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I'm a bit confused because I'm questioning the definition of 'online' in OLTP and OLAP. I used to think that 'online' here means that we want our answer in bounded time and based on the data available at an instance in time.

But OLAP queries can take a some hours to compute - isn't this offline?

And a quick search indicates offline OLAP which sounds rather confusing (offline online analytical processing)...?

What exactly is 'online'?

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    Not really working with SSAS - no experience on that. To my knowledge, this is a classic database textbook reference since the 80s. I am trying to better understand what 'online' in OLAP/OLTP means today this many years later. – Zeruno Jun 19 at 13:05
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    Also, why is Online abbreviated as OL, while analytics is A, and processing is P. The mysteries abound. – Max Vernon Jun 19 at 16:36
  • Online in this context means a remote terminal connected to a mainframe. – user207421 Jun 20 at 1:10
  • @MaxVernon This is also a historical artifact: "on-line" used to be the preferred spelling, hence the two-letter abbr. – mustaccio Jun 20 at 14:58
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It is simply a remnant of olden times, when it was used in contrast to batch processing. "Online" here means "interactive", that is, requests to the database are processed as they come and responses are given more or less immediately, or at least as soon as they are available. Batch processing would collect requests into, well, batches, and execute them on schedule; responses would be given after the entire batch execution (e.g. next morning).

Abbreviations OLAP and OLTP hint at another historical artifact: "on-line" used to be the more common spelling until mid-1980s.

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    +1. Greybeards who programmed COBOL on IBM mainframes will remember stuff like this. – RonJohn Jun 19 at 21:58
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    @RonJohn "Greybeards"... I don't know where you've seen my photo... The concepts of interactive and batch processing are still alive and well, it's the "online" part that has become confusing over time. – mustaccio Jun 19 at 22:54
  • Pretty sure this is wrong. According to my cs teaches (And wikipedia): "an online algorithm is one that can process its input piece-by-piece in a serial fashion". So an online algorithem is one who newer have access to all data at the same time. (So it's streaming data) – MTilsted Jun 20 at 15:02
  • @RonJohn, Those of us who worked on Hitachi mainframes remember this too. ;-) – B540Glenn Jun 20 at 15:10
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    @MTilsted This isn't about an online algorithm, it's about an online data-processing system. The word "online" has different meanings depending on the context. – Barmar Jun 20 at 16:05

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