According to PostgreSQL: Documentation: 38.7. Function Volatility Categories:
A STABLE function cannot modify the database and is guaranteed to return the same results given the same arguments for all rows within a single statement. This category allows the optimizer to optimize multiple calls of the function to a single call.
What about when using CTEs? If I call a STABLE function inside a CTE and then again inside the primary SELECT query, will the optimizer optimize both calls of the function to a single call?
How can you tell? (I don't know how to use EXPLAIN.)
In the impractical example below, I want to make sure that the function get_user_by_id() is only called once.
CREATE TABLE users (
id bigserial PRIMARY KEY,
username varchar(64) NOT NULL
);
CREATE FUNCTION get_user_by_id(_id bigint) RETURNS users AS $$
SELECT *
FROM users
WHERE id = _id
$$ LANGUAGE SQL STABLE;
INSERT INTO users (username) VALUES ('user1');
WITH error AS (
SELECT -1 AS code
WHERE (SELECT get_user_by_id(3) IS NULL)
)
SELECT code AS id, NULL AS username FROM error
UNION ALL
SELECT * FROM get_user_by_id(3) WHERE id IS NOT NULL;
I think the answer is no, multiple calls to a STABLE function are not optimized across CTEs. I don't know how to prove it, but according to PostgreSQL: Documentation: 11: 7.8. WITH Queries (Common Table Expressions), CTEs seem to be separate "auxiliary statements". And, they don't seem to be well-integrated with the parent query. For example:
. . . the optimizer is less able to push restrictions from the parent query down into a WITH query than an ordinary subquery. The WITH query will generally be evaluated as written, without suppression of rows that the parent query might discard afterwards.
Related
I have a table with common word values to match against brands - so when someone types in "coke" I want to match any possible brand names associated with it as well as the original term.
CREATE TABLE word_association ( commonterm TEXT, assocterm TEXT);
INSERT INTO word_association ('coke', 'coca-cola'), ('coke', 'cocacola'), ('coke', 'coca-cola');
I have a function to create a list of these values in a pipe-delim string for pattern matching:
CREATE OR REPLACE FUNCTION usp_get_search_terms(userterm text)
RETURNS text AS
$BODY$DECLARE
returnstr TEXT DEFAULT '';
BEGIN
SET DATESTYLE TO DMY;
returnstr := userterm;
IF EXISTS (SELECT 1 FROM word_association WHERE LOWER(commonterm) = LOWER(userterm)) THEN
SELECT returnstr || '|' || string_agg(assocterm, '|') INTO returnstr
FROM word_association
WHERE commonterm = userterm;
END IF;
RETURN returnstr;
END;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;
ALTER FUNCTION usp_get_search_terms(text)
OWNER TO customer_role;
If you call SELECT * FROM usp_get_search_terms('coke') you end up with
coke|coca-cola|cocacola|coca cola
EDIT: this function runs <100ms so it works fine.
I want to run a query with this text inserted e.g.
SELECT X.article_number, X.online_description
FROM articles X
WHERE LOWER(X.online_description) % usp_get_search_terms ('coke');
This takes approx 56s to run against my table of ~500K records.
If I get the raw text and use it in the query it takes ~300ms e.g.
SELECT X.article_number, X.online_description
FROM articles X
WHERE X.online_description % '(coke|coca-cola|cocacola|coca cola)';
The result sets are identical.
I've tried modifying what the output string from the function to e.g. enclose it in quotes and parentheses but it doesn't seem to make a difference.
Can someone please advise why there is a difference here? Is it the data type or something about calling functions inside queries? Thanks.
Your function might take 100ms, but it's not calling your function once; it's calling it 500,000 times.
It's because your function is declared VOLATILE. This tells Postgres that either the function returns different values when called multiple times within a query (like clock_timestamp() or random()), or that it alters the state of the database in some way (for example, by inserting records).
If your function contains only SELECTs, with no INSERTs, calls to other VOLATILE functions, or other side-effects, then you can declare it STABLE instead. This tells the planner that it can call the function just once and reuse the result without affecting the outcome of the query.
But your function does have side-effects, due to the SET DATESTYLE statement, which takes effect for the rest of the session. I doubt this was the intention, however. You may be able to remove it, as it doesn't look like date formatting is relevant to anything in there. But if it is necessary, the correct approach is to use the SET clause of the CREATE FUNCTION statement to change it only for the duration of the function call:
...
$BODY$
LANGUAGE plpgsql STABLE
SET DATESTYLE TO DMY
COST 100;
The other issue with the slow version of the query is the call to LOWER(X.online_description), which will prevent the query from utilising the index (since online_description is indexed, but LOWER(online_description) is not).
With these changes, the performance of both queries is the same; see this SQLFiddle.
So the answer came to me about dawn this morning - CTEs to the rescue!
Particularly as this is the "simple" version of a very large query, it helps to get this defined once in isolation, then do the matching against it. The alternative (given I'm calling this from a NodeJS platform) is to have one request retrieve the string of terms, then make another request to pass the string back. Not elegant.
WITH matches AS
( SELECT * FROM usp_get_search_terms('coke') )
, main AS
( SELECT X.article_number, X.online_description
FROM articles X
JOIN matches M ON X.online_description % M.usp_get_search_terms )
SELECT * FROM main
Execution time is somewhere around 300-500ms depending on term searched and articles returned.
Thanks for all your input guys - I've learned a few things about PostGres that my MS-SQL background didn't necessarily prepare me for :)
Have you tried removing the IF EXISTS() and simply using:
SELECT returnstr || '|' || string_agg(assocterm, '|') INTO returnstr
FROM word_association
WHERE LOWER(commonterm) = LOWER(userterm)
In instead of calling the function for each row call it once:
select x.article_number, x.online_description
from
woolworths.articles x
cross join
woolworths.usp_get_search_terms ('coke') c (s)
where lower(x.online_description) % s
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
Using Postgres 9.3:
I am attempting to automatically populate a table when an insert is performed on another table. This seems like a good use for rules, but after adding the rule to the first table, I am no longer able to perform inserts into the second table using the writable CTE. Here is an example:
CREATE TABLE foo (
id INT PRIMARY KEY
);
CREATE TABLE bar (
id INT PRIMARY KEY REFERENCES foo
);
CREATE RULE insertFoo AS ON INSERT TO foo DO INSERT INTO bar VALUES (NEW.id);
WITH a AS (SELECT * FROM (VALUES (1), (2)) b)
INSERT INTO foo SELECT * FROM a
When this is run, I get the error
"ERROR: WITH cannot be used in a query that is rewritten by rules
into multiple queries".
I have searched for that error string, but am only able to find links to the source code. I know that I can perform the above using row-level triggers instead, but it seems like I should be able to do this at the statement level. Why can I not use the writable CTE, when queries like this can (in this case) be easily re-written as:
INSERT INTO foo SELECT * FROM (VALUES (1), (2)) a
Does anyone know of another way that would accomplish what I am attempting to do other than 1) using rules, which prevents the use of "with" queries, or 2) using row-level triggers? Thanks,
TL;DR: use triggers, not rules.
Generally speaking, prefer triggers over rules, unless rules are absolutely necessary. (Which, in practice, they never are.)
Using rules introduces heaps of problems which will needlessly complicate your life down the road. You've run into one here. Another (major) one is, for instance, that the number of affected rows will correspond to that of the very last query -- if you're relying on FOUND somewhere and your query is incorrectly reporting that no rows were affected by a query, you'll be in for painful bugs.
Moreover, there's occasional talk of deprecating Postgres rules outright:
http://postgresql.nabble.com/Deprecating-RULES-td5727689.html
As the other answer I definitely recommend using INSTEAD OF triggers before RULEs.
However if for some reason you don't want to change existing VIEW RULEs and still want use WITH you can do so by wrapping the VIEW in a stored procedure:
create function insert_foo(int) returns void as $$
insert into foo values ($1)
$$ language sql;
WITH a AS (SELECT * FROM (VALUES (1), (2)) b)
SELECT insert_foo(a.column1) from a;
This could be useful when using some legacy db through some system that wraps statements with CTEs.
I have the following problem with PostgreSQL 9.3.
There is a view encapsulating a non-trivial query to some resources (e.g., documents). Let's illustrate it as simple as
CREATE VIEW vw_resources AS
SELECT * FROM documents; -- there are several joined tables in fact...
The client application uses the view usually with some WHERE conditions on several fields, and might also use paging of the results, so OFFSET and LIMIT may also be applied.
Now, on top of the actual resource list computed by vw_resources, I only want to display resources which the current user is allowed for. There is quite a complex set of rules regarding privileges (they depend on several attributes of the resources in question, explicit ACLs, implicit rules based on user roles or relations to other users...) so I wanted to encapsulate all of them in a single function. To prevent repetitive costly queries for each resource, the function takes a list of resource IDs, evaluates the privileges for all of them at once, and returns the set of the requested resource IDs together with the according privileges (read/write is distinguished). It looks roughly like this:
CREATE FUNCTION list_privileges(resource_ids BIGINT[])
RETURNS TABLE (resource_id BIGINT, privilege TEXT)
AS $function$
BEGIN
-- the function lists privileges for a user that would get passed in an argument - omitting that for simplicity
RAISE NOTICE 'list_privileges called'; -- for diagnostic purposes
-- for illustration, let's simply grant write privileges for any odd resource:
RETURN QUERY SELECT id, (CASE WHEN id % 2 = 1 THEN 'write' ELSE 'none' END)
FROM unnest(resource_ids) id;
END;
$function$ LANGUAGE plpgsql STABLE;
The question is how to integrate such a function in the vw_resources view for it to give only resources the user is privileged for (i.e., has 'read' or 'write' privilege).
A trivial solution would use a CTE:
CREATE VIEW vw_resources AS
WITH base_data AS (
SELECT * FROM documents
)
SELECT base_data.*, priv.privilege
FROM base_data
JOIN list_privileges((SELECT array_agg(resource_id) FROM base_data)) AS priv USING (resource_id)
WHERE privilege IN ('read', 'write');
The problem is that the view itself gives too much rows - some WHERE conditions and OFFSET/LIMIT clauses are only applied to the view itself, like SELECT * FROM vw_resources WHERE id IN (1,2,3) LIMIT 10 (any complex filtering might be requested by the client application). And since PostgreSQL is unable to push the conditions down the CTE, the list_privileges(BIGINT[]) function ends up with evaluating privileges for all resources in the database, which effectively kills the performance.
So I attempted to use a window function which would collect resource IDs from the whole result set and join the list_privileges(BIGINT[]) function in an outer query, like illustrated below, but the list_privileges(BIGINT[]) function ends up being called repetitively for each row (as testified by 'list_privileges called' notices), which kinda ruins the previous effort:
CREATE VIEW vw_resources AS
SELECT d.*, priv.privilege
FROM (
SELECT *, array_agg(resource_id) OVER () AS collected
FROM documents
) AS d
JOIN list_privileges(d.collected) AS priv USING (resource_id)
WHERE privilege IN ('read', 'write');
I would resort to forcing clients to give two separate queries, the first taking the vw_resources without privileges applied, the second calling the list_privileges(BIGINT[]) function passing it the list of resource IDs fetched by the first query, and filtering the disallowed resources on the client side. It is quite clumsy for the client, though, and obtaining e.g. the first 20 allowed resources would be practically impossible as limiting the first query simply does not get it - if some resources are filtered out due to privileges then we simply don't have 20 rows in the overall result...
Any help welcome!
P.S. For the sake of completeness, I append a sample documents table:
CREATE TABLE documents (resource_id BIGINT, content TEXT);
INSERT INTO documents VALUES (1,'a'),(2,'b'),(3,'c');
If you must use plpgsql then create the function taking no arguments
create function list_privileges()
returns table (resource_id bigint, privilege text)
as $function$
begin
raise notice 'list_privileges called'; -- for diagnostic purposes
return query select 1, case when 1 % 2 = 1 then 'write' else 'none' end
;
end;
$function$ language plpgsql stable;
And join it to the other complex query to form the vw_resources view
create view vw_resources as
select *
from
documents d
inner join
list_privileges() using(resource_id)
The filter conditions will be added at query time
select *
from vw_resources
where
id in (1,2,3)
and
privilege in ('read', 'write')
Let the planner do its optimization magic and check the explain output before any "premature optimization".
This is just a conjecture: The function might make it harder or impossible for the planner to optimize.
If plpgsql is not really necessary, and that is very frequent, I would just create a view in instead of the function
create view vw_list_privileges as
select
1 as resource_id,
case when 1 % 2 = 1 then 'write' else 'none' end as privilege
And join it the same way to the complex query
create view vw_resources as
select *
from
documents d
inner join
vw_list_privileges using(resource_id)
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.