Differential D25007 Diff 21 src/docs/contributor/n_plus_one.diviner

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src/docs/contributor/n_plus_one.diviner

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The problem with this is that each query has quite a bit of overhead. **It is		The problem with this is that each query has quite a bit of overhead. **It is
//much faster// to issue 1 query which returns 100 results than to issue 100		//much faster// to issue 1 query which returns 100 results than to issue 100
queries which each return 1 result.** This is particularly true if your database		queries which each return 1 result.** This is particularly true if your database
is on a different machine which is, say, 1-2ms away on the network. In this		is on a different machine which is, say, 1-2ms away on the network. In this
case, issuing 100 queries serially has a minimum cost of 100-200ms, even if they		case, issuing 100 queries serially has a minimum cost of 100-200ms, even if they
can be satisfied instantly by MySQL. This is far higher than the entire		can be satisfied instantly by MySQL. This is far higher than the entire
server-side generation cost for most Phabricator pages should be.		server-side generation cost for most Phorge pages should be.

= Batching Queries =		= Batching Queries =

Fix the N+1 query problem by batching queries. Load all your data before		Fix the N+1 query problem by batching queries. Load all your data before
iterating through it (this is oversimplified and omits error checking):		iterating through it (this is oversimplified and omits error checking):

$cats = load_cats();		$cats = load_cats();
$hats = load_all_hats_for_these_cats($cats);		$hats = load_all_hats_for_these_cats($cats);
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