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MDL-26458 sqlsrv - limit impl using scrollable sets

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Eloy Lafuente (stronk7) 2011-02-18 01:08:58 +01:00 committed by Aparup Banerjee
parent 48fe5c6cf9
commit 3479ba4d48
1 changed files with 15 additions and 110 deletions
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125
lib/dml/sqlsrv_native_moodle_database.php
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@ -347,13 +347,18 @@ class sqlsrv_native_moodle_database extends moodle_database {
* @param mixed $params array of params for binding. If NULL, they are ignored.
* @param mixed $sql_query_type - Type of operation
* @param mixed $free_result - Default true, transaction query will be freed.
* @param mixed $scrollable - Default false, to use for quickly seeking to target records
*/
private function do_query($sql, $params, $sql_query_type, $free_result = true) {
private function do_query($sql, $params, $sql_query_type, $free_result = true, $scrollable = false) {
list($sql, $params, $type) = $this->fix_sql_params($sql, $params);
$sql = $this->emulate_bound_params($sql, $params);
$this->query_start($sql, $params, $sql_query_type);
$result = sqlsrv_query($this->sqlsrv, $sql);
if (!$scrollable) { // Only supporting next row
$result = sqlsrv_query($this->sqlsrv, $sql);
} else { // Suporting absolute/relative rows
$result = sqlsrv_query($this->sqlsrv, $sql, array(), array('Scrollable' => SQLSRV_CURSOR_STATIC));
}
if ($result === false) {
// TODO do something with error or just use if DEV or DEBUG?
@ -756,118 +761,18 @@ class sqlsrv_native_moodle_database extends moodle_database {
$limitnum = max(0, $limitnum);
if ($limitfrom or $limitnum) {
$sql = $this->limit_to_top_n($sql, $limitfrom, $limitnum);
}
$result = $this->do_query($sql, $params, SQL_QUERY_SELECT, false);
return $this->create_recordset($result);
}
/**
* Perform a emulation for LIMIT(offset, limit)
*
* @param string $sql
* @param int $offset
* @param int $limit
* @return string sql
*/
private function limit_to_top_n($sql, $offset, $limit) {
// If there is no limit we can return immediately
if ($limit < 1 && $offset < 1) {
return $sql;
}
// Make sure they are at least 0
$limit = max(0, (int)$limit);
$offset = max(0, (int)$offset);
// This is an sqlserver bigint - -1 and will be used as a value
// for top when essentially we want everything.
// This needs to be a string so that it doesn't get malformed.
$bigint = '9223372036854775806';
// If limit is 0 set it to BITINT - 1
if (empty($limit)) {
$limit = $bigint;
} else {
$limit = $offset + $limit;
}
// Set up defaults for the next bit.
$columns = '*'; // Default to all columns
$columnnames = '*'; // As above
$firstcolumn = 'id'; // The default first column is id
$orderby = ''; // The order by of the main query
// We need to do a couple of maintenance tasks on the columns seeing as
// they are going to be wrapped in a sub select.
//
// 1. Collect the first column to use for an order by if there isn't an
// explicit order by within the query.
// 2. Give all constant columns a proper alias, this is required because
// of the subselect.
if (preg_match('#^(\s*SELECT\s+)(DISTINCT\s+)?(.*?)(\sFROM\s)#is', $sql, $columnmatches)) {
// Make sure we have some columns
if (!empty($columnmatches[3])) {
$columns = explode(',', $columnmatches[3]);
$firstcolumn = null;
$constantcount = 0;
foreach ($columns as $key=>$column) {
// Trim, trim, trim, except during Movember.
$column = trim($column);
if (preg_match('#\sAS\s+(\w+)\s*$#si', $column, $matches)) {
// Make sure we use the column alias if available.
$column = $matches[1];
} else if (preg_match("#^('[^']*'|\d+)$#", $column)) {
// Give constants an alias in the main query and use the
// alias for the new outer queries.
$constantcount++;
$column = 'constant_'.$constantcount;
$columns[$key] .= ' AS '.$column;
}
// Store the first column for later abuse.
if ($firstcolumn === null) {
$firstcolumn = $column;
}
}
// Glue things back together
$columns = join(', ', $columns);
// Switch out the fixed main columns (added constant aliases).
$sql = str_replace($columnmatches[0], $columnmatches[1].$columnmatches[2].$columns.$columnmatches[4], $sql);
if ($limitnum >= 1) { // Only apply TOP clause if we have any limitnum (limitfrom offset is handled later)
$fetch = $limitfrom + $limitnum;
$sql = preg_replace('/^([\s(])*SELECT([\s]+(DISTINCT|ALL))?(?!\s*TOP\s*\()/i',
"\\1SELECT\\2 TOP $fetch", $sql);
}
}
$result = $this->do_query($sql, $params, SQL_QUERY_SELECT, false, (bool)$limitfrom);
// Collect the orderby from the main query to use in the row number order by.
if (preg_match('#\sORDER\s+BY\s+([^)]+?)(GROUP\s+BY|$)#i', $sql, $matches)) {
// We need to remove it from the main query as well.
$sql = str_replace($matches[0], ' '.$matches[2], $sql);
$orderby = $matches[1];
} else {
// Default orderby to the first column.
$orderby = $firstcolumn;
if ($limitfrom) { // Skip $limitfrom records
sqlsrv_fetch($result, SQLSRV_SCROLL_ABSOLUTE, $limitfrom - 1);
}
// Remove any table aliases from the order by.
$orderby = preg_replace('#[^\s,]*\.([^\s,]*)#', '$1', $orderby);
// If the orderby is all tables everything will break, default to id.
if ($orderby == '*') {
$orderby = 'id';
}
// Now we need to build the queries up so that we collect a row count field and then sort on it.
// To do this we need to nest the query twice. The first nesting selects all the rows from the
// query and then proceeds to use OVER to generate a row number.
// The second nesting we limit by selecting where rownumber between offset and limit
// In both cases we will select the original query fields using q.* this is important
// as there can be any number of crafty things going on. It does however mean that we
// end up with the first field being sqlsrvrownumber however sqlsrv_native_moodle_recordset
// strips that off during processing if it exists.
// Build the inner outer query.
$sql = "SELECT TOP $bigint ROW_NUMBER() OVER(ORDER BY $orderby) AS sqlsrvrownumber, q.* FROM ($sql) AS q";
// Build the outer most query.
$sql = "SELECT q.* FROM ($sql) AS q WHERE q.sqlsrvrownumber > $offset AND q.sqlsrvrownumber <= $limit";
// Return the now mangled query for use.
return $sql;
return $this->create_recordset($result);
}
/**