use estimated bbox for very large tables

bbox.js

 // based on https://raw.githubusercontent.com/tobinbradley/dirt-simple-postgis-http-api/master/routes/bbox.js
 
 const sqlTableName = require('./utils/sqltablename.js');
-const DirCache = require('./utils/dircache.js')
-const cache = new DirCache('./cache');
 
+function splitTableName(params) {
+  const parts = params.table.split('.');
+  if (parts.length === 1) {
+    return {
+      namespace: 'public',
+      tablename: parts[0]
+    }
+  } else {
+    return {
+      namespace: parts[0],
+      tablename: parts[1]
+    }
+  }
+}
+
+const getSqlEstimateRows = (params) =>{
+  const ts = splitTableName(params);
+  return `select ((c.reltuples/case when c.relpages=0 then 1 else c.relpages end) * (pg_relation_size(c.oid) / (current_setting('block_size')::integer)))::integer as estimated_rows
+     from pg_class c JOIN pg_namespace n on c.relnamespace = n.oid
+       where n.nspname='${ts.namespace}' and c.relname='${ts.tablename}'
+  `;
+}
+
+const sqlEstimateBbox = (params, query, estimatedRows) => {
+  const ts = splitTableName(params);
+  return `
+    with srid as
+      (select st_srid(${query.geom_column}) srid
+         from ${sqlTableName(params.table)} 
+            where ${query.geom_column} is not null limit 1)
+    ,bboxsrid as
+      (select ST_EstimatedExtent('${ts.namespace}', '${ts.tablename}', '${query.geom_column}') as bboxsrid)
+    ,bboxll as
+      (select st_extent(st_transform(st_setsrid(st_envelope(bboxsrid), srid),4326)) bboxll 
+         from bboxsrid, srid)
+    select ${estimatedRows} as allrows, ${estimatedRows} as geomrows, bboxll,srid,bboxsrid 
+       from bboxll,srid,bboxsrid      
+  `;
+} 
 
-const sql = (params, query) => {
+const sqlBbox = (params, query) => {
   return `
-with srid as 
-  (select st_srid(${query.geom_column}) srid from ${sqlTableName(params.table)} where ${query.geom_column} is not null limit 1)
-,bboxll as 
-(select ST_Extent(ST_Transform(${query.geom_column}, 4326)) as bboxll, count(*) allrows, count(${query.geom_column}) geomrows from ${sqlTableName(params.table)}
+    with srid as 
+      (select st_srid(${query.geom_column}) srid
+        from ${sqlTableName(params.table)}
+          where ${query.geom_column} is not null limit 1)
+    ,bboxll as 
+      (select ST_Extent(ST_Transform(${query.geom_column}, 4326)) as bboxll, count(*) allrows, count(${query.geom_column}) geomrows 
+          from ${sqlTableName(params.table)}
             -- Optional where filter
             ${query.filter ? `WHERE ${query.filter}` : '' }
-)
-,bboxsrid as
-(select st_extent(st_transform(st_setsrid(st_envelope(bboxll),4326),srid)) bboxsrid from bboxll,srid)
-select allrows, geomrows, bboxll,srid,bboxsrid from bboxll,srid,bboxsrid
-      `
+      )
+    ,bboxsrid as
+      (select st_extent(st_transform(st_setsrid(st_envelope(bboxll),4326),srid)) bboxsrid 
+         from bboxll,srid)
+    select allrows, geomrows, bboxll,srid,bboxsrid 
+      from bboxll,srid,bboxsrid
+  `;
 }
 
-let cacheMiddleware = async(req, res, next) => {
+module.exports = function(app, pool, cache) {
+  let cacheMiddleware = async(req, res, next) => {
+    if (!cache) {
+      next();
+      return;
+    }
     const cacheDir = `${req.params.table}/bbox`
     const key = ((req.query.geom_column?req.query.geom_column:'geom') 
       + (req.query.columns?'_'+req.query.columns:'')
@@ -42,9 +89,7 @@ let cacheMiddleware = async(req, res, next) => {
       }
       next();
     }
-}
-
-module.exports = function(app, pool) {
+  }
 /**
  * @swagger
  *
@@ -77,6 +122,9 @@ module.exports = function(app, pool) {
  *         schema:
  *            type: object
  *            properties:
+ *               estimated:
+ *                 type: boolean
+ *                 description: result is estimated (true) or precise (false)
  *               allrows:
  *                 type: integer
  *                 description: number of rows in table
@@ -118,12 +166,22 @@ module.exports = function(app, pool) {
       if (!req.query.srid) {
         req.query.srid = 4326
       }
-      const sqlString = sql(req.params, req.query);
+      let sql = getSqlEstimateRows(req.params);
+      let estimated = false;
       try {
-        const result = await pool.query(sqlString);
+        const estimateResult = await pool.query(sql);
+        const estimatedRows = estimateResult.rows[0].estimated_rows;
+        if (estimatedRows < 5000000 || req.query.filter) {
+          sql = sqlBbox(req.params, req.query);
+        } else {
+          sql = sqlEstimateBbox(req.params, req.query, estimatedRows);
+          estimated = true;
+        }
+        const result = await pool.query(sql);
         if (result.rows.length === 1) {
           const row = result.rows[0];
           res.json({
+            estimated: estimated,
             allrows: Number(row.allrows), 
             geomrows: Number(row.geomrows),
             bboxll: row.bboxll?row.bboxll.match(/BOX\((.*)\)/)[1].split(',').map(coord=>coord.split(' ').map(c=>parseFloat(c))):null,