From e60f73a31b26ccaac3354c9ccf1d3f0e72ba26a7 Mon Sep 17 00:00:00 2001
From: Rafa de la Torre <rtorre@cartodb.com>
Date: Mon, 21 Dec 2015 13:16:57 +0100
Subject: [PATCH 1/3] A bit of internal documentation

---
 scripts-available/CDB_Overviews.sql | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/scripts-available/CDB_Overviews.sql b/scripts-available/CDB_Overviews.sql
index 3c97c8a..e1da1fe 100644
--- a/scripts-available/CDB_Overviews.sql
+++ b/scripts-available/CDB_Overviews.sql
@@ -1,3 +1,9 @@
+-- Determine the max feature density of a given dataset.
+-- Scope: private.
+-- Parameters
+--   reloid: oid of the input table. It must be a cartodbfy'ed table.
+--   nz: number of zoom levels to consider from z0 upward.
+-- Return value: feature density (num_features / webmercator_squared_meters).
 CREATE OR REPLACE FUNCTION _CDB_Feature_Density(reloid REGCLASS, nz integer)
 RETURNS FLOAT8
 AS $$

From 5010109c7dfc17c061bd3edc6ca9f6a840cfd7a4 Mon Sep 17 00:00:00 2001
From: Rafa de la Torre <rtorre@cartodb.com>
Date: Mon, 21 Dec 2015 17:41:28 +0100
Subject: [PATCH 2/3] Add _cdb_estimated_extent to get the extent from stats

---
 scripts-available/CDB_Overviews.sql | 37 +++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/scripts-available/CDB_Overviews.sql b/scripts-available/CDB_Overviews.sql
index e1da1fe..a72d48d 100644
--- a/scripts-available/CDB_Overviews.sql
+++ b/scripts-available/CDB_Overviews.sql
@@ -1,3 +1,40 @@
+-- Calculate the estimated extent of a cartodbfy'ed table.
+-- Scope: private.
+-- Parameters
+--   reloid: oid of the input table.
+-- Return value A box2d extent in 3857.
+CREATE OR REPLACE FUNCTION _cdb_estimated_extent(reloid REGCLASS)
+RETURNS box2d
+AS $$
+  DECLARE
+    ext box2d;
+    ext_query text;
+    table_id record;
+  BEGIN
+
+    SELECT n.nspname AS schema_name, c.relname table_name INTO STRICT table_id
+      FROM pg_class c JOIN pg_namespace n on n.oid = c.relnamespace WHERE c.oid = reloid::oid;
+
+    ext_query = format(
+      'SELECT ST_EstimatedExtent(''%1$I'', ''%2$I'', ''%3$I'');',
+      table_id.schema_name, table_id.table_name, 'the_geom_webmercator'
+    );
+
+    BEGIN
+      EXECUTE ext_query INTO ext;
+      EXCEPTION
+        -- This is the typical ERROR: stats for "mytable" do not exist
+        WHEN internal_error THEN
+          -- Get stats and execute again
+          EXECUTE format('ANALYZE %1$I', reloid);
+          EXECUTE ext_query INTO ext;
+    END;
+
+    RETURN ext;
+  END;
+$$ LANGUAGE PLPGSQL VOLATILE;
+
+
 -- Determine the max feature density of a given dataset.
 -- Scope: private.
 -- Parameters

From 564ab75d2d4cc3797aa8c24262d2598e41eb22bd Mon Sep 17 00:00:00 2001
From: Rafa de la Torre <rtorre@cartodb.com>
Date: Mon, 21 Dec 2015 18:34:26 +0100
Subject: [PATCH 3/3] Use _cdb_estimated_extent instead of ST_Extent

With a 3.8M points table, this was a ~30% gain in my local env.
---
 scripts-available/CDB_Overviews.sql | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/scripts-available/CDB_Overviews.sql b/scripts-available/CDB_Overviews.sql
index a72d48d..5d94b09 100644
--- a/scripts-available/CDB_Overviews.sql
+++ b/scripts-available/CDB_Overviews.sql
@@ -71,7 +71,7 @@ AS $$
   -- considered tiles.
   EXECUTE Format('
     WITH RECURSIVE t(x, y, z, e) AS (
-      WITH ext AS (SELECT ST_Extent(the_geom_webmercator) g FROM %1$s),
+      WITH ext AS (SELECT _cdb_estimated_extent(%6$s) as g),
       base AS (
         SELECT (-floor(log(2, (greatest(ST_XMax(ext.g)-ST_XMin(ext.g), ST_YMax(ext.g)-ST_YMin(ext.g))/(%4$s*%5$s))::numeric)))::integer z
         FROM ext
@@ -101,7 +101,7 @@ AS $$
       WHERE t.e > %2$s AND t.z < (base.z + %3$s)
     )
     SELECT MAX(e/ST_Area(CDB_XYZ_Extent(x,y,z))) FROM t where e > 0;
-  ', reloid::text, min_features, nz, n, c)
+  ', reloid::text, min_features, nz, n, c, reloid::oid)
   INTO fd;
   RETURN fd;
   END