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    <p>Transactions need to be ACID, but the "Durability" needs to be enforced only at the transaction's end. So physical IO to the disk may be postponed after the apparent SQL statement execution and actually done in the background, while your transaction is processing other statements.</p> <p>As a consequence, issuing SQL statements <em>serially</em> may not be much slower than issuing them <em>concurrently</em>. Consider this scenario:</p> <ul> <li>Execute the SQL statement [A] that writes data. The disk is not actually touched, writes are simply queued for later, so the execution flow returns very quickly to the client (i.e. [A] does not block for long).</li> <li>Execute the SQL statement [B] that writes data. Writes are queued and [B] does not block for long, just as before. The physical I/O of [A] may already be happening in the background at this point.</li> <li>Other processing takes place in the transaction, while DBMS performs the physical I/O to the disk in the background.</li> <li>The transaction is committed. <ul> <li>If queued writes are finished, there is no need to wait.</li> <li>If queued writes are not finished by now, wait until they are. BTW, some databases can relax the "Durability" requirements to avoid this wait, but not MS SQL Server (AFAIK).</li> </ul></li> </ul> <p>Of course there are scenarios where this "auto-parallelism" of DBMS would not work well, for example when there is a <code>WHERE</code> clause that for different statements touches different partitions on different disks - DBMS would love to parallelize these clauses but can't if they are fed to it one-by-one.</p> <p>In any case, don't guess where your performance bottleneck is. Measure it instead!</p> <hr> <p>BTW, MARS will not help you in parallelizing your statements - according to <a href="http://msdn.microsoft.com/en-us/library/ms131686.aspx" rel="nofollow">MSDN</a>: <em>"Note, however, that MARS is defined in terms of interleaving, not in terms of parallel execution."</em></p>
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    1. COLots of great info in here. To your point of measurement, what I end up with is, I fear, apples to oranges: I can compare 1 connection & 1 transaction executing serially to 80,000 connections & transactions in parallel (w/ pooling). Between those, there is a 15 second difference (about a minute and a half altogether) Depending on how efficiently pooling works, I was hope for substantial savings if I could maintain the same connection.
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    2. COAddendum: The scenario above with 1 connection + blocking comes out to about 10 seconds faster than serial, and about 5 seconds slower than 'full parallel'
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