MySQL how does SELECT * actually work in MySQL/Query Efficiency - mysql

Question

I am building some AJAX queries that need to gather META data (column names) to query one input across several columns on the database...such as if you search for John Anderson and an email address of jand#email.com, "and" would match ANDerson and jAND#email.com
The query looks something like this:
// The joins might not make sense I took the database name out, since it is
// the same as the company I work for...
$querySyntax = "SELECT
idAccount,
FirstName,
LastName,
Email,
Phone,
CCCity,
CCState
FROM
account Right Join
states On account.CCState =
states.ID WHERE ";
$cols = $dbo->query("SELECT * FROM account");
$colcount = (count($cols) > 0 ? $cols->columnCount() : count($cols));
for ($ii=0; $ii < count($search); $ii++) {
if ($ii>0)
$querySyntax = $querySyntax . " AND ";
$querySyntax = $querySyntax . "(";
for ($i=0; $i<$colcount; $i++) {
if ($i>0)
$querySyntax = $querySyntax . " OR ";
$meta = $cols->getColumnMeta($i);
$colNames[$i] = $meta['name'];
$querySyntax = $querySyntax . $colNames[$i] . " LIKE '%" . $search[$ii] . "%'";
}
$querySyntax = $querySyntax . ")";
}
$querySyntax .= " LIMIT 50";
$found = $dbo->query($querySyntax);
for($i=0; $row=$found->fetch();$i++) {
$result[$i] = $row;
}
Ok, with that said, is there a better way to get the column names than the "SELECT * ..." query?
When you run SELECT * does it in fact read and store each record to memory? This could be extremely slow once the database is populated with the thousands of records we have.

Answer 1

SHOW COLUMNS (and most other SHOW commands that query metadata) function internally as queries against the INFORMATION_SCHEMA. When you do that against InnoDB tables, MySQL attempts to refresh table statistics by querying random pages from the respective table. The result is that SHOW commands and queries against INFORMATION_SCHEMA are terribly slow.
There are a few workarounds:
SET GLOBAL innodb_stats_on_metadata=0 which disables the automatic recalc of table stats every time you view the metadata for InnoDB tables.
See Peter Zaitsev's recent blog: Solving INFORMATION_SCHEMA slowness, and the manual Dynamically Changing innodb_stats_on_metadata.
Use DESC account which is much faster than querying INFORMATION_SCHEMA.
Use PDOStatement::getColumnMeta() as you are doing, but use prepare() instead of query(). So it will parse your query but not execute it or attempt to fetch rows of results. Test this carefully because getColumnMeta() is "experimental" and probably buggy.
However, you will find that LIKE '%search%' predicates will perform poorly anyway. You should use a real fulltext search solution like Sphinx Search or Apache Solr.
PS: When coding a loop in PHP, it's better to calculate the count once before the loop, instead of on every iteration of the loop.
for ($ii=0; $ii < count($search); $ii++) {
Should be
$c = count($search);
for ($ii=0; $ii < $c; $ii++) {

Answer 2

This is pretty wasteful, and yes, this retrieves all records from that table. If you are interested in only listing the columns, you could probably use the SHOW COLUMNS FROM <your table> statement (docs) to retrieve just the list of columns. Moreover, you definitely should not do it before every query. Better to do it just once and then store the column list for future use, as they are really unlikely to change during the user session, aren't they?