of function */
  SELECT INTO rettype, argtypes, allargtypes, argmodes, argnames
         CASE
         WHEN pg_proc.proretset
         THEN 'setof ' || pg_catalog.format_type(pg_proc.prorettype, NULL)
         ELSE pg_catalog.format_type(pg_proc.prorettype, NULL) END,
         pg_proc.proargtypes,
         pg_proc.proallargtypes,
         pg_proc.proargmodes,
         pg_proc.proargnames
    FROM pg_catalog.pg_proc
         JOIN pg_catalog.pg_namespace
         ON (pg_proc.pronamespace = pg_namespace.oid)
   WHERE pg_proc.prorettype <> 'pg_catalog.cstring'::pg_catalog.regtype
     AND (pg_proc.proargtypes[0] IS NULL
      OR pg_proc.proargtypes[0] <> 'pg_catalog.cstring'::pg_catalog.regtype)
     AND NOT pg_proc.proisagg
     AND pg_proc.proname = funcname
     AND pg_namespace.nspname = schema
     AND pg_catalog.pg_function_is_visible(pg_proc.oid);
 
  /* bail out if not found */
  IF NOT FOUND THEN
    RETURN;
  END IF;
 
  /* return a row for the return value */
  pos = 0;
  direction = 'o'::char;
  argname = 'RETURN VALUE';
  datatype = rettype;
  RETURN NEXT;
 
  /* unfortunately allargtypes is NULL if there are no OUT parameters */
  IF allargtypes IS NULL THEN
    mini = array_lower(argtypes, 1); maxi = array_upper(argtypes, 1);
  ELSE
    mini = array_lower(allargtypes, 1); maxi = array_upper(allargtypes, 1);
  END IF;
  IF maxi < mini THEN RETURN; END IF;
 
  /* loop all the arguments */
  FOR i IN mini .. maxi LOOP
    pos = i - mini + 1;
    IF argnames IS NULL THEN
      argname = NULL;
    ELSE
      argname = argnames[pos];
    END IF;
    IF allargtypes IS NULL THEN
      direction = 'i'::char;
      datatype = pg_catalog.format_type(argtypes[i], NULL);
    ELSE
      direction = argmodes[i];
      datatype = pg_catalog.format_type(allargtypes[i], NULL);
    END IF;
    RETURN NEXT;
  END LOOP;
 
  RETURN;
END;$$ LANGUAGE plpgsql STABLE STRICT SECURITY INVOKER;
COMMENT ON FUNCTION public.function_args(character varying, character
varying)
IS $$For a function name and schema, this procedure selects for each
argument the following data:
- position in the argument list (0 for the return value)
- direction 'i', 'o', or 'b'
- name (NULL if not defined)
- data type$$;

Show PROCEDURE definition

SELECT p.proname AS procedure_name,
          p.pronargs AS num_args,
          t1.typname AS return_type,
          a.rolname AS procedure_owner,
          l.lanname AS language_type,
          p.proargtypes AS argument_types_oids,
          prosrc AS body
     FROM pg_proc p
LEFT JOIN pg_type t1 ON p.prorettype=t1.oid   
LEFT JOIN pg_authid a ON p.proowner=a.oid 
LEFT JOIN pg_language l ON p.prolang=l.oid
    WHERE proname = :PROCEDURE_NAME;

pg_proc.proargtypes contains an array of oids pointing to pg_type.oid. You can use unnest(), generate_procedure() or the function in the previous paragraph to retrieve the data type of each parameter.

What else?

If you'd like to see some other example queries, or have some comments and/or suggestions, just drop me a mail (you can find my address in the footer of this page) and I'll add them to this list.
HTH.

   SELECT c.conname AS constraint_name,
          t.relname AS table_name,
          a.attname AS field_name,
          t2.relname AS references_table,
          a2.attname AS references_field
     FROM pg_constraint c
LEFT JOIN pg_class t  ON c.conrelid  = t.oid
LEFT JOIN pg_class t2 ON c.confrelid = t2.oid
LEFT JOIN pg_attribute a ON t.oid = a.attrelid AND a.attnum = c.conkey[1]
LEFT JOIN pg_attribute a2 ON t2.oid = a2.attrelid AND a2.attnum = c.confkey[1]
    WHERE t.relname = 'testconstraints2'
     AND c.conname = 'testconstraints_id_fk'
     AND c.contype = 'f';

SELECT * FROM information_schema.table_constraints  WHERE table_name = 'test'

SELECT * FROM information_schema.key_column_usage WHERE table_name = 'test'

SELECT constraint_name
FROM information_schema.table_constraints
WHERE table_name = 'test' and constraint_type = 'PRIMARY KEY';

   SELECT tc.constraint_name,
          tc.constraint_type,
          tc.table_name,
          kcu.column_name,
	  tc.is_deferrable,
          tc.initially_deferred,
          rc.match_option AS match_type,
          rc.update_rule AS on_update,
          rc.delete_rule AS on_delete,
          ccu.table_name AS references_table,
          ccu.column_name AS references_field
     FROM information_schema.table_constraints tc
LEFT JOIN information_schema.key_column_usage kcu
       ON tc.constraint_catalog = kcu.constraint_catalog
      AND tc.constraint_schema = kcu.constraint_schema
      AND tc.constraint_name = kcu.constraint_name
LEFT JOIN information_schema.referential_constraints rc
       ON tc.constraint_catalog = rc.constraint_catalog
      AND tc.constraint_schema = rc.constraint_schema
      AND tc.constraint_name = rc.constraint_name
LEFT JOIN information_schema.constraint_column_usage ccu
       ON rc.unique_constraint_catalog = ccu.constraint_catalog
      AND rc.unique_constraint_schema = ccu.constraint_schema
      AND rc.unique_constraint_name = ccu.constraint_name
    WHERE tc.table_name = 'test'
      AND tc.constraint_type = 'PRIMARY KEY';

Lorenzo Alberton

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