Add safety limit to sample metadata

The sampling probability is now being computed using an estimate of the table row count
This could led to too high probabilities (to large samples) if the estimate is not accurate.
To avoid potential problems with large samples we've added a LIMIT to the sampling queries.

lib/cartodb/backends/layer-stats/mapnik-layer-stats.js

@@ -172,13 +172,14 @@ function mergeColumns(results) {
     }
 }
 
-
 function _sample(ctx, numRows) {
     if (ctx.metaOptions.sample) {
         const sampleProb = Math.min(ctx.metaOptions.sample / numRows, 1);
+        // We'll use a safety limit just in case numRows is a bad estimate
+        const limit = Math.ceil(ctx.metaOptions.sample * 1.5);
         return queryPromise(
             ctx.dbConnection,
-            _getSQL(ctx, sql => queryUtils.getQuerySample(sql, sampleProb)),
+            _getSQL(ctx, sql => queryUtils.getQuerySample(sql, sampleProb, limit)),
             res => ({ sample: res.rows })
         );
     }

lib/cartodb/utils/query-utils.js

@@ -88,11 +88,12 @@ module.exports.getQueryTopCategories = function(query, column, topN, includeNull
     `;
 };
 
-module.exports.getQuerySample = function(query, sampleProb, randomSeed = 0.5) {
+module.exports.getQuerySample = function(query, sampleProb, limit = null, randomSeed = 0.5) {
     const singleTable = simpleQueryTable(query);
     if (singleTable) {
         return getTableSample(singleTable.table, singleTable.columns, sampleProb, randomSeed);
     }
+    const limitClause = limit ? `LIMIT ${limit}` : '';
     return `
         WITH __cdb_rndseed AS (
             SELECT setseed(${randomSeed})
@@ -100,14 +101,16 @@ module.exports.getQuerySample = function(query, sampleProb, randomSeed = 0.5) {
         SELECT *
             FROM (${query}) AS __cdb_query
             WHERE random() < ${sampleProb}
+            ${limitClause}
     `;
 };
 
-function getTableSample(table, columns, sampleProb, randomSeed) {
+function getTableSample(table, columns, sampleProb, limit = null, randomSeed = 0.5) {
+    const limitClause = limit ? `LIMIT ${limit}` : '';
     sampleProb *= 100;
     randomSeed *= Math.pow(2, 31) -1;
     return `
-        SELECT ${columns} FROM ${table} TABLESAMPLE BERNOULLI (${sampleProb}) REPEATABLE (${randomSeed})
+        SELECT ${columns} FROM ${table} TABLESAMPLE BERNOULLI (${sampleProb}) REPEATABLE (${randomSeed}) ${limitClause}
     `;
 }