Subtle little query caching performance issue

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I'm doing a bit of performance tuning (via CFML) for a client and I ran into a little subtle bug that I thought I'd share with others. One of the methods returned all rows that were within a day based range. By that I mean it allowed you to get entries that were created less than 1 day ago, 2 days ago, 5 days ago, and so forth. The query runs a bit slow but was using query caching.

I noticed something interesting though. The debugging was the query always said that the cache was not enabled. Looking over the query I didn't see anything wrong - at least not at first. Can you see why the query never cached?

<cfquery datasource="#datasource#" cachedWithin="#createTimeSpan(0,0,5,0)#" name="q"> SELECT Post.postId, Post.feed FROM Post WHERE dateTimeAggregated > #dateAdd("h", ((arguments.daysBack * 24) * -1), dateConvert("local2utc", now()))# ORDER BY clicks DESC, Post.dateTimeAggregated desc LIMIT #arguments.offset#, #arguments.limit# </cfquery>

I've trimmed the query a bit - but do you see the issue yet? The WHERE clause is the culprit. Remember, the idea was - return records that were made N days ago or sooner. You can do this in SQL completely, but the code did it in ColdFusion with the dateAdd function. Notice how it creates the date value - it subtracts N*24 (N being the number of days) from the current time. So if I run this query at 12:01:01 and then again at 12:01:02, the value of now() has changed. Since the WHERE clause changed, the cached query from before wasn't used. I'm not sure this is the best fix, but I simply tried a hard coded time value:

dateTimeAggregated > #dateAdd("h", ((arguments.daysBack * 24) * -1), "#dateFormat(now(), 'mm/dd/yyyy')#")#

It seemed to work fine. I need to double check that it returns the values right - but the point is - when using dynamic queries and caching, be sure to note exactly how the variables can change. While the "daysBack" variable only has a few options, now() is - obviously - pretty varied.

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Raymond is a developer advocate for HERE Technologies. He focuses on JavaScript, serverless and enterprise cat demos. If you like this article, please consider visiting my Amazon Wishlist or donating via PayPal to show your support. You can even buy me a coffee!

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