It’s easy to find resources about improving Elasticsearch performance, but what if you wanted to reduce it? In Part I of this two-part series we looked under the hood in order to learn how ES works internally. Now, in Part II, is the time to apply this knowledge in practice and ruin our ES performance. Most tips should also be applicable to Solr, raw Lucene, or, for that matter, to any other full-text search engine as well.

Using complex boolean queries

A consequence of the algorithms outlined in Part I is that simple queries, such as finding a document containing two or three specific words, are relatively cheap to compute. We can easily increase the cost by making the queries more complex. This complexity is easily achieved by using boolean queries which allow arbitrary boolean expressions, including nested subexpressions.

A “flat” query, even with many words, boils down to a single AND/OR operation (though potentially with many lists). Since a search engine is usually smart enough to sort these lists by length, it can execute such queries really fast. But what happens if we use a complex boolean expression? Let’s compare two example queries:

• Q1: find books which contain all of the words: “I like apples”
• Q2: find books which contain all of the words: “I like apples” or “I like oranges” and were published in 2020 or 2021 and are at least 100 pages long

Let us now look at the boolean expressions which correspond to these queries:

• Q1: I AND like AND apples
• Q2: ((I AND like AND apples) OR (I AND like AND oranges)) AND (2020 OR 2021) AND (pages >= 100)

Clearly, there is much more stuff to compute in the case of Q2, and the query will take more time, accordingly. Even though from the end user’s perspective, we only added a few filters, the complexity of the query rose dramatically. For Q1, we need to perform 2 AND operations. For Q2, we end up with 6 AND operations and 2 OR operations. However, this query is probably still much worse for performance than it may seem.

Let’s start analyzing this query from the bottom up.

The expression 2020 OR 2021 is a little gem that looks innocent, but is actually quite expensive. As you remember, the cost of an OR operation is proportional to the sum of the sizes of input lists. The lists of books published in a year are probably quite long, so the cost of merging two will be quite high. As a bonus, we get an even longer list as a result and this long list will take part in any computations that follow. So here are the takeaways:

• OR operations are costly,
• even more so when inputs are large document sets;
• subqueries (parentheses in the logical expression) cause temporary postings lists to be created, which then take part in further calculations and so their sizes affect query performance.

Looking further at our query, we see that even more temporary document ID lists will have to be created: one for each pair of parentheses. These results have to be computed, and since they are temporary partial results, they will have to be stored in memory since they cannot be retrieved from the index directly.

Also note that subqueries can hinder many optimizations search engines employ. I mentioned earlier that Lucene sorts postings lists by length when AND-ing them together. This can only work reliably if list lengths are known. For a postings list of a single word, its length is stored in the index and known exactly up-front. For a nested subexpression, however, the number of matches is not known before the subexpression is evaluated. But, Lucene needs the number of matches in order to prepare the optimal query plan. This leads to a chicken-and-egg problem which Lucene solves by estimating the size of subquery results list based on the sizes of its constituents. For example, it estimates the result size of a subquery with OR-s as the sum of its input sizes. Being just an estimate, this number may differ from the actual value, and thus cause suboptimal query performance further up the stack. Takeaway:

• subqueries are great at hindering global query optimizations.

Another reason why subqueries may negatively affect performance becomes apparent with queries such as (a AND b) AND (c AND d). Since AND is an associative operation, the expression above gives the same result as a AND b AND c AND d. In the version without parentheses, the optimization of sorting lists by size works globally since all inputs are at the same nesting level, potentially achieving better performance than the version with nested subexpressions which can only sort the lists within each pair of parentheses separately.

You may wonder why anyone would add these parentheses, but such constructs may arise naturally due to the way your code is structured if individual subqueries are built by separate methods or classes because they serve different business needs.

Looking at how long postings lists affect query performance, especially with OR operator, you can see one of the reasons for introducing stopwords into search configuration. Words such as the are very common and on one hand they introduce practically no meaning at all to the query (with rare exceptions), matching almost all documents anyway, and on the other, they could add immense computational cost.

Obviously, the longest postings list possible is the one containing all documents in the index. And indeed, pure negative queries such as “all documents but those with the word x” tend to be very expensive. Surprisingly, AND-ing the full set of documents (the result of a match_all query) with results of another query is very fast. This is because of a special optimization which uses the identity ALL AND a = a to simplify those queries so that the expensive computation can be completely avoided. This kind of query rewriting can transform a number of query patterns to queries with the same result but better performance characteristics. However, this only works for a set of rather simple cases: for example if you do not use match_all query, but create some other query which also happens to match all documents, this optimization will not be triggered. Complex query structure with subqueries can effectively disable such optimizations as well.

Thinking about indexing and index segments, you have to notice that merging partial results from each segment is an operation similar to OR-ing (though it additionally has to account for document removal and updates). This leads to the conclusion that having many segments hurts search performance, especially for popular keywords whose postings lists are large to start with. Indeed, this actually happens. Performance may vary significantly depending on the number of segments, and the optimum is having just a single segment in your index. In Elastic, you can use the force merge API to reduce the number of segments after indexing. I have actually worked with a product in which data was never indexed incrementally, but instead the whole index was rebuilt from scratch and force-merged to a single segment after each update. This was a relatively small index with high search traffic, so big gains in search performance (on the order of two times shorter response times) were a justifiable reason for this seemingly wasteful indexation process.

Complex queries in disguise

Some queries seem simple, but are actually very complex for the search engine to handle. One example is prefix queries such as cat* (which matches documents containing any words starting with cat). It turns out that unless you do something special, such a query is likely to be handled as an OR-query with all words matching the prefix: (cat OR catamaran OR catapult OR category OR ...). Keeping in mind that queries with the OR operator can be expensive, you see the risk: there may be lots and lots of words in the resulting expression, increasing the cost of merging their corresponding postings lists. In most datasets, a query such as a*, with probably thousands of individual postings lists, each containing millions of documents, can take ages to finish and even bring down the whole cluster.

Another type of query that looks simple at first glance but can (or rather, used to) be very costly is range searches in numeric and date fields. Let’s say you want to limit your query to only documents modified between 2020-01-01 and 2020-12-31. How costly could that be? The inverted index maps individual values to lists of document IDs. If each value in the index corresponds to a single day, and the documents are evenly spread throughout the year, there will be 366 lists to join with the OR operator. If the data is indexed with millisecond resolution, there will be many more, with performance becoming even worse.

Fortunately, these issues have been known for a long time, and there are a number of solutions in place. For text fields, you can enable prefix indexing which creates special structures in the index which contain merged postings lists so that they don’t have to be computed at query time. Range queries on numeric and date fields are now optimized by default in Elasticsearch by creating additional structures in the index as well, though with a particularly nasty data set, you might still be able to trigger some issues. Note that these solutions are space-time tradeoffs (speeding up searches at the cost of larger index), and as with any tradeoff, there is always some risk of shooting yourself in the foot. Also, new versions introduce new optimizations, so behavior may well change between ES versions. Interestingly, some preconceptions related to performance are very persistent (not only in the full-text search field), and you may run into people recommending optimizations which made sense ten years ago, but may be counterproductive now. For example, range searches have been efficient for ten years, and apart from extreme cases, you should not need to worry about them too much now.

As a side note, ES tries to protect you from yourself and by default disables some types of queries that are likely to be costly: you have to explicitly enable them if you know what you’re doing and want to use them.

Returning lots of search results

Elasticsearch indexes may be huge, often searching millions and billions of documents, but usually only a tiny fraction of these documents match each query’s criteria, and out of those, only a handful (10 or so) are returned to the end user. Increasing the number of documents returned is detrimental to search performance in many ways:

• Some algorithms, such as finding the top N results when sorting, have complexity which depends on N: they are faster if N is much smaller than the total number of matches, and become slower as N grows.
• Some operations a full-text search engine performs are proportional to the number of documents returned (linear complexity). As you remember, just finding matches is very fast since it uses inverted indices, but in order to actually return the documents’ contents, they have to be fetched from document store, and this operation scales linearly with the number of documents returned. So, if you fetch 100 documents instead of 10, this part takes around ten times longer. Same goes for highlighting query terms. The amount of data transferred over the network scales similarly.
• Aggregations such as grouping documents by a field’s value may also have linear complexity (the number of documents returned being the input size).

Also note that paging the results (e.g. retrieving 100 pages of 10 documents each instead of a single request asking for 1000 documents) helps only a little. The problem is that in order to find documents on positions 991-1000, Elastic has to find the complete list of results 1-1000 first, and only then take the last 10 items. This means the cost of fetching documents from storage is indeed proportional to 10, but the cost of performing set operations on postings lists and aggregations as well as memory usage is still proportional to 1000.

So, if you think you can have millions of documents in ES and can just retrieve them all (or some large subset) using a simple query, you may be in for a surprise. There are specialized APIs for such a use case, but they all have their limitations.

Assuming Elastic knows as much about your data as you do

Much of the discussion up to this point revolved around replacing boolean expressions with their equivalents that have different performance characteristics. Some of these transformations are always correct since both expressions can be proven equal by means of boolean algebra. However, sometimes two forms of a query are equivalent only within a specific data set. Despite many smart optimizations used by modern full-text search engines, by using knowledge about your dataset you can often achieve more in terms of increasing or decreasing search performance than by relying on mathematics alone.

A simple example of using this knowledge in practice is improving performance by removing subqueries which are redundant due to the nature of the data. Suppose your index contains both printed and online publications. Only online publications have a URL. By following the simplest logic, if you wanted to find an online publication by URL, you would issue a query such as type:online AND url:value. This will work, although you could query just url:value as well. However, it requires that you know something about your data, namely that only online publications have any value set in the url field. Obviously, this simplified query will be faster than the original.

Where you can’t avoid complex queries, you can still use your domain knowledge to improve or reduce performance. For example, since the cost of merge operations depends on sizes of inputs, knowing that a particular subquery is likely to return many or few results (query selectivity) and modifying the query in a way in which the sizes of consecutive intermediate results diminish faster may boost performance, while relying only on Elastic’s optimizations may result in sub-par performance.

Suppose (a real-world example) there is an index with two types of documents whose counts differ wildly: documents of type 1 make up 99% of the index while type 2 amounts to just 1% of all documents. Certain queries must be limited to just a single type. The obvious way to filter these results is to add a clause such as ... AND type:1 and ... AND type:2, correspondingly, but replacing the first one with ... AND NOT type:2 may be faster since the results list for type 2 is much shorter than for type 1. If the filters can be combined (e.g. by the user checking checkboxes in a GUI), and the user selects both types, meaning effectively no filtering by type, it is probably much more efficient to simply remove the filter from the query than to add a ... AND (type:1 OR type:2) clause.

As you may have already realized, not only boolean queries’ but also range queries’ performance may depend a lot on your data, for example on a field’s cardinality (the number of unique values). One of the more spectacular ways of shooting yourself in the foot is applying a pattern which normally helps performance, but in your particular case, due to a specific distribution of a field’s values, does just the opposite. Such situations may be very difficult to discover if you do not precisely track performance before and after each significant change. Sneakily placing such a pattern in your code can be a great way to end up with low performance difficult to explain.

For a real life example, consider the rule of thumb that if you don’t care about a subquery’s score, using filter subqueries within a bool query results in faster response times than using must subqueries since the former do not need to update matching documents’ scores. In our advertising system, we match ads in a way mostly consistent with the way we match organic results. We match ads by keywords, but we also take into account criteria such as delivery methods selected by the user. In the latter case, the fact that a sponsored offer is available with some delivery method only affects which offers match, but does not affect their scores. This is a perfect use case for filter queries. However, we also use function score query. Function score query allows us to combine a document’s score resulting from how well it matches our keywords with additional factors. Function score query accepts an embedded query — only documents matching this query have their scores modified. Symbolically, we could express it as our complete query being: function_score_query(keyword_subquery AND filters_subquery). At one point, I wanted to optimize the performance of this query, and following the abovementioned rule of thumb, thought that it would make sense to move filters_subquery outside of function_score_query since filters need not participate in score calculations. This resulted in the query filters_subquery AND function_score_query(keyword_subquery) and should have improved search performance. However, upon running performance tests, to my surprise I realized these changes actually made performance worse. The reason was, with the filters moved outside function_score_query, function_score_query had to modify the scores of a larger number of documents and for the particular data I had in my index, the added cost of rescoring more documents was greater than the savings achieved by not having to calculate the score for these documents in the first place. This just shows that with performance tuning, YMMV, always.

Treating search and indexing as two separate problems

You might be tempted to think of Elasticsearch as yet another database. If you do, you are likely to run into many issues, including performance problems. One of the main things that set ES apart from most databases, whether they be SQL or NoSQL, is the search-indexing asymmetry. In contrast to a normal database, in Elasticsearch you can’t just insert a bunch of documents: this process triggers indexing, creates new segments, potentially triggers segment merges, has to propagate replicas and handle consistency within the cluster, etc. This may all affect performance in interesting ways. While indices of some kind are used in pretty much all databases, in ES they play a central role. Another important difference to databases is that Elastic data model favors, and often forces, very much denormalized data. This is common with NoSQL databases but in ES, it is even more extreme.

In particular, since ES is — in most cases — more about search performance than anything else, it is a common optimization to move cost from search time to indexing time when possible, and many such optimizations result in an even more denormalized data model.

For example, let’s consider an index of offers such as you may find in an online store. Each offer may be available with a free return option, but there’s a catch: while the client only sees a single checkbox in the UI, internally there are several types of free returns, e.g. free return by package locker and free return by post. The natural way to handle this would be to index each of these two flags and then to search for offers having either of those flags. It would also be a step towards our goal of ruining Elasticsearch search performance, especially if the number of values was 200 rather than 2.

The reason it works this way is that there are lots and lots of offers matching any of these flags: probably around 90% match one and around 90% match the other (with, obviously, a large number matching both). Going back to the section about OR operator, you will notice that having two very long input lists is about the worst case for OR operator efficiency. A usually reasonable trade-off in such a case is to move the cost to indexing-time, and to index with the document just a single flag, “free return”, which will improve search performance (at the cost of reducing indexing performance by just a tiny amount). Note that this was a very simple case and sometimes indexing denormalized data may increase index size significantly, in which case the trade-off may become less obvious.

Another quirk is the mutual interaction between indexing and search performance. Interaction between reads and writes happens in practically any database, but with Elastic, it is easier for it to become an issue due to the relatively high CPU and I/O cost of indexing. Ignoring this fact and treating search and indexing performance as two independent issues is a recipe for poor performance in both areas.

Jumping right into optimization without checking first

One effective method of achieving inferior performance, which works not only with Elasticsearch, is jumping right into optimization without first analyzing the problem, and, even better, not checking if there is a problem at all. It is a boring thing to repeat over and over, but the only way to improve performance is to:

• first, measure the baseline you are starting from (avoiding common pitfalls along the way),
• decide whether the values are satisfactory or not,
• define target values if they are not, and
• systematically measure and improve until success or surrender.

Optimizing without measurement and without defining goals, on the other hand, is a good method of wasting your time, and consequently, achieving sub-par performance. While there are some simple improvements which amount to “don’t do stupid things” and can be applied practically always without any risk, most are trade-offs: you gain something at the expense of something else. If you apply them inappropriately, you may end up with expenses but without the gains. Many optimizations’ effectiveness varies a lot depending on the kind of data in the index or specific query patterns generated by your users, so, for example you may introduce an optimization whose effect is negligible, but whose cost (e.g. in increased complexity and thus maintenance cost) is significant.

Blindly trusting what you read on the web

This leads us to the last tip: if you really want to ruin your ES performance, always trust strangers on the internet and apply their advice duly and without hesitation. Obviously, this applies to this very post as well. Another good practice is to never check publication dates, or the ES versions that particular tips apply to.

Summary

I hope Part I gave you some background on how Elastic works under the hood. In Part II, we discussed various techniques which can affect its performance in real-world scenarios. Armed with this knowledge, you will be able to make or break Elasticsearch performance: the choice is yours.