Paging to Arbitrary Query Depth in Java

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Paging to Arbitrary Query Depth
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A frequent requirement, when displaying the results of a query, is to enable the user to navigate through the results in pagesThe basic function of fetching another page of results is built into SimpleDB with the NextToken Unfortunately, there are many things you might want to do with search results pages that SimpleDB does not directly support
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One of the common ways to display search results in pages is to begin by showing the results for the current page, and also showing links to each of the other pagesThis has the benefit of quickly allowing the user to see how many pages there are and going to one specific page quickly However, as a practical matter, when there are many pages, there is seldom a good reason for the user to navigate to an arbitrary middle page Usually, either the next page or the last page is the next navigation targetAs such, it has become common, on the web, to see search results that do not include full pagination but only the more common first, last, next, and previous linksThis can serve to reduce visual clutter, and it is well suited to SimpleDB, where getting the full query result count can be time consuming for a large domainWhen going to the next page, you may be able to simplify the use case by only allowing a next page and not arbitrary pagingYou can do this in SimpleDB by using the LIMIT clause:
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SELECT title, summary, votecount FROM posts WHERE userid = '000022656' LIMIT 25
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You already know how to handle the NextToken, but if you use this tactic, you can support previous page by storing the breadcrumb trail of next tokens (for example, in the web session) and re-issuing the query with a previous NextToken rather than a subsequent one However, the general case for handling arbitrary pagination in SimpleDB is the same for previous and next In the general case, the user may click on an arbitrary page number, like five, without ever having visited page four or six You handle this in SimpleDB by using the fact that NextToken only requires the WHERE clause and ORDER BY clause to be the same to work properly So, rather than querying through every page in sequence and pulling down all the intervening items, you can usually do it in two steps, as follows: 1 Issue your query with a limit value of where the desired page should start, and SELECT count(*) instead of the actual attributes you want 2 Use the NextToken from step one to fetch the actual page data using the desired attributes and the page size as the LIMIT Listing 6-1 shows a first pass at the pseudocode to implement this
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Listing 6-1 First-Pass Pseudocode for Jumping to an Arbitrary Search Page
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int targetPage, pageSize; int jumpLimit = pageSize * (targetPage - 1); String query = "SELECT %1 FROM posts WHERE userid = '000022656' LIMIT %2"; String output = "title, summary, votecount"; Result temp = sdbselect(query, "count(*)", jumpLimit); Result data = sdbselect(query, output, pageSize, tempgetToken());
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In this code, %1 and %2 are String substitutions and sdbselect() is a fictitious method that includes the String substitution code along with the SimpleDB call Whether or not you can accomplish this in two calls to SimpleDB (as shown in the code) will depend on the complexity of your WHERE clause and the size of your data set The preceding code is simplified in that the temporary result may have returned a partial count if the query took more than five seconds to runYou would really want to put that line in a loop until the proper count is reachedTo make the code a little more realistic, I have added the loop, placed it within methods, and abandoned the String substitutions The updated code is shown in Listing 6-2
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