Windshaft-cartodb/lib/cartodb/api/tables_extent_api.js

function TablesExtentApi(pgQueryRunner) {
    this.pgQueryRunner = pgQueryRunner;
}

module.exports = TablesExtentApi;

/**
 * Given a username and a list of tables it will return the estimated extent in SRID 4326 for all the tables based on
 * the_geom_webmercator (SRID 3857) column.
 *
 * @param {String} username
 * @param {Array} tableNames The named can be schema qualified, so this accepts both `schema_name.table_name` and
 *  `table_name` format as valid input
 * @param {Function} callback function(err, result) {Object} result with `west`, `south`, `east`, `north`
 */
TablesExtentApi.prototype.getBounds = function (username, tableNames, callback) {
    var estimatedExtentSQLs = tableNames.map(function(tableName) {
        var schemaTable = tableName.split('.');
        if (schemaTable.length > 1) {
            return "ST_EstimatedExtent('" + schemaTable[0] + "', '" + schemaTable[1] + "', 'the_geom_webmercator')";
        }
        return "ST_EstimatedExtent('" + schemaTable[0] + "', 'the_geom_webmercator')";
    });

    var query = [
        "WITH ext as (" +
            "SELECT ST_Transform(ST_SetSRID(ST_Extent(ST_Union(ARRAY[",
                estimatedExtentSQLs.join(','),
            "])), 3857), 4326) geom)",
        "SELECT",
            "ST_XMin(geom) west,",
            "ST_YMin(geom) south,",
            "ST_XMax(geom) east,",
            "ST_YMax(geom) north",
        "FROM ext"
    ].join(' ');

    this.pgQueryRunner.run(username, query, handleBoundsResult, callback);
};

function handleBoundsResult(err, rows, callback) {
    if (err) {
        var msg = err.message ? err.message : err;
        return callback(new Error('could not fetch source tables: ' + msg));
    }
    var result = null;
    if (rows.length > 0) {
        result = {
            bounds: rows[0]
        };
    }
    callback(null, result);
}