Hello, In magento, You can manually reindex data from System -> Index Management. But if you get any problem in indexing then you can do it through code/programming. Currently, there are 9 indexes. They are as give n below: (with their respective key number):

1. Product Attributes
2. Product Prices
3. Catalog URL Rewrites
4. Product Flat Data
5. Category Flat Data
6. Category Products
7. Catalog Search index
8. Tag Aggregation Data
9. Stock Status

So, if you want to reindex only “Category Products” index, then you can use the code given

$process = Mage::getModel('index/process')->load(6);
$process->reindexAll();

below:- If you want to reindex all indexes, then you can loop through these indices to reindex them.

for ($i = 1; $i <= 9; $i++) {
    $process = Mage::getModel('index/process')->load($i);
    $process->reindexAll();
}

Check the code given below:- Thanks,