Question

I see from the Postgres 8.1 docs that EXPLAIN generated something like tabular data:

Prior to PostgreSQL 7.3, the plan was emitted in the form of a NOTICE
message. Now it appears as a query result (formatted like a table with
a single text column).

I’m working with 9.0, for which, the docs say, the output can be a variety of types, including TEXT and XML. What I’d really like to do is treat the output as a standard query result so that I could generate a simple report for a query or a set of queries, e.g.,

SELECT maxcost FROM (
    EXPLAIN VERBOSE 
    SELECT COUNT(*) 
      FROM Mytable
     WHERE value>17);

The above doesn’t work in any form that I’ve tried, and I made up the attribute maxcost to demonstrate how neat it would be to pull out specific bits of data (in this case, the maximum estimated cost of the query). Is there anything I can do that would get me part of the way there? I’d prefer to be able to work within a simple SQL console.

Answer 1

No other answers so far, so here’s my own stab at it.

It’s possible to read the results of explain into a variable within plpgsql, and since the output can be in XML one can wrap EXPLAIN in a stored function to yield the top-level costs using xpath:

CREATE OR REPLACE FUNCTION estimate_cost(IN query text, 
OUT startup numeric,
OUT totalcost numeric, 
OUT planrows numeric, 
OUT planwidth numeric)
AS
$BODY$
DECLARE
    query_explain  text;
    explanation       xml;
    nsarray text[][];
BEGIN
nsarray := ARRAY[ARRAY['x', 'http://www.postgresql.org/2009/explain']];
query_explain :=e'EXPLAIN(FORMAT XML) ' || query;
EXECUTE query_explain INTO explanation;
startup := (xpath('/x:explain/x:Query/x:Plan/x:Startup-Cost/text()', explanation, nsarray))[1];
totalcost := (xpath('/x:explain/x:Query/x:Plan/x:Total-Cost/text()', explanation, nsarray))[1];
planrows := (xpath('/x:explain/x:Query/x:Plan/x:Plan-Rows/text()', explanation, nsarray))[1];
planwidth := (xpath('/x:explain/x:Query/x:Plan/x:Plan-Width/text()', explanation, nsarray))[1];
RETURN;
END;
$BODY$
LANGUAGE plpgsql;

Hence the example from the question becomes:

SELECT totalcost
FROM estimate_cost('SELECT COUNT(*) 
      FROM Mytable
     WHERE value>17');