Thread: table column information
Currently using Postgresql 7.2.4-5.80 with php 4.2.2.-8.0.8 on a redhat 8.0 system. I am writing some php scripts where I want to generate a list of the column names in a particular table that the user selects. I could take the brute force method and hard code the column names but then every time I add a new table or modify an existing one I would have to modify the code. What I want is to have a generic function that given the table name it will pull the column names for my use. I need to get the table column names for several tables I have setup. I know if I do a select * from tablename I can then use the pg_fieldname function to pull the column names for all columns. But I don't think I want to select the entire contents of the table every time I want to get the names of the columns. I know this will work but I think performance will be very poor. Trying to find something the equivalent of doing a \d tablename in psql. I did see a function to pull meta data but that is in a 4.3 version of php. I have also been trying to track down some information on the pga_layout table. This appears to be a system table that might contain the information I want but it does not list every table I have created. Not sure what that is. The books I have do not say much if anything about such system tables. Any help or pointers would be appreciated. -- Scot L. Harris <webid@cfl.rr.com>
webid@cfl.rr.com wrote: > Currently using Postgresql 7.2.4-5.80 with php 4.2.2.-8.0.8 on a redhat > 8.0 system. > > I am writing some php scripts where I want to generate a list of the > column names in a particular table that the user selects. I could take > the brute force method and hard code the column names but then every > time I add a new table or modify an existing one I would have to modify > the code. What I want is to have a generic function that given the > table name it will pull the column names for my use. > > I need to get the table column names for several tables I have setup. I > know if I do a select * from tablename I can then use the pg_fieldname > function to pull the column names for all columns. > > But I don't think I want to select the entire contents of the table > every time I want to get the names of the columns. I know this will > work but I think performance will be very poor. >... You almost got it - just do "select * from tablename where 0=1", which returns no rows but will give you the fieldnames. A portable and (I think) efficient way to get table column names.
On Sun, 2004-05-16 at 20:19, ljb wrote: > You almost got it - just do "select * from tablename where 0=1", which returns > no rows but will give you the fieldnames. A portable and (I think) > efficient way to get table column names. Thanks. That should do it. I was not able to get to the list archives earlier. -- Scot L. Harris <webid@cfl.rr.com>
On Mon, 2004-05-17 at 00:19 +0000, ljb wrote:
> >
> > I am writing some php scripts where I want to generate a list of the
> > column names in a particular table that the user selects. I could take
> > the brute force method and hard code the column names but then every
> > time I add a new table or modify an existing one I would have to modify
> > the code. What I want is to have a generic function that given the
> > table name it will pull the column names for my use.
> >
> > I need to get the table column names for several tables I have setup. I
> > know if I do a select * from tablename I can then use the pg_fieldname
> > function to pull the column names for all columns.
> >
> > But I don't think I want to select the entire contents of the table
> > every time I want to get the names of the columns. I know this will
> > work but I think performance will be very poor.
> >...
>
> You almost got it - just do "select * from tablename where 0=1", which returns
> no rows but will give you the fieldnames. A portable and (I think)
> efficient way to get table column names.
It can be a cute trick, and I use it myself from time to time
(especially for "CREATE TABLE AS SELECT ..." where I want an empty table
with the same structure, pre v 7.4 which can do this anyway). You
should be aware however that as written above it will almost invariably
force a full-table scan!
You can also select the column names from the database metadata
directly:
SELECT attname
FROM pg_class c join pg_attribute a on c.oid = a.attrelid
WHERE c.relname = '<your table name>'
AND a.attnum >= 0;
This approach won't get killed by the efficiency problems above.
Cheers,
Andrew.
-------------------------------------------------------------------------
Andrew @ Catalyst .Net .NZ Ltd, PO Box 11-053, Manners St, Wellington
WEB: http://catalyst.net.nz/ PHYS: Level 2, 150-154 Willis St
DDI: +64(4)916-7201 MOB: +64(21)635-694 OFFICE: +64(4)499-2267
You have many friends and very few living enemies.
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On Sun, 2004-05-16 at 22:55, Andrew McMillan wrote: > On Mon, 2004-05-17 at 00:19 +0000, ljb wrote: > > > But I don't think I want to select the entire contents of the table > > > every time I want to get the names of the columns. I know this will > > > work but I think performance will be very poor. > > >... > > > > You almost got it - just do "select * from tablename where 0=1", which returns > > no rows but will give you the fieldnames. A portable and (I think) > > efficient way to get table column names. > > It can be a cute trick, and I use it myself from time to time > (especially for "CREATE TABLE AS SELECT ..." where I want an empty table > with the same structure, pre v 7.4 which can do this anyway). You > should be aware however that as written above it will almost invariably > force a full-table scan! > > You can also select the column names from the database metadata > directly: > > SELECT attname > FROM pg_class c join pg_attribute a on c.oid = a.attrelid > WHERE c.relname = '<your table name>' > AND a.attnum >= 0; > > This approach won't get killed by the efficiency problems above. > > Cheers, > Andrew. Thanks. Most of the tables I have are fairly small (for now) but at least one of them has many thousands of rows and I did not want to have to scan all of them for this information. I understand why the 0=1 trick will scan every row. I like the idea of getting the meta data directly. None of the books I have seem to discuss this kind of thing. Is the pg_class and pg_attribute tables hidden? I see pga_layout and some others but not the first two when I do a \d. I do get a column listing when I do a \d pg_class so they are there. And this worked great on my test database/tables. Thanks! -- Scot L. Harris <webid@cfl.rr.com>
On Sun, 2004-05-16 at 23:16 -0400, Scot L. Harris wrote: > > > > You can also select the column names from the database metadata > > directly: > > > > SELECT attname > > FROM pg_class c join pg_attribute a on c.oid = a.attrelid > > WHERE c.relname = '<your table name>' > > AND a.attnum >= 0; > > > > This approach won't get killed by the efficiency problems above. > > > > Cheers, > > Andrew. > > Thanks. Most of the tables I have are fairly small (for now) but at > least one of them has many thousands of rows and I did not want to have > to scan all of them for this information. I understand why the 0=1 > trick will scan every row. I like the idea of getting the meta data > directly. > > None of the books I have seem to discuss this kind of thing. Is the > pg_class and pg_attribute tables hidden? I see pga_layout and some > others but not the first two when I do a \d. I do get a column listing > when I do a \d pg_class so they are there. > > And this worked great on my test database/tables. When I want to figure out something like this I tend to use "psql -E" so that all queries are echoed before being sent to the backend. Then I do something like "\d <table>" and see what SQL psql generates internally. Also, dig here for detailed information on the postgresql data dictionary tables: http://www.postgresql.org/docs/7.4/interactive/catalogs.html the most useful ones are pg_class and pg_attribute usually (for obvious reasons :-). With 7.4 I also find myself looking at the pg_stat_activity view from time to time as well. Cheers, Andrew. ------------------------------------------------------------------------- Andrew @ Catalyst .Net .NZ Ltd, PO Box 11-053, Manners St, Wellington WEB: http://catalyst.net.nz/ PHYS: Level 2, 150-154 Willis St DDI: +64(4)916-7201 MOB: +64(21)635-694 OFFICE: +64(4)499-2267 You possess a mind not merely twisted, but actually sprained. -------------------------------------------------------------------------
On Sunday 16 May 2004 8:16 pm, Scot L. Harris wrote: > On Sun, 2004-05-16 at 22:55, Andrew McMillan wrote: > > On Mon, 2004-05-17 at 00:19 +0000, ljb wrote: > > > > But I don't think I want to select the entire contents of the > > > > table every time I want to get the names of the columns. I > > > > know this will work but I think performance will be very > > > > poor. > > > > > > You almost got it - just do "select * from tablename where > > > 0=1", which returns no rows but will give you the fieldnames. A > > > portable and (I think) efficient way to get table column names. > > > > It can be a cute trick, and I use it myself from time to time > > (especially for "CREATE TABLE AS SELECT ..." where I want an > > empty table with the same structure, pre v 7.4 which can do this > > anyway). You should be aware however that as written above it > > will almost invariably force a full-table scan! Depending on your needs you can also use: select * from tablename limit 0; Slow machine, 3.3 million row table: 4 milliseconds. No full table scan there. Cheers, Steve
On Mon, 2004-05-17 at 06:26, Andrew McMillan wrote: > On Sun, 2004-05-16 at 23:16 -0400, Scot L. Harris wrote: > > > > > > You can also select the column names from the database metadata > > > directly: > > > > > > SELECT attname > > > FROM pg_class c join pg_attribute a on c.oid = a.attrelid > > > WHERE c.relname = '<your table name>' > > > AND a.attnum >= 0; > > > > > > This approach won't get killed by the efficiency problems above. > > > > > > Cheers, > > > Andrew. > > > When I want to figure out something like this I tend to use "psql -E" so > that all queries are echoed before being sent to the backend. Then I do > something like "\d <table>" and see what SQL psql generates internally. > > Also, dig here for detailed information on the postgresql data > dictionary tables: > > http://www.postgresql.org/docs/7.4/interactive/catalogs.html > > the most useful ones are pg_class and pg_attribute usually (for obvious > reasons :-). With 7.4 I also find myself looking at the > pg_stat_activity view from time to time as well. > > Cheers, > Andrew. Thanks to everyone that responded to my question. I have my application working as I wanted. I really appreciate all the help that was provided. -- Scot L. Harris <webid@cfl.rr.com>
On Monday 17 May 2004 17:31, Scot L. Harris wrote: > On Mon, 2004-05-17 at 06:26, Andrew McMillan wrote: > > On Sun, 2004-05-16 at 23:16 -0400, Scot L. Harris wrote: > > > > You can also select the column names from the database metadata > > > > directly: > > > > > > > > SELECT attname > > > > FROM pg_class c join pg_attribute a on c.oid = a.attrelid > > > > WHERE c.relname = '<your table name>' > > > > AND a.attnum >= 0; > > > > > > > > This approach won't get killed by the efficiency problems above. > > > > > > > > Cheers, > > > > Andrew. > > > > When I want to figure out something like this I tend to use "psql -E" so > > that all queries are echoed before being sent to the backend. Then I do > > something like "\d <table>" and see what SQL psql generates internally. > > > > Also, dig here for detailed information on the postgresql data > > dictionary tables: > > > > http://www.postgresql.org/docs/7.4/interactive/catalogs.html > > > > the most useful ones are pg_class and pg_attribute usually (for obvious > > reasons :-). With 7.4 I also find myself looking at the > > pg_stat_activity view from time to time as well. > > > > Cheers, > > Andrew. > > Thanks to everyone that responded to my question. I have my application > working as I wanted. I really appreciate all the help that was > provided. A little late to the party, but it seems worth mentioning that this information is also available in the information_schema, which has the benefits of being sql complient, stable across releases, and keeps you out of the system catalogs. for example: cms74=# select column_name from information_schema.columns where table_name = 'current_downloads'; column_name ------------- start_time entity_id (2 rows) for more on information schema check out http://www.postgresql.org/docs/7.4/interactive/information-schema.html Robert Treat -- Build A Brighter Lamp :: Linux Apache {middleware} PostgreSQL
On Tue, 2004-05-18 at 17:53, Robert Treat wrote: > A little late to the party, but it seems worth mentioning that this > information is also available in the information_schema, which has the > benefits of being sql complient, stable across releases, and keeps you out of > the system catalogs. for example: > > cms74=# select column_name from information_schema.columns where table_name = > 'current_downloads'; > column_name > ------------- > start_time > entity_id > (2 rows) > > for more on information schema check out > http://www.postgresql.org/docs/7.4/interactive/information-schema.html > > Robert Treat That looks like a nice way to get that information. Thanks for another good tip. -- Scot L. Harris <webid@cfl.rr.com>
ljb wrote: >webid@cfl.rr.com wrote: > > >>Currently using Postgresql 7.2.4-5.80 with php 4.2.2.-8.0.8 on a redhat >>8.0 system. >> >>I am writing some php scripts where I want to generate a list of the >>column names in a particular table that the user selects. I could take >>the brute force method and hard code the column names but then every >>time I add a new table or modify an existing one I would have to modify >>the code. What I want is to have a generic function that given the >>table name it will pull the column names for my use. >> >>I need to get the table column names for several tables I have setup. I >>know if I do a select * from tablename I can then use the pg_fieldname >>function to pull the column names for all columns. >> >>But I don't think I want to select the entire contents of the table >>every time I want to get the names of the columns. I know this will >>work but I think performance will be very poor. >>... >> >> > >You almost got it - just do "select * from tablename where 0=1", which returns >no rows but will give you the fieldnames. A portable and (I think) >efficient way to get table column names. > >---------------------------(end of broadcast)--------------------------- >TIP 6: Have you searched our list archives? > > http://archives.postgresql.org > > even more efficient (possibly not quite as portable but does work with pgsql and mysql) select * from tablename limit 0