I have written the below code:
create or replace function max_price()
returns real
volatile
as
$$
select
max(main_amount)
from
table
$$
language sql;
I am receiving this error message:
ERROR: The select expression can not have aggregate or window function.
CONTEXT: Create SQL function "max_price" body
How can I work around this?
No, Redshift UDFs are scalar - each "row" of input values returns one output.
https://docs.aws.amazon.com/redshift/latest/dg/udf-creating-a-scalar-sql-udf.html
You may be able to use a Stored Procedure to obtain the result you are looking for.
https://docs.aws.amazon.com/redshift/latest/dg/stored-procedure-create.html
A scalar User-Defined Function in Amazon Redshift cannot issue a SELECT command that retrieves data from a table. It is intended as a means of calculating a number, rather than querying the database.
From Creating a scalar SQL UDF - Amazon Redshift:
The SELECT clause can't include any of the following types of clauses: FROM, INTO, WHERE, GROUP BY, ORDER BY, LIMIT
If you need to consult another table as part of the function, use a Stored procedure.
Related
I googled but everyone was asking for how to pass tables or how to use the return result into a Function; I want to do neither. I simply want to take the value of a Column (lets assume col2 below is of the text datatype) of a table, and pass that data into a Function, so I can manipulate the data, but in the SELECT... statement itself, i.e.
SELECT t.col1, "myCustomFunction"(t.col2)
FROM tbl t
WHERE t.col1 = 'someCondition';
CREATE OR REPLACE FUNCTION myCustomFunction(myArg text)
RETURNS text AS $$
DECLARE
BEGIN
RETURN UPPER(myArg);
END
$$ LANGUAGE plpgsql;
... So if myCustomerFunction()'s job was capitalize letters (its not, just an example), the output would be the table with col2 data all capitalized.
Is this possible? I supposed it would be no different than embedding a CASE expression there, which I know works, and a Function returns a result, so I assumed it would be the same, but I am getting SQL Error
You cannot to pass named column to some function and you cannot to return this named column like table with this column. The table is set of rows, and almost all processing in Postgres is based on rows processing. Usually you need to hold only data of one row in memory, so you can process much bigger dataset than is your memory.
Inside PL/pgSQL function you have not informations about outer. You can get just data of scalar types, arrays of scalars, or composite or arrays of composites (or ranges and multiranges - this special kind of composite and array of composite). Nothing else.
Theoretically you can aggregate data in one column to array, and later you can expand this array to table. But these operations are memory expensive and can be slow. You need it only in few cases (like computing of median function), but it is slow, and there is risk of out of memory exception.
When object names are not doubled quoted Postgres processes then internally as lower case. When doubled quoted the are processed exactly as quoted. The thing is these may not be the same. You defined the function as FUNCTION myCustomFunction(myArg text) Not doubled quoted, but attempt to call it via "myCustomFunction"(t.col2). Unfortunately myCustomFunction processed as mycustomfunction but "myCustomFunction" is processed exactly as it appears. Those are NOT the same. Either change your select to:
SELECT t.col1,myCustomFunction(t.col2)
FROM tbl t
WHERE t.col1 = 'someCondition';
or change the function definition to:
CREATE OR REPLACE FUNCTION "myCustomFunction"(myArg text)
RETURNS text AS $$
DECLARE
BEGIN
RETURN UPPER(myArg);
END
$$ LANGUAGE plpgsql;
Are composite type arrays the correct way to simulate a "function scoped" table in plpgsql?
I want to store the results of a query for access throughout the function,
but I don't want to use a temp table because a single session may execute multiple function calls concurrently.
DECLARE
projectDocuments document_tracking.docmaster[];
SELECT * INTO projectDocuments FROM document_tracking.docmaster WHERE jobnumber = jobnumberparam;
Produces the malformed array literal error, am I supposed to use aggregate functions, or :=?
consider using
ON COMMIT DELETE ROWS
https://www.postgresql.org/docs/current/static/sql-createtable.html
All rows in the temporary table will be deleted at the end of each
transaction block. Essentially, an automatic TRUNCATE is done at each
commit.
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
I have a very simple query that is not much more complicated than:
select *
from table_name
where id = 1234
...it takes less than 50 milliseconds to run.
Took that query and put it into a function:
CREATE OR REPLACE FUNCTION pie(id_param integer)
RETURNS SETOF record AS
$BODY$
BEGIN
RETURN QUERY SELECT *
FROM table_name
where id = id_param;
END
$BODY$
LANGUAGE plpgsql STABLE;
This function when executed select * from pie(123); takes 22 seconds.
If I hard code an integer in place of id_param, the function executes in under 50 milliseconds.
Why does the fact that I am using a parameter in the where statement cause my function to run slow?
Edit to add concrete example:
CREATE TYPE test_type AS (gid integer, geocode character varying(9))
CREATE OR REPLACE FUNCTION geocode_route_by_geocode(geocode_param character)
RETURNS SETOF test_type AS
$BODY$
BEGIN
RETURN QUERY EXECUTE
'SELECT gs.geo_shape_id AS gid,
gs.geocode
FROM geo_shapes gs
WHERE geocode = $1
AND geo_type = 1
GROUP BY geography, gid, geocode' USING geocode_param;
END;
$BODY$
LANGUAGE plpgsql STABLE;
ALTER FUNCTION geocode_carrier_route_by_geocode(character)
OWNER TO root;
--Runs in 20 seconds
select * from geocode_route_by_geocode('999xyz');
--Runs in 10 milliseconds
SELECT gs.geo_shape_id AS gid,
gs.geocode
FROM geo_shapes gs
WHERE geocode = '9999xyz'
AND geo_type = 1
GROUP BY geography, gid, geocode
Update in PostgreSQL 9.2
There was a major improvement, I quote the release notes here:
Allow the planner to generate custom plans for specific parameter
values even when using prepared statements (Tom Lane)
In the past, a prepared statement always had a single "generic" plan
that was used for all parameter values, which was frequently much
inferior to the plans used for non-prepared statements containing
explicit constant values. Now, the planner attempts to generate custom
plans for specific parameter values. A generic plan will only be used
after custom plans have repeatedly proven to provide no benefit. This
change should eliminate the performance penalties formerly seen from
use of prepared statements (including non-dynamic statements in
PL/pgSQL).
Original answer for PostgreSQL 9.1 or older
A plpgsql functions has a similar effect as the PREPARE statement: queries are parsed and the query plan is cached.
The advantage is that some overhead is saved for every call.
The disadvantage is that the query plan is not optimized for the particular parameter values it is called with.
For queries on tables with even data distribution, this will generally be no problem and PL/pgSQL functions will perform somewhat faster than raw SQL queries or SQL functions. But if your query can use certain indexes depending on the actual values in the WHERE clause or, more generally, chose a better query plan for the particular values, you may end up with a sub-optimal query plan. Try an SQL function or use dynamic SQL with EXECUTE to force a the query to be re-planned for every call. Could look like this:
CREATE OR REPLACE FUNCTION pie(id_param integer)
RETURNS SETOF record AS
$BODY$
BEGIN
RETURN QUERY EXECUTE
'SELECT *
FROM table_name
where id = $1'
USING id_param;
END
$BODY$
LANGUAGE plpgsql STABLE;
Edit after comment:
If this variant does not change the execution time, there must be other factors at play that you may have missed or did not mention. Different database? Different parameter values? You would have to post more details.
I add a quote from the manual to back up my above statements:
An EXECUTE with a simple constant command string and some USING
parameters, as in the first example above, is functionally equivalent
to just writing the command directly in PL/pgSQL and allowing
replacement of PL/pgSQL variables to happen automatically. The
important difference is that EXECUTE will re-plan the command on each
execution, generating a plan that is specific to the current parameter
values; whereas PL/pgSQL normally creates a generic plan and caches it
for re-use. In situations where the best plan depends strongly on the
parameter values, EXECUTE can be significantly faster; while when the
plan is not sensitive to parameter values, re-planning will be a
waste.
I'm using PostgreSQL database.
I've a my plpgsql FUNCTION that returns a single record with an arbitrary number of columns.
Due to this arbitrariness I would need to use something like:
SELECT * FROM my_function(97)
But this doesn't work as Postgres gives me the following error:
a column definition list is required for functions returning "record"
But if I do:
SELECT my_function(97)
I can see the expected result but encapsulated in a single column.
Is there a way to fetch the expected result as a set of columns as intended by the function and not a single column encapsulating all of them?
When a function just RETURNS record or SETOF record (and no OUT parameters to go with it), PostgreSQL does not know the names and types of its elements and you are required to provide a column definition list with every call.
Avoid that if at all possible and return a well known (row) type instead. There are a several ways to declare the return type. See:
PostgreSQL: ERROR: 42601: a column definition list is required for functions returning "record"
Refactor a PL/pgSQL function to return the output of various SELECT queries
There are quite a few related questions on SO. Try a search!
When using a set returning function (setof) in the select list, on the left hand side of the FROM, the function returns a composite type. Using a function in the select list can be hard to avoid when using a table as input to a function.
A way to SELECT items from a single column of composite type follows:
SELECT
(my_function).field1,
(my_function).field2,
(my_function).field3
FROM
(SELECT my_function(*)
FROM sometable) t
You have a few options here:
Return a REFCURSOR and fetch from that cursor in the application. Note you can actually return multiple REFCURSORS if you need to return multiple result sets.
Return an XML document and parse it in the application.
Use a bunch of OUT variables, return RECORD, and determine which of these to select from
The basic problem is that the actual return results need to be known at planning time so you can't just return an arbitrary number of columns. The planner needs to know what is going to be returned.
In order to return a "set of columns" you will have define a return type as TABLE or SETOF in which case you actually return a SET of records which you should be able to SELECT from.
For more information about functions returning SETOF take a look at this link to documentation
I'm not certain that I follow what you're after, but does this work?
SELECT (my_function(97)).my_column