Trying to be lazy when looking at an example
SELECT realestate.address, realestate.parcel, s.sale_year, s.sale_amount,
FROM realestate INNER JOIN
dblink('dbname=somedb port=5432 host=someserver
user=someuser password=somepwd',
'SELECT parcel_id, sale_year,
sale_amount FROM parcel_sales')
AS s(parcel_id char(10),sale_year int, sale_amount int)
Is there a way of getting the AS section filled in from the table?
I'm copying data from tables of the same name and structure on different servers.
If I can get the structure to copy from the existing table, it will save me a lot of time
Thanks
Bruce
The answer is: No. See the doc:
Since dblink can be used with any query, it is declared to return record, rather than specifying any particular set of columns. This means that you must specify the expected set of columns in the calling query — otherwise PostgreSQL would not know what to expect.
http://www.postgresql.org/docs/9.1/static/contrib-dblink-function.html
Edit: by the way, for a table or a view, you can get the fields name and type in a first query:
select column_name
from information_schema.columns
where table_name = 'your_table_or_view';
You could then use it to fill the fields declaration.
Alexis
Related
I feel the need to get the column names and data types of the table returned by any function that has a 'record' return data type, because...
A key process in an existing SQL Server-based system makes use of a stored procedure that takes a user-defined function as a parameter. An initial step gets the column names and types of the table returned by the function that was passed as a parameter.
In Postgres 13 I can use pg_proc.prorettype and the corresponding pg_type to find functions that return record types...that's a start. I can also use pg_get_function_result() to get the string containing the information I need. But, it's a string, and while I ultimately will have to assemble a very similar string, this is just one application of the info. Is there a tabular equivalent containing (column_name, data_type, ordinal_position), or do I need to do that myself?
Is there access to a composite data type the system may have created when such a function is created?
One option that I think will work for me, but I think it's a little weird, is to:
> create temp table t as select * from function() limit 0;
then look that table up in info_schema.columns, assemble what I need and drop the temp table...putting all of this into a function.
You can query the catalog table pg_proc, which contains all the required information:
SELECT coalesce(p.na, 'column' || p.i),
p.ty::regtype,
p.i
FROM pg_proc AS f
CROSS JOIN LATERAL unnest(
coalesce(f.proallargtypes, ARRAY[f.prorettype]),
f.proargmodes,
f.proargnames
)
WITH ORDINALITY AS p(ty,mo,na,i)
WHERE f.proname = 'interval_ok'
AND coalesce(p.mo, 'o') IN ('o', 't')
ORDER BY p.i;
I'm using a PostgreSQL with a Go driver. Sometimes I need to query not existing fields, just to check - maybe something exists in a DB. Before querying I can't tell whether that field exists. Example:
where size=10 or length=10
By default I get an error column "length" does not exist, however, the size column could exist and I could get some results.
Is it possible to handle such cases to return what is possible?
EDIT:
Yes, I could get all the existing columns first. But the initial queries can be rather complex and not created by me directly, I can only modify them.
That means the query can be simple like the previous example and can be much more complex like this:
WHERE size=10 OR (length=10 AND n='example') OR (c BETWEEN 1 and 5 AND p='Mars')
If missing columns are length and c - does that mean I have to parse the SQL, split it by OR (or other operators), check every part of the query, then remove any part with missing columns - and in the end to generate a new SQL query?
Any easier way?
I would try to check within information schema first
"select column_name from INFORMATION_SCHEMA.COLUMNS where table_name ='table_name';"
And then based on result do query
Why don't you get a list of columns that are in the table first? Like this
select column_name
from information_schema.columns
where table_name = 'table_name' and (column_name = 'size' or column_name = 'length');
The result will be the columns that exist.
There is no way to do what you want, except for constructing an SQL string from the list of available columns, which can be got by querying information_schema.columns.
SQL statements are parsed before they are executed, and there is no conditional compilation or no short-circuiting, so you get an error if a non-existing column is referenced.
I have a table named conditions on a schema named test. I created a hypertable and inserted hundreds of rows.
When I run select show_chunks(), it works and displays chunks but I cannot use the table name as parameter as suggested in the manual. This does not work:
SELECT show_chunks("test"."conditions");
How can I fix this?
Ps: I want to query the chunk itself by its name? How can I do this?
The show_chunks expects a regclass, which depending on your current search path means you need to schema qualify the table.
The following should work:
SELECT public.show_chunks('test.conditions');
The double quotes are only necessary if your table is a delimited identifier, for example if your tablename contains a space, you would need to add the double quotes for the identifier. You will still need to wrap it in single quotes though:
SELECT public.show_chunks('test."equipment conditions"');
SELECT public.show_chunks('"test schema"."equipment conditions"');
For more information about identifier quoting:
https://www.postgresql.org/docs/current/sql-syntax-lexical.html#SQL-SYNTAX-IDENTIFIERS
Edit: Addressing the PS:
I want to query the chunk itself by its name? How can I do this?
feike=# SELECT public.show_chunks('test.conditions');
show_chunks
--------------------------------------------
_timescaledb_internal._hyper_28_1176_chunk
_timescaledb_internal._hyper_28_1177_chunk
[...]
SELECT * FROM _timescaledb_internal._hyper_28_1176_chunk;
I am new to using cursors for looping through a set of rows. But so far I had prior knowledge of which columns I am about to read.
E.g.
DECLARE db_cursor FOR
SELECT Column1, Column2
FROM MyTable
DECLARE #ColumnOne VARCHAR(50), #ColumnTwo VARCHAR(50)
OPEN db_cursor
FETCH NEXT FROM db_cursor INTO #ColumnOne, #ColumnTwo
...
But the tables I am about to read into my key/value table have no specific structure and I should be able to process them one row at a time. How, using a nested cursor, can I loop through all the columns of the fetched row and process them according to their type and name?
TSQL cursors are not really designed to read data from tables of unknown structure. The two possibilities I can think of to achieve something in that direction are:
First read the column names of an unknown table from the Information Schema Views (see System Information Schema Views (Transact-SQL)). Then use dynamic SQL to create the cursor.
If you simply want to get any columns as a large string value, you might also try a simple SELECT * FROM TABLE_NAME FOR XML AUTO and further process the retrieved data for your purposes (see FOR XML (SQL Server)).
SQL is not very good in dealing with sets generically. In most cases you must know column names, data types and much more in advance. But there is XQuery. You can transform any SELECT into XML rather easily and use the mighty abilities to deal with generic structures there. I would not recommend this, but it might be worth a try:
CREATE PROCEDURE dbo.Get_EAV_FROM_SELECT
(
#SELECT NVARCHAR(MAX)
)
AS
BEGIN
DECLARE #tmptbl TABLE(TheContent XML);
DECLARE #cmd NVARCHAR(MAX)= N'SELECT (' + #SELECT + N' FOR XML RAW, ELEMENTS XSINIL);';
INSERT INTO #tmptbl EXEC(#cmd);
SELECT r.value('*[1]/text()[1]','nvarchar(max)') AS RowID
,c.value('local-name(.)','nvarchar(max)') AS ColumnKey
,c.value('text()[1]','nvarchar(max)') AS ColumnValue
FROM #tmptbl t
CROSS APPLY t.TheContent.nodes('/row') A(r)
CROSS APPLY A.r.nodes('*[position()>1]') B(c)
END;
GO
EXEC Get_EAV_FROM_SELECT #SELECT='SELECT TOP 10 o.object_id,o.* FROM sys.objects o';
GO
--Clean-Up for test purpose
DROP PROCEDURE Get_EAV_FROM_SELECT;
The idea in short
The select is passed into the procedure as string. With the SP we create a statement dynamically and create XML from it.
The very first column is considered to be the Row's ID, if not (like in sys.objects) we can write the SELECT and force it that way.
The inner SELECT will read each row and return a classical EAV-list.
I have found solutions (I think) to the problem I'm about to ask for on Oracle and SQL Server, but can't seem to translate this into a Postgres solution. I am using Postgres 9.3.6.
The idea is to be able to generate "metadata" about the table content for profiling purposes. This can only be done (AFAIK) by having queries run for each column so as to find out, say... min/max/count values and such. In order to automate the procedure, it is preferable to have the queries generated by the DB, then executed.
With an example salesdata table, I'm able to generate a select query for each column, returning the min() value, using the following snippet:
SELECT 'SELECT min('||column_name||') as minval_'||column_name||' from salesdata '
FROM information_schema.columns
WHERE table_name = 'salesdata'
The advantage being that the db will generate the code regardless of the number of columns.
Now there's a myriad places I had in mind for storing these queries, either a variable of some sort, or a table column, the idea being to then have these queries execute.
I thought of storing the generated queries in a variable then executing them using the EXECUTE (or EXECUTE IMMEDIATE) statement which is the approach employed here (see right pane), but Postgres won't let me declare a variable outside a function and I've been scratching my head with how this would fit together, whether that's even the direction to follow, perhaps there's something simpler.
Would you have any pointers, I'm currently trying something like this, inspired by this other question but have no idea whether I'm headed in the right direction:
CREATE OR REPLACE FUNCTION foo()
RETURNS void AS
$$
DECLARE
dyn_sql text;
BEGIN
dyn_sql := SELECT 'SELECT min('||column_name||') from salesdata'
FROM information_schema.columns
WHERE table_name = 'salesdata';
execute dyn_sql
END
$$ LANGUAGE PLPGSQL;
System statistics
Before you roll your own, have a look at the system table pg_statistic or the view pg_stats:
This view allows access only to rows of pg_statistic that correspond
to tables the user has permission to read, and therefore it is safe to
allow public read access to this view.
It might already have some of the statistics you are about to compute. It's populated by ANALYZE, so you might run that for new (or any) tables before checking.
-- ANALYZE tbl; -- optionally, to init / refresh
SELECT * FROM pg_stats
WHERE tablename = 'tbl'
AND schemaname = 'public';
Generic dynamic plpgsql function
You want to return the minimum value for every column in a given table. This is not a trivial task, because a function (like SQL in general) demands to know the return type at creation time - or at least at call time with the help of polymorphic data types.
This function does everything automatically and safely. Works for any table, as long as the aggregate function min() is allowed for every column. But you need to know your way around PL/pgSQL.
CREATE OR REPLACE FUNCTION f_min_of(_tbl anyelement)
RETURNS SETOF anyelement
LANGUAGE plpgsql AS
$func$
BEGIN
RETURN QUERY EXECUTE (
SELECT format('SELECT (t::%2$s).* FROM (SELECT min(%1$s) FROM %2$s) t'
, string_agg(quote_ident(attname), '), min(' ORDER BY attnum)
, pg_typeof(_tbl)::text)
FROM pg_attribute
WHERE attrelid = pg_typeof(_tbl)::text::regclass
AND NOT attisdropped -- no dropped (dead) columns
AND attnum > 0 -- no system columns
);
END
$func$;
Call (important!):
SELECT * FROM f_min_of(NULL::tbl); -- tbl being the table name
db<>fiddle here
Old sqlfiddle
You need to understand these concepts:
Dynamic SQL in plpgsql with EXECUTE
Polymorphic types
Row types and table types in Postgres
How to defend against SQL injection
Aggregate functions
System catalogs
Related answer with detailed explanation:
Table name as a PostgreSQL function parameter
Refactor a PL/pgSQL function to return the output of various SELECT queries
Postgres data type cast
How to set value of composite variable field using dynamic SQL
How to check if a table exists in a given schema
Select columns with particular column names in PostgreSQL
Generate series of dates - using date type as input
Special difficulty with type mismatch
I am taking advantage of Postgres defining a row type for every existing table. Using the concept of polymorphic types I am able to create one function that works for any table.
However, some aggregate functions return related but different data types as compared to the underlying column. For instance, min(varchar_column) returns text, which is bit-compatible, but not exactly the same data type. PL/pgSQL functions have a weak spot here and insist on data types exactly as declared in the RETURNS clause. No attempt to cast, not even implicit casts, not to speak of assignment casts.
That should be improved. Tested with Postgres 9.3. Did not retest with 9.4, but I am pretty sure, nothing has changed in this area.
That's where this construct comes in as workaround:
SELECT (t::tbl).* FROM (SELECT ... FROM tbl) t;
By casting the whole row to the row type of the underlying table explicitly we force assignment casts to get original data types for every column.
This might fail for some aggregate function. sum() returns numeric for a sum(bigint_column) to accommodate for a sum overflowing the base data type. Casting back to bigint might fail ...
#Erwin Brandstetter, Many thanks for the extensive answer. pg_stats does indeed provide a few things, but what I really need to draw a complete profile is a variety of things, min, max values, counts, count of nulls, mean etc... so a bunch of queries have to be ran for each columns, some with GROUP BY and such.
Also, thanks for highlighting the importance of data types, i was sort of expecting this to throw a spanner in the works at some point, my main concern was with how to automate the query generation, and its execution, this last bit being my main concern.
I have tried the function you provide (I probably will need to start learning some plpgsql) but get a error at the SELECT (t::tbl) :
ERROR: type "tbl" does not exist
btw, what is the (t::abc) notation referred as, in python this would be a list slice, but it’s probably not the case in PLPGSQL