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AvocadoDB query language (DRAFT!)
 

AvocadoDB query language (DRAFT!)

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The is the RFC for AvocadoDB's query language. AvocadoDB is an open source nosql database (see www.avocadodb.org) offering a mixture of data models like key value pairs, documents and graphs.

The is the RFC for AvocadoDB's query language. AvocadoDB is an open source nosql database (see www.avocadodb.org) offering a mixture of data models like key value pairs, documents and graphs.

The REST API for AvocadoDB is already available and stable and people are writing APIs using it. Awesome. As AvocacoDB offers more complex data structures like graphs and lists REST is not enough. We implemented a first version of a query language some time ago which is very similar to SQL and UNQL.

Then we realized that this approach was not completely satisfying as some queries cannot expressed very well with it, especially multi-valued attributes/lists. UNQL addresses this partly, but does not go far enough. Another issue are graphs. AvocadoDB supports querying graphs, neither SQL nor UNQL offer any "natural" graph traversal facilities.

As we did not find any existing query language that addresses the problems we found we had to define a new query language which is presented in the presentation.

Have some feedback on this? Come to www.avocadodb.org and tell us what you think about it. :-)

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    AvocadoDB query language (DRAFT!) AvocadoDB query language (DRAFT!) Presentation Transcript

    • AvocadoDB query language Jan Steemann (triAGENS) © 2012 triAGENS GmbH | 2012-04-13 1
    • Database query languages / paradigms There are many database query languages and paradigms around Some examples:  SQL declarative query language for relational databases, well-known and popular  UNQL declarative query language for document databases, SQL-syntax like, embeds JSON  graph query languages (Cypher, Gremlin, ...) declarative languages focusing on graph queries  fluent query languages/interfaces e.g. db.user.find(...).sort(...)  map/reduce imperative query formulation/programming  ... © 2012 triAGENS GmbH | 2012-04-13 2
    • AvocadoDB query language: status quo There is a query language in AvocadoDB The language syntax is very similar to SQL / UNQL The language currently supports reading data from collections (i.e. equivalent to an SQL/UNQL SELECT query) Some complex access patterns (e.g. joins using multiple collections) are also supported There are some specials as creating inline lists from a list of documents (named: LIST JOIN) © 2012 triAGENS GmbH | 2012-04-13 3
    • AvocadoDB query language: status quo There is a query language in AvocadoDB The language syntax is very similar to SQL / UNQL The language currently supports reading data from collections (i.e. equivalent to an SQL/UNQL SELECT query) Some complex access patterns (e.g. joins using multiple collections) are also supported There are some specials as creating inline lists from a list of documents (named: LIST JOIN) © 2012 triAGENS GmbH | 2012-04-13 4
    • AvocadoDB query language: status quo Syntax example: SELECT { "user": u, "friends": f } FROM users u LIST JOIN friends f ON (u.id == f.uid) WHERE u.type == 1 ORDER BY u.name © 2012 triAGENS GmbH | 2012-04-13 5
    • Language problems The current query language has the problem that some queries cannot be expressed very well with it This might be due to the query language being based on SQL, and SQL being a query language for relational databases AvocadoDB is mainly a document-oriented database and its object model does only partly overlap with the SQL object model: SQL (relational):  AvocadoDB (document-oriented):  tables  collections  (homogenous) rows  (inhomogenous) documents  columns  attributes  scalars  scalars  lists  references  edges © 2012 triAGENS GmbH | 2012-04-13 6
    • Language problems: multi-valued attributes Attributes in AvocadoDB can and shall be stored denormalised (multi-valued attributes, lists, ...): { "user": { "name": "Fred", "likes": [ "Fishing", "Hiking", "Swimming" ] } } In an SQL database, this storage model would be an anti-pattern Problem: SQL is not designed to access multi-valued attributes/lists but in AvocadoDB we want to support them via the language UNQL addresses this partly, but does not go far enough © 2012 triAGENS GmbH | 2012-04-13 7
    • Language problems: graph queries AvocadoDB also supports querying graphs Neither SQL nor UNQL offer any „natural“ graph traversal facilities Instead, there are:  SQL language extensions: e.g. CONNECT BY, proprietary  SQL stored procedures: e.g. PL/SQL imperative code, does not match well with the declarative nature of SQL Neither SQL nor UNQL are the languages of choice for graph queries, but we want to support graph queries in AvocadoDB © 2012 triAGENS GmbH | 2012-04-13 8
    • AvocadoDB query language, version 2 During the past few weeks we thought about moving AvocadoDBs query language from the current SQL-/ UNQL-based syntax to something else We did not find an existing query language that addresses the problems we had too well So we tried to define a syntax for a new query language © 2012 triAGENS GmbH | 2012-04-13 9
    • AvocadoDB query language, version 2 The new AvocadoDB query language should  have an easy-to-understand syntax for the end user  offer a way to declaratively express queries  avoid ASCII art queries  still allow more complex queries (joins, sub-queries etc.)  allow accessing lists and list elements more naturally  be usable with the different data models AvocadoDB supports (e.g. document-oriented, graph, „relational“)  be consistent and easy to process  have one syntax regardless of the underlying client language © 2012 triAGENS GmbH | 2012-04-13 10
    • AvocadoDB query language, version 2 A draft of the new language version is presented as follows It is not yet finalized and not yet implemented Your feedback on it is highly appreciated Slides will be uploaded to http://www.avocadodb.org/ © 2012 triAGENS GmbH | 2012-04-13 11
    • Data types The language has the following data types:  absence of a value: null  boolean truth values: false, true  numbers (signed double precision): 1, -34.24  strings, e.g. "John", "goes fishing"  lists (with elements accessible by their position), e.g. [ "one", "two", false, -1 ]  documents (with elements accessible by their name), e.g. { "user": { "name": "John", "age": 25 } } Note: names of document attributes can also be used without surrounding quotes © 2012 triAGENS GmbH | 2012-04-13 12
    • Bind parameters Queries can be parametrized using bind parameters This allows separation of query text and actual query values Any literal values, including lists and documents can be bound Collection names can also be bound Bind parameters can be accessed in the query using the @ prefix Example: @age u.name == @name u.state IN @states © 2012 triAGENS GmbH | 2012-04-13 13
    • Operators The language has the following operators:  logical: will return a boolean value or an error && || !  arithmetic: will return a numeric value or an error + - * / %  relational: will return a boolean value or an error == != < <= > >= IN  ternary: will return the true or the false part ? : String concatentation will be provided via a function © 2012 triAGENS GmbH | 2012-04-13 14
    • Type casts Typecasts can be achieved by explicitly calling typecast functions No implicit type cast will be performed Performing an operation with invalid/inappropriate types will result in an error When performing an operation that does not have a valid or defined result, the outcome will be an error: 1 / 0 => error 1 + "John" => error Errors might be caught and converted to null in a query or bubble up to the top, aborting the query. This depends on settings © 2012 triAGENS GmbH | 2012-04-13 15
    • Null When referring to something non-existing (e.g. a non-existing attribute of a document), the result will be null: users.nammme => null Using the comparison operators, null can be compared to other values and also null itself. The result will be a boolean (not null as in SQL) © 2012 triAGENS GmbH | 2012-04-13 16
    • Type comparisons When comparing two values, the following algorithm is used If the types of the compared values are not equal, the compare result is as follows: null < boolean < number < string < list < document Examples: null < false 0 != null false < 0 null != false true < 0 false != "" true < [ 0 ] "" != [ ] true < [ ] null != [ ] 0 < [ ] [ ] < { } © 2012 triAGENS GmbH | 2012-04-13 17
    • Type comparisons If the types are equal, the actual values are compared For boolean values, the order is: false < true For numeric values, the order is determined by the numeric value For string values, the order is determined by bytewise comparison of the strings characters Note: at some point, collations will need to be introduced for string comparisons © 2012 triAGENS GmbH | 2012-04-13 18
    • Type comparisons For list values, the elements from both lists are compared at each position. For each list element value, the described comparisons will be done recursively: [ 1 ] > [ 0 ] [ 2, 0 ] > [ 1, 2 ] [ 99, 4 ] > [ 99, 3 ] [ 23 ] > [ true ] [ [ 1 ] ] > 99 [ ] > 1 [ true ] > [ ] [ null ] > [ ] [ true, 0 ] > [ true ] © 2012 triAGENS GmbH | 2012-04-13 19
    • Type comparisons For document values, the attribute names from both documents are collected and sorted. The sorted attribute names are then checked individually: if one of the documents does not have the attribute, it will be considered „smaller“. If both documents have the attribute, a value comparison will be done recursively: { } < { "age": 25 } { "age": 25 } < { "age": 26 } { "age": 25 } > { "name": "John" } { "name": "John", == { "age": 25, "age": 25 } "name": "John" } { "age": 25 } < { "age": 25, "name": "John" } © 2012 triAGENS GmbH | 2012-04-13 20
    • Base building block: lists A good part of the query language is about processing lists There are several types of lists:  statically declared lists, e.g. [ { "user": { "name": "Fred" } }, { "user": { "name": "John" } } ]  lists of documents from collections, e.g.  users  locations  result lists from filters/queries, e.g.  NEAR(locations, [ 43, 10 ], 100) © 2012 triAGENS GmbH | 2012-04-13 21
    • FOR: List iteration The FOR keyword can be used to iterate over all elements from a list Example (collection-based, collection „users“): FOR u IN users A result document (named: u) is produced on each iteration The above example produces the following result list: [ u1, u2, u3, ..., un ] Note: this is comparable to the following SQL: SELECT * FROM users u In each iteration, the individual element is accessible via its name (u) © 2012 triAGENS GmbH | 2012-04-13 22
    • FOR: List iteration Nesting of multiple FOR blocks is possible Example: cross product of users and locations (u x l): FOR u IN users FOR l IN locations A result document containing both variables (u, l) is produced on each iteration of the inner loop The result document contains both u and l Note: this is equivalent to the following SQL queries: SELECT * FROM users u, locations l SELECT * FROM users u INNER JOIN locations l ON (1=1) © 2012 triAGENS GmbH | 2012-04-13 23
    • FOR: List iteration Example: cross product of years & quarters (non collection-based): FOR year IN [ 2011, 2012, 2013 ] FOR quarter IN [ 1, 2, 3, 4 ] Note: this is equivalent to the following SQL query: SELECT * FROM (SELECT 2011 UNION SELECT 2012 UNION SELECT 2013) year, (SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4) quarter © 2012 triAGENS GmbH | 2012-04-13 24
    • FILTER: results filtering The FILTER keyword can be used to restrict the results to elements that match some definable condition Example: retrieve all users that are active Access to the individual FOR list elements in FOR list u IN users using variable name u FILTER u.active == true Note: this is equivalent to the following SQL: SELECT * FROM users u WHERE u.active = true © 2012 triAGENS GmbH | 2012-04-13 25
    • FILTER: results filtering The FILTER keyword in combination with nested FOR blocks can be used to perform joins Example: retrieve all users that have matching locations FOR u IN users FOR Access to the individual l IN locations list elements using FILTER variable names u.a == l.b Note: this is equivalent to the following SQL queries: SELECT * FROM users u, locations l WHERE u.a == l.b SELECT * FROM users u (INNER) JOIN locations l ON u.a == l.b © 2012 triAGENS GmbH | 2012-04-13 26
    • Base building block: scopes The query language is scoped Variables can only be used after they have been declared Example: Introduces u FOR u IN users FOR Introduces l l IN locations FILTER u.a == l.b Can use both u and l Scopes can be made explicit using brackets (will be shown later) © 2012 triAGENS GmbH | 2012-04-13 27
    • FILTER: results filtering Thanks to scopes, the FILTER keyword can be used everywhere where SQL needs multiple keywords:  ON  WHERE  HAVING © 2012 triAGENS GmbH | 2012-04-13 28
    • FILTER: results filtering That means: in AvocadoDB you would use FILTER FOR u IN users FOR l IN locations FILTER u.a == l.b whereas in SQL you would use either ON SELECT * FROM users (INNER) JOIN locations l ON u.a == l.b or WHERE: SELECT * FROM users, locations l WHERE u.a == l.b © 2012 triAGENS GmbH | 2012-04-13 29
    • FILTER: results filtering FILTER can be used to model both an SQL ON and an SQL WHERE in one go: FOR u IN users FOR l IN locations FILTER u.active == 1 && u.a == l.b This is equivalent to the following SQL query: SELECT * FROM users u (INNER) JOIN locations l ON u.a == l.b WHERE u.active = 1 © 2012 triAGENS GmbH | 2012-04-13 30
    • FILTER: results filtering More than one FILTER condition allowed per query The following queries are all equivalent Optimizers job is to figure out best positions for applying FILTERs FOR FOR FOR u IN users u IN users u IN users FILTER FOR FOR u.c == 1 == l IN locations == l IN locations FOR FILTER FILTER l IN locations u.c == 1 && l.d == 2 && FILTER l.d == 2 && u.a == l.b l.d == 2 u.a == l.b FILTER FILTER u.c == 1 u.a == l.b © 2012 triAGENS GmbH | 2012-04-13 31
    • RETURN: results projection The RETURN keyword produces the end result documents from the intermediate results produced by the query Comparable to the SELECT part in an SQL query RETURN part is mandatory at the end of a query (and at the end of each subquery) RETURN is partly left out in this presentation for space reasons © 2012 triAGENS GmbH | 2012-04-13 32
    • RETURN: results projection Example: FOR u IN users RETURN { "name" : u.name, "likes" : u.likes, "numFriends": LENGTH(u.friends) } Produces such document for each u found © 2012 triAGENS GmbH | 2012-04-13 33
    • RETURN: results projection To return all documents as they are in the original list, there is the following variant: FOR u IN users RETURN u Would produce: [ { "name": "John", "age": 25 }, { "name": "Tina", "age": 29 }, ... ] Note: this is similar to SQLs SELECT u.* © 2012 triAGENS GmbH | 2012-04-13 34
    • RETURN: results projection To return just the names for all users, the following query would do: FOR u IN users RETURN u.name Would produce: [ "John", "Tina", ... ] Note: this is similar to SQLs SELECT u.name © 2012 triAGENS GmbH | 2012-04-13 35
    • RETURN: results projection To return a hierchical result (e.g. data from multiple collections), the following query could be used: FOR u IN users FOR l IN locations RETURN { "user": u, "location" : l } Would produce: [ { "user": { "name": "John", "age": 25 }, "location": { "x": 1, "y“: -1 } }, { "user": { "name": "Tina", "age": 29 }, "location": { "x": -2, "y": 3 } }, ... ] © 2012 triAGENS GmbH | 2012-04-13 36
    • RETURN: results projection To return a flat result from hierchical data (e.g. data from multiple collections), the MERGE() function can be employed: FOR u IN users FOR l IN locations RETURN MERGE(u, l) Would produce: [ { "name": "John", "age": 25, "x": 1, "y": -1 }, { "name": "Tina", "age": 29, "x": -2, "y": 3 }, ... ] © 2012 triAGENS GmbH | 2012-04-13 37
    • SORT: Sorting The SORT keyword will force a sort of the list of intermediate results according to one or multiple criteria Example (sort by first and last name first, then by id): FOR u IN users FOR l IN locations SORT u.first, u.last, l.id DESC This is very similar to ORDER BY in SQL © 2012 triAGENS GmbH | 2012-04-13 38
    • LIMIT: Result set slicing The LIMIT keyword allows slicing the list of result documents using an offset and a count Example for top 3 (offset = 0, count = 3): FOR u IN users SORT u.first, u.last LIMIT 0, 3 © 2012 triAGENS GmbH | 2012-04-13 39
    • LET: variable creation The LET keyword can be used to create a variable using data from a subexpression (e.g. a FOR expression) Example (will populate variable t with the result of the FOR): LET t = ( FOR explicit scope bounds u IN users ) This will populate t with [ u1, u2, u3, u4, ... un ] © 2012 triAGENS GmbH | 2012-04-13 40
    • LET: variable creation The results created using LET can be filtered afterwards using the FILTER keyword This is then similar to the behaviour of HAVING in SQL Example using a single collection (users): FOR Iterates over an attribute („friends“) of each u u IN users LET friends = ( FOR function to retrieve the f IN u.friends length of a list ) FILTER LENGTH(friends) > 5 © 2012 triAGENS GmbH | 2012-04-13 41
    • LET: variable creation Example using two collections (users, friends): FOR u IN users LET friends = ( FOR f IN friends FILTER u.id == f.uid ) FILTER LENGTH(friends) > 5 Differences to previous one collection example:  replaced f IN u.friends with just f IN friends  added inner filter condition © 2012 triAGENS GmbH | 2012-04-13 42
    • LET: variable creation SQL approach: SELECT u.*, GROUP_CONCAT(f.uid) AS friends FROM users u (INNER) JOIN friends f ON u.id = f.uid GROUP BY u.id HAVING COUNT(f.uid) > 5 Notes:  we are using 2 different tables now  the GROUP_CONCAT() aggregate function will create the friend list as a comma-separated string  need to use GROUP BY to aggregate  non-portable: GROUP_CONCAT is available in MySQL only © 2012 triAGENS GmbH | 2012-04-13 43
    • LET: variable creation More complex example (selecting users along with logins and group membership): FOR u IN users LET logins = ( for each user, all users FOR logins are put into l IN logins_2012 variable „logins“ FILTER u.id == l.uid for each user, all group ) memberships are put into variable „groups“ LET groups = ( FOR logins and groups are g IN group_memberships independent of each FILTER other u.id == g.uid ) RETURN { "user": u, "logins": logins, "groups": groups } © 2012 triAGENS GmbH | 2012-04-13 44
    • COLLECT: grouping The COLLECT keyword can be used to group a list by one or multiple group criteria Difference to SQL: in AvocadoDB COLLECT performs grouping, but no aggregation Aggregation can be performed later using LET or RETURN The result of COLLECT is a (grouped/hierarchical) list of documents, containing one document for each group This document contains the group criteria values The list of documents for the group can optionally be retrieved by using the INTO keyword © 2012 triAGENS GmbH | 2012-04-13 45
    • COLLECT: grouping Example: retrieve the users per city (non-aggregated): FOR u IN users group criterion COLLECT (name: „city“, value: u.city) city = u.city captures group values into INTO g variable g RETURN { "c": city, "u": g } g contains all group members Produces the following result: [ { "c": "cgn", "u": [ { "u": {..} }, { "u": {..} }, { "u": {..} } ] }, { "c": "ffm", "u": [ { "u": {..} }, { "u": {..} } ], { "c": "ddf", "u": [ { "u": {..} } ] } ] © 2012 triAGENS GmbH | 2012-04-13 46
    • COLLECT: grouping Example: retrieve the number of users per city (aggregated): FOR u IN users COLLECT city = u.city INTO g RETURN { "c": city, "numUsers": LENGTH(g) } Produces the following result: [ { "c": "cgn", "numUsers": 3 }, { "c": "ffm", "numUsers": 2 }, { "c": "ddf", "numUsers": 1 } ] © 2012 triAGENS GmbH | 2012-04-13 47
    • Aggregate functions Query language should provide some aggregate functions, e.g.  MIN()  MAX()  SUM()  LENGTH() Input to aggregate functions is a list of values to process. Example: [ { "user": { "type": 1, "rating": 1 } }, { "user": { "type": 1, "rating": 4 } }, { "user": { "type": 1, "rating": 3 } } ] Problem: how to access the „user.rating“ attribute of each value inside the aggregate function? © 2012 triAGENS GmbH | 2012-04-13 48
    • Aggregate functions Solution 1: use „access to all list members“ shortcut: FOR u IN [ { "user": { "type": 1, "rating": 1 } }, { "user": { "type": 1, "rating": 4 } }, { "user": { "type": 1, "rating": 3 } } ] COLLECT type = u.type g[*] will iterate over all elements INTO g in g and return each elements u.user.rating attribute RETURN { "type": type, "maxRating": MAX(g[*].u.user.rating) } © 2012 triAGENS GmbH | 2012-04-13 49
    • Aggregate functions Solution 2: use FOR sub-expression to iterate over group elements FOR u IN users capture group values COLLECT city = u.city g is a variable containing INTO g all group members RETURN { "c" : city, "numUsers" : LENGTH(g), "maxRating": MAX((FOR sub-expression to iterate over e IN g all elements in the group RETURN e.user.rating)) } © 2012 triAGENS GmbH | 2012-04-13 50
    • Unions and intersections Unions and intersections can be created by invoking functions on lists:  UNION(list1, list2)  INTERSECTION(list1, list2) There will not be special keywords as in SQL © 2012 triAGENS GmbH | 2012-04-13 51
    • Graph queries In AvocadoDB, relations between documents can be stored using graphs Graphs can be used to model tree structures, networks etc. Popular use cases:  find friends of friends  find similarities  find recommendations © 2012 triAGENS GmbH | 2012-04-13 52
    • Graph queries In AvocadoDB, a graph is composition of  vertices: the nodes in the graph  edges: the relations between nodes in the graph Vertices are stored as documents in regular collections Edges are stored as documents in special edge collections, with each edge having the following attributes:  _from id of linked vertex (incoming relation)  _to id of linked vertex (outgoing relation) Additionally, all document have an _id attribute The _id values are used for linking in the edges collections © 2012 triAGENS GmbH | 2012-04-13 53
    • Graph queries Task: find direct friends of users Data: users are related (friend relationships) to other users Example data (vertex collection „users“): [ { "_id": 123, "name": "John", "age": 25 }, { "_id": 456, "name": "Tina", "age": 29 }, { "_id": 235, "name": "Bob", "age": 15 }, { "_id": 675, "name": "Phil", "age": 12 } ] Example data (edge collection „relations“): [ { "_id": 1, "_from": 123, "_to": 456 }, { "_id": 2, "_from": 123, "_to": 235 }, { "_id": 3, "_from": 456, "_to": 123 }, { "_id": 4, "_from": 456, "_to": 235 }, { "_id": 5, "_from": 235, "_to": 456 }, { "_id": 6, "_from": 235, "_to": 675 } ] © 2012 triAGENS GmbH | 2012-04-13 54
    • Graph queries To traverse the graph, the PATHS function can be used It traverses a graphs edges defined in an edge collection and produces a list of paths found Each path object will have the following properties:  _from id of vertex the path started at  _to id of vertex the path ended with  _edges edges visited along the path  _vertices vertices visited along the path © 2012 triAGENS GmbH | 2012-04-13 55
    • Graph queries Example: edge collection: relations direction: OUTBOUND FOR max path length: 1 u IN users LET friends = ( path variable name: p FOR p IN PATHS(relations, OUTBOUND, 1) FILTER p._from == u._id only consider paths starting at ) the current user (using the users _id attribute) © 2012 triAGENS GmbH | 2012-04-13 56
    • Graph queries Produces: [ { "u": { "_id": 123, "name": "John", "age": 25 }, "p": [ { "_from": 123, "_to": 456, ... }, { "_from": 123, "_to": 235, ... } ] }, { "u": { "_id": 456, "name": "Tina", "age": 29 }, "p": [ { "_from": 456, "_to": 123, ... }, { "_from": 456, "_to": 235, ... } ] }, { "u": { "_id": 235, "name": "Bob", "age": 15}, "p": [ { "_from": 235, „_to": 456, ... }, { "_from": 235, „_to": 675, ... } ] }, { "u": { "_id": 675, "name": "Phil", "age": 12 }, "p": [ ] } ] Note: _edges and _vertices attributes for each p left out for space reasons © 2012 triAGENS GmbH | 2012-04-13 57
    • Summary: main keywords Keyword Use case FOR ... IN  List iteration FILTER  Results filtering RETURN  Results projection SORT  Sorting LIMIT  Results set slicing LET  Variable creation COLLECT ... INTO  Grouping © 2012 triAGENS GmbH | 2012-04-13 58
    • Q&A Your feedback on the draft is highly appreciated Please let us know what you think:  m.schoenert@triagens.de f.celler@triagens.de j.steemann@triagens.de #AvocadoDB And please try out AvocadoDB:  http://www.avocadodb.org/  https://github.com/triAGENS/AvocadoDB © 2012 triAGENS GmbH | 2012-04-13 59