Distributed searching

Manticore is designed to scale effectively through its distributed searching capabilities. Distributed searching is beneficial for improving query latency (i.e., search time) and throughput (i.e., max queries/sec) in multi-server, multi-CPU, or multi-core environments. This is crucial for applications that need to search through vast amounts of data (i.e., billions of records and terabytes of text).

The primary concept is to horizontally partition the searched data across search nodes and process it in parallel.

Partitioning is done manually. To set it up, you should:

  • Set up multiple instances of Manticore on different servers
  • Distribute different parts of your dataset to different instances
  • Configure a special distributed table on some of the searchd instances
  • Route your queries to the distributed table

This type of table only contains references to other local and remote tables - so it cannot be directly reindexed. Instead, you should reindex the tables that it references.

When Manticore receives a query against a distributed table, it performs the following steps:

  1. Connects to the configured remote agents
  2. Sends the query to them
  3. Simultaneously searches the configured local tables (while the remote agents are searching)
  4. Retrieves the search results from the remote agents
  5. Merges all the results together, removing duplicates
  6. Sends the merged results to the client

From the application's perspective, there are no differences between searching through a regular table or a distributed table. In other words, distributed tables are fully transparent to the application, and there's no way to tell whether the table you queried was distributed or local.

Learn more about remote nodes.


Multi-queries, or query batches, allow you to send multiple search queries to Manticore in a single network request.

👍 Why use multi-queries?

The primary reason is performance. By sending requests to Manticore in a batch instead of one by one, you save time by reducing network round-trips. Additionally, sending queries in a batch allows Manticore to perform certain internal optimizations. If no batch optimizations can be applied, queries will be processed individually.

⛔ When not to use multi-queries?

Multi-queries require all search queries in a batch to be independent, which isn't always the case. Sometimes query B depends on query A's results, meaning query B can only be set up after executing query A. For example, you might want to display results from a secondary index only if no results were found in the primary table, or you may want to specify an offset into the 2nd result set based on the number of matches in the 1st result set. In these cases, you'll need to use separate queries (or separate batches).

You can run multiple search queries with SQL by separating them with a semicolon. When Manticore receives a query formatted like this from a client, all inter-statement optimizations will be applied.

Multi-queries don't support queries with FACET. The number of multi-queries in one batch shouldn't exceed max_batch_queries.

  • SQL
SELECT id, price FROM products WHERE MATCH('remove hair') ORDER BY price DESC; SELECT id, price FROM products WHERE MATCH('remove hair') ORDER BY price ASC

Multi-queries optimizations

There are two major optimizations to be aware of: common query optimization and common subtree optimization.

Common query optimization means that searchd will identify all those queries in a batch where only the sorting and group-by settings differ, and only perform searching once. For example, if a batch consists of 3 queries, all of them are for "ipod nano", but the 1st query requests the top-10 results sorted by price, the 2nd query groups by vendor ID and requests the top-5 vendors sorted by rating, and the 3rd query requests the max price, full-text search for "ipod nano" will only be performed once, and its results will be reused to build 3 different result sets.

Faceted search is a particularly important case that benefits from this optimization. Indeed, faceted searching can be implemented by running several queries, one to retrieve search results themselves, and a few others with the same full-text query but different group-by settings to retrieve all the required groups of results (top-3 authors, top-5 vendors, etc). As long as the full-text query and filtering settings stay the same, common query optimization will trigger, and greatly improve performance.

Common subtree optimization is even more interesting. It allows searchd to exploit similarities between batched full-text queries. It identifies common full-text query parts (subtrees) in all queries and caches them between queries. For example, consider the following query batch:

donald trump president
donald trump barack obama john mccain
donald trump speech

There's a common two-word part donald trump that can be computed only once, then cached and shared across the queries. And common subtree optimization does just that. Per-query cache size is strictly controlled by subtree_docs_cache and subtree_hits_cache directives (so that caching all sixteen gazillions of documents that match "i am" does not exhaust the RAM and instantly kill your server).

How can you tell if the queries in the batch were actually optimized? If they were, the respective query log will have a "multiplier" field that specifies how many queries were processed together:

Note the "x3" field. It means that this query was optimized and processed in a sub-batch of 3 queries.

  • log
[Sun Jul 12 15:18:17.000 2009] 0.040 sec x3 [ext/0/rel 747541 (0,20)] [lj] the
[Sun Jul 12 15:18:17.000 2009] 0.040 sec x3 [ext/0/ext 747541 (0,20)] [lj] the
[Sun Jul 12 15:18:17.000 2009] 0.040 sec x3 [ext/0/ext 747541 (0,20)] [lj] the

For reference, this is how the regular log would look like if the queries were not batched:

  • log
[Sun Jul 12 15:18:17.062 2009] 0.059 sec [ext/0/rel 747541 (0,20)] [lj] the
[Sun Jul 12 15:18:17.156 2009] 0.091 sec [ext/0/ext 747541 (0,20)] [lj] the
[Sun Jul 12 15:18:17.250 2009] 0.092 sec [ext/0/ext 747541 (0,20)] [lj] the

Notice how the per-query time in the multi-query case improved by a factor of 1.5x to 2.3x, depending on the specific sorting mode.


Manticore supports SELECT subqueries via SQL in the following format:


The outer select allows only ORDER BY and LIMIT clauses. Sub-select queries currently have two use cases:

  1. When you have a query with two ranking UDFs, one very fast and the other slow, and perform a full-text search with a large match result set. Without subselect, the query would look like:

     SELECT id,slow_rank() as slow,fast_rank() as fast FROM index
         WHERE MATCH(‘some common query terms’) ORDER BY fast DESC, slow DESC LIMIT 20
         OPTION max_matches=1000;

    With sub-selects, the query can be rewritten as:

         (SELECT id,slow_rank() as slow,fast_rank() as fast FROM index WHERE
             MATCH(‘some common query terms’)
             ORDER BY fast DESC LIMIT 100 OPTION max_matches=1000)
     ORDER BY slow DESC LIMIT 20;

    In the initial query, the slow_rank() UDF is computed for the entire match result set. With SELECT sub-queries, only fast_rank() is computed for the entire match result set, while slow_rank() is computed for a limited set.

  2. The second case is useful for large result sets coming from a distributed table.

    For this query:

     SELECT * FROM my_dist_index WHERE some_conditions LIMIT 50000;

    If you have 20 nodes, each node can send back to the master a maximum of 50K records, resulting in 20 x 50K = 1M records. However, since the master sends back only 50K (out of 1M), it might be good enough for the nodes to send only the top 10K records. With sub-select, you can rewrite the query as:

          (SELECT * FROM my_dist_index WHERE some_conditions LIMIT 10000)
      ORDER by some_attr LIMIT 50000;

    In this case, the nodes receive only the inner query and execute it. This means the master will receive only 20x10K=200K records. The master will take all the records received, reorder them by the OUTER clause, and return the best 50K records. The sub-select helps reduce the traffic between the master and the nodes, as well as reduce the master's computation time (since it processes only 200K instead of 1M records).