I'm catching features from my PostGIS database with PHP, which generates dynamic sql requests. Since I can't manage to wrap my response into an ol.layer.Image I thought about using ST_UNION or ST_COLLECT to improve map rendering performance. My current generated SQL is looking like this:
SELECT krs,class, st_asgeojson(st_transform(ST_SimplifyPreserveTopology(the_geom,0),4326),6) AS geojson
FROM tbl_detailansicht_f
WHERE krs = 'Rotenburg (Wümme)' AND class = 5 AND the_geom && st_transform(st_makeenvelope(7.981262207031247,52.23004870271677,11.112365722656248,53.695628373000176,4326),4326)
I would like to get a sql statement like this:
SELECT krs,class, st_asgeojson(st_transform(ST_SimplifyPreserveTopology(st_collect(the_geom),0),4326),6) AS geojson
FROM tbl_detailansicht_f
WHERE krs = 'Rotenburg (Wümme)' AND class = 5 AND the_geom && st_transform(st_makeenvelope(7.981262207031247,52.23004870271677,11.112365722656248,53.695628373000176,4326),4326)
GROUP BY krs, class
So my PHP (by https://github.com/bmcbride/PHP-Database-GeoJSON) needs to be adjusted in order to consider this change, which I fail at.
<?php
/**
* GET GeoJSON from PostGIS
* Query a PostGIS table or view and return the results in GeoJSON format, suitable for use in OpenLayers, Leaflet, etc.
* Author: Bryan R. McBride, GISP, adapted by G.Moes
* Contact: bryanmcbride.com
* GitHub: https://github.com/bmcbride/PHP-Database-GeoJSON
*
* @param string $geotable The PostGIS layer name *REQUIRED*
* @param string $geomfield The PostGIS geometry field *REQUIRED*
* @param string $srid The SRID of the returned GeoJSON *OPTIONAL (If omitted, EPSG: 2169 will be used)*
* @param string $fields Fields to be returned *OPTIONAL (If omitted, all fields will be returned)*
* NOTE- Uppercase field names should be wrapped in double quotes
* @param string $parameters SQL WHERE clause parameters *OPTIONAL*
* @param string $orderby SQL ORDER BY constraint *OPTIONAL*
* @param string $sort SQL ORDER BY sort order (ASC or DESC) *OPTIONAL*
* @param string $limit Limit number of results returned *OPTIONAL*
* @param integer $precision digits of returned geojson 6 = 0.111 m submeter as DEFAULT *OPTIONAL*
* @param real $simplify simplify geometry to >5.0m as DEFAULT *OPTIONAL*
* @param string $offset Offset used in conjunction with limit *OPTIONAL*
* @return string resulting geojson string
*/
# Connect to PostgreSQL database You need to pass here the credentials to connect to Your database
require("./connect.php");
function escapeJsonString($value) { # list from www.json.org: (\b backspace, \f formfeed)
$escapers = array("\\", "/", "\"", "\n", "\r", "\t", "\x08", "\x0c");
$replacements = array("\\\\", "\\/", "\\\"", "\\n", "\\r", "\\t", "\\f", "\\b");
$result = str_replace($escapers, $replacements, $value);
return $result;
}
# Retrive URL variables
if (empty($_GET['geotable'])) {
echo "missing required parameter: <i>geotable</i>";
exit;
} else
$geotable = $_GET['geotable'];
if (empty($_GET['geomfield'])) {
$geomfield='the_geom';
} else
$geomfield = $_GET['geomfield'];
if (empty($_GET['srid'])) {
$srid = 4326; // changethis if You need another standard SRID
} else
$srid = $_GET['srid'];
if (empty($_GET['fields'])) {
$fields = '*';
} else
$fields = $_GET['fields'];
$parameters = $_GET['where'];
$bbox = $_GET['bbox'];
if (empty($_GET['precision'])) {
$precision = 6; // change this to Your needs
} else
$precision = $_GET['precision'];
if (empty($_GET['simplify'])) {
$simplify = 0; // change this to Your needs
} else
$simplify = $_GET['simplify'];
$orderby = $_GET['orderby'];
if (empty($_GET['sort'])) {
$sort = 'ASC';
} else
$sort = $_GET['sort'];
$limit = $_GET['limit'];
$offset = $_GET['offset'];
# Build SQL SELECT statement and return the geometry as a GeoJSON element in EPSG: 4326
$sql = "SELECT " . pg_escape_string($fields) . ", st_asgeojson(st_transform(ST_SimplifyPreserveTopology(" . pg_escape_string($geomfield) . ",".$simplify."),4326),".$precision.") AS geojson FROM " . pg_escape_string($geotable);
if (strlen(trim($parameters)) > 0) {
$sql .= " WHERE " . str_replace("''", "'", pg_escape_string($parameters));
}
if (strlen(trim($parameters)) > 0 AND strlen(trim($bbox)) > 0){
$sql .= " AND the_geom && st_transform(st_makeenvelope(".pg_escape_string ($bbox).",4326),".$srid.")";
}
if (strlen(trim($parameters)) <= 0 AND strlen(trim($bbox)) > 0) {
$sql .= " WHERE the_geom && st_transform(st_makeenvelope(".pg_escape_string ($bbox).",4326),".$srid.")";
}
if (strlen(trim($orderby)) > 0) {
$sql .= " ORDER BY " . pg_escape_string($orderby) . " " . $sort;
}
if (strlen(trim($limit)) > 0) {
$sql .= " LIMIT " . pg_escape_string($limit);
}
if (strlen(trim($offset)) > 0) {
$sql .= " OFFSET " . pg_escape_string($offset);
}
//echo $sql;
# Try query or error
$rs = pg_query($db_connection, $sql);
if (!$rs) {
echo "An SQL error occured.\n";
echo $sql;
exit;
}
# Build GeoJSON
$output = '';
$rowOutput = '';
while ($row = pg_fetch_assoc($rs)) {
$rowOutput = (strlen($rowOutput) > 0 ? ',' : '') . '{"type": "Feature", "geometry": ' . $row['geojson'] . ', "properties": {';
$props = '';
$id = '';
foreach ($row as $key => $val) {
if ($key != "geojson") {
$props .= (strlen($props) > 0 ? ',' : '') . '"' . $key . '":"' . escapeJsonString($val) . '"';
}
if ($key == "id") {
$id .= ',"id":"' . escapeJsonString($val) . '"';
}
}
$rowOutput .= $props . '}';
$rowOutput .= $id;
$rowOutput .= '}';
$output .= $rowOutput;
}
$output = '{"type": "FeatureCollection", "features": [ ' . $output . ' ]}';
header('Content-type:application/json;charset=utf-8');
echo json_encode( $output);
?>
