Add CDB_EstimateRowCount function

See #295

NEWS.md

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+0.19.0 (2017-04-XX)
+
+* Add new function `CDB_EstimateRowCount` #295
+
 0.18.5 (2016-11-30)
 
 * Add to new overview creation strategies #290

doc/CDB_EstimateRowCount.md

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+Estimate the number of rows of a query.
+
+
+#### Using the function
+
+```sql
+SELECT CDB_EstimateRowCount($$
+  UPDATE addresses SET the_geom = cdb_geocode_street_point(addr, city, state, 'US');
+$$) AS row_count;
+```
+
+Result:
+
+```
+ row_count
+-----------
+         5
+(1 row)
+```
+
+#### Arguments
+
+CDB_EstimateRowCount(query)
+
+* **query** text: the SQL query to estimate the row count for.

scripts-available/CDB_EstimateRowCount.sql

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+-- Internal function to generate stats for a table if they don't exist
+CREATE OR REPLACE FUNCTION _CDB_GenerateStats(reloid REGCLASS)
+RETURNS VOID
+AS $$
+DECLARE
+  num_cols INTEGER;
+BEGIN
+  SELECT COUNT(*) FROM pg_catalog.pg_statistic WHERE starelid = reloid INTO num_cols;
+  IF num_cols = 0 THEN
+    EXECUTE Format('ANALYZE %s;', reloid);
+  END IF;
+END
+$$ LANGUAGE 'plpgsql' VOLATILE STRICT SECURITY DEFINER;
+
+-- Return a row count estimate of the result of a query using statistics
+CREATE OR REPLACE FUNCTION CDB_EstimateRowCount(query text)
+RETURNS Numeric
+AS $$
+DECLARE
+  plan JSON;
+BEGIN
+  -- Make sure statistics exist for all the tables of the query
+  PERFORM _CDB_GenerateStats(tabname) FROM  unnest(CDB_QueryTablesText(query)) AS tabname;
+
+  -- Use the query planner to obtain an estimate of the number of result rows
+  EXECUTE 'EXPLAIN (FORMAT JSON) ' || query INTO STRICT plan;
+  RETURN plan->0->'Plan'->'Plan Rows';
+END
+$$ LANGUAGE 'plpgsql' VOLATILE STRICT;

scripts-enabled/280-CDB_EstimateRowCount.sql

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+../scripts-available/CDB_EstimateRowCount.sql

test/CDB_EstimateRowCountTest.sql

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+SET client_min_messages TO error;
+\set VERBOSITY terse
+CREATE TABLE tmptab1(id INT);
+INSERT INTO tmptab1(id) VALUES (1), (2), (3);
+CREATE TABLE tmptab2(id INT, value NUMERIC);
+INSERT INTO tmptab2(id, value) VALUES (1, 10.0), (2, 20.0);
+SELECT CDB_EstimateRowCount('SELECT SUM(value) FROM tmptab1 INNER JOIN tmptab2 ON (tmptab1.id = tmptab2.id);') AS row_count;
+SELECT CDB_EstimateRowCount('UPDATE tmptab2 SET value = 30 WHERE id=2;') AS row_count;
+DROP TABLE tmptab2;
+DROP TABLE tmptab1;

test/CDB_EstimateRowCountTest_expect

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+SET
+CREATE TABLE
+INSERT 0 3
+CREATE TABLE
+INSERT 0 2
+1
+1
+DROP TABLE
+DROP TABLE