cubicweb: comparison doc/tutorials/dataimport/data_import

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+Importing relational data into a CubicWeb instance
+==================================================
+Introduction
+~~~~~~~~~~~~
+This tutorial explains how to import data from an external source (e.g. a collection of files)
+into a CubicWeb cube instance.
+First, once we know the format of the data we wish to import, we devise a
+*data model*, that is, a CubicWeb (Yams) schema which reflects the way the data
+is structured. This schema is implemented in the ``schema.py`` file.
+In this tutorial, we will describe such a schema for a particular data set,
+the Diseasome data (see below).
+Once the schema is defined, we create a cube and an instance.
+The cube is a specification of an application, whereas an instance
+is the application per se.
+Once the schema is defined and the instance is created, the import can be performed, via
+the following steps:
+1. Build a custom parser for the data to be imported. Thus, one obtains a Python
+memory representation of the data.
+2. Map the parsed data to the data model defined in ``schema.py``.
+3. Perform the actual import of the data. This comes down to "populating"
+the data model with the memory representation obtained at 1, according to
+the mapping defined at 2.
+This tutorial illustrates all the above steps in the context of relational data
+stored in the RDF format.
+More specifically, we describe the import of Diseasome_ RDF/OWL data.
+.. _Diseasome: http://datahub.io/dataset/fu-berlin-diseasome
+Building a data model
+~~~~~~~~~~~~~~~~~~~~~
+The first thing to do when using CubicWeb for creating an application from scratch
+is to devise a *data model*, that is, a relational representation of the problem to be
+modeled or of the structure of the data to be imported.
+In such a schema, we define
+an entity type (``EntityType`` objects) for each type of entity to import. Each such type
+has several attributes. If the attributes are of known CubicWeb (Yams) types, viz. numbers,
+strings or characters, then they are defined as attributes, as e.g. ``attribute = Int()``
+for an attribute named ``attribute`` which is an integer.
+Each such type also has a set of
+relations, which are defined like the attributes, except that they represent, in fact,
+relations between the entities of the type under discussion and the objects of a type which
+is specified in the relation definition.
+For example, for the Diseasome data, we have two types of entities, genes and diseases.
+Thus, we create two classes which inherit from ``EntityType``::
+class Disease(EntityType):
+# Corresponds to http://www.w3.org/2000/01/rdf-schema#label
+label = String(maxsize=512, fulltextindexed=True)
+...
+#Corresponds to http://www4.wiwiss.fu-berlin.de/diseasome/resource/diseasome/associatedGene
+associated_genes = SubjectRelation('Gene', cardinality='**')
+...
+#Corresponds to 'http://www4.wiwiss.fu-berlin.de/diseasome/resource/diseasome/chromosomalLocation'
+chromosomal_location = SubjectRelation('ExternalUri', cardinality='?*', inlined=True)
+class Gene(EntityType):
+...
+In this schema, there are attributes whose values are numbers or strings. Thus, they are
+defined by using the CubicWeb / Yams primitive types, e.g., ``label = String(maxsize=12)``.
+These types can have several constraints or attributes, such as ``maxsize``.
+There are also relations, either between the entity types themselves, or between them
+and a CubicWeb type, ``ExternalUri``. The latter defines a class of URI objects in
+CubicWeb. For instance, the ``chromosomal_location`` attribute is a relation between
+a ``Disease`` entity and an ``ExternalUri`` entity. The relation is marked by the CubicWeb /
+Yams ``SubjectRelation`` method. The latter can have several optional keyword arguments, such as
+``cardinality`` which specifies the number of subjects and objects related by the relation type
+specified. For example, the ``'?*'`` cardinality in the ``chromosomal_relation`` relation type says
+that zero or more ``Disease`` entities are related to zero or one ``ExternalUri`` entities.
+In other words, a ``Disease`` entity is related to at most one ``ExternalUri`` entity via the
+``chromosomal_location`` relation type, and that we can have zero or more ``Disease`` entities in the
+data base.
+For a relation between the entity types themselves, the ``associated_genes`` between a ``Disease``
+entity and a ``Gene`` entity is defined, so that any number of ``Gene`` entities can be associated
+to a ``Disease``, and there can be any number of ``Disease`` s if a ``Gene`` exists.
+Of course, before being able to use the CubicWeb / Yams built-in objects, we need to import them::
+from yams.buildobjs import EntityType, SubjectRelation, String, Int
+from cubicweb.schemas.base import ExternalUri
+Building a custom data parser
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The data we wish to import is structured in the RDF format,
+as a text file containing a set of lines.
+On each line, there are three fields.
+The first two fields are URIs ("Universal Resource Identifiers").
+The third field is either an URI or a string. Each field bares a particular meaning:
+- the leftmost field is an URI that holds the entity to be imported.
+Note that the entities defined in the data model (i.e., in ``schema.py``) should
+correspond to the entities whose URIs are specified in the import file.
+- the middle field is an URI that holds a relation whose subject is the  entity
+defined by the leftmost field. Note that this should also correspond
+to the definitions in the data model.
+- the rightmost field is either an URI or a string. When this field is an URI,
+it gives the object of the relation defined by the middle field.
+When the rightmost field is a string, the middle field is interpreted as an attribute
+of the subject (introduced by the leftmost field) and the rightmost field is
+interpreted as the value of the attribute.
+Note however that some attributes (i.e. relations whose objects are strings)
+have their objects defined as strings followed by ``^^`` and by another URI;
+we ignore this part.
+Let us show some examples:
+- of line holding an attribute definition:
+``<http://www4.wiwiss.fu-berlin.de/diseasome/resource/genes/CYP17A1>
+<http://www.w3.org/2000/01/rdf-schema#label> "CYP17A1" .``
+The line contains the definition of the ``label`` attribute of an
+entity of type ``gene``. The value of ``label`` is '``CYP17A1``'.
+- of line holding a relation definition:
+``<http://www4.wiwiss.fu-berlin.de/diseasome/resource/diseases/1>
+<http://www4.wiwiss.fu-berlin.de/diseasome/resource/diseasome/associatedGene>
+<http://www4.wiwiss.fu-berlin.de/diseasome/resource/genes/HADH2> .``
+The line contains the definition of the ``associatedGene`` relation between
+a ``disease`` subject entity identified by ``1`` and a ``gene`` object
+entity defined by ``HADH2``.
+Thus, for parsing the data, we can (:note: see the ``diseasome_parser`` module):
+1. define a couple of regular expressions for parsing the two kinds of lines,
+``RE_ATTS`` for parsing the attribute definitions, and ``RE_RELS`` for parsing
+the relation definitions.
+2. define a function that iterates through the lines of the file and retrieves
+(``yield`` s) a (subject, relation, object) tuple for each line.
+We called it ``_retrieve_structure`` in the ``diseasome_parser`` module.
+The function needs the file name and the types for which information
+should be retrieved.
+Alternatively, instead of hand-making the parser, one could use the RDF parser provided
+in the ``dataio`` cube.
+.. XXX To further study and detail the ``dataio`` cube usage.
+Once we get to have the (subject, relation, object) triples, we need to map them into
+the data model.
+Mapping the data to the schema
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+In the case of diseasome data, we can just define two dictionaries for mapping
+the names of the relations as extracted by the parser, to the names of the relations
+as defined in the ``schema.py`` data model. In the ``diseasome_parser`` module
+they are called ``MAPPING_ATTS`` and ``MAPPING_RELS``.
+Given that the relation and attribute names are given in CamelCase in the original data,
+mappings are necessary if we follow the PEP08 when naming the attributes in the data model.
+For example, the RDF relation ``chromosomalLocation`` is mapped into the schema relation
+``chromosomal_location``.
+Once these mappings have been defined, we just iterate over the (subject, relation, object)
+tuples provided by the parser and we extract the entities, with their attributes and relations.
+For each entity, we thus have a dictionary with two keys, ``attributes`` and ``relations``.
+The value associated to the ``attributes`` key is a dictionary containing (attribute: value)
+pairs, where "value" is a string, plus the ``cwuri`` key / attribute holding the URI of
+the entity itself.
+The value associated to the ``relations`` key is a dictionary containing (relation: value)
+pairs, where "value" is an URI.
+This is implemented in the ``entities_from_rdf`` interface function of the module
+``diseasome_parser``. This function provides an iterator on the dictionaries containing
+the ``attributes`` and ``relations`` keys for all entities.
+However, this is a simple case. In real life, things can get much more complicated, and the
+mapping can be far from trivial, especially when several data sources (which can follow
+different formatting and even structuring conventions) must be mapped into the same data model.
+Importing the data
+~~~~~~~~~~~~~~~~~~
+The data import code should be placed in a Python module. Let us call it
+``diseasome_import.py``. Then, this module should be called via
+``cubicweb-ctl``, as follows::
+cubicweb-ctl shell diseasome_import.py -- <other arguments e.g. data file>
+In the import module, we should use a *store* for doing the import.
+A store is an object which provides three kinds of methods for
+importing data:
+- a method for importing the entities, along with the values
+of their attributes.
+- a method for importing the relations between the entities.
+- a method for committing the imports to the database.
+In CubicWeb, we have four stores:
+1. ``ObjectStore`` base class for the stores in CubicWeb.
+It only provides a skeleton for all other stores and
+provides the means for creating the memory structures
+(dictionaries) that hold the entities and the relations
+between them.
+2. ``RQLObjectStore``: store which uses the RQL language for performing
+database insertions and updates. It relies on all the CubicWeb hooks
+machinery, especially for dealing with security issues (database access
+permissions).
+2. ``NoHookRQLObjectStore``: store which uses the RQL language for
+performing database insertions and updates, but for which
+all hooks are deactivated. This implies that
+certain checks with respect to the CubicWeb / Yams schema
+(data model) are not performed. However, all SQL queries
+obtained from the RQL ones are executed in a sequential
+manner, one query per inserted entity.
+4. ``SQLGenObjectStore``: store which uses the SQL language directly.
+It inserts entities either sequentially, by executing an SQL query
+for each entity, or directly by using one PostGRES ``COPY FROM``
+query for a set of similarly structured entities.
+For really massive imports (millions or billions of entities), there
+is a cube ``dataio`` which contains another store, called
+``MassiveObjectStore``. This store is similar to ``SQLGenObjectStore``,
+except that anything related to CubicWeb is bypassed. That is, even the
+CubicWeb EID entity identifiers are not handled. This store is the fastest,
+but has a slightly different API from the other four stores mentioned above.
+Moreover, it has an important limitation, in that it doesn't insert inlined [#]_
+relations in the database.
+.. [#] An inlined relation is a relation defined in the schema
+with the keyword argument ``inlined=True``. Such a relation
+is inserted in the database as an attribute of the entity
+whose subject it is.
+In the following section we will see how to import data by using the stores
+in CubicWeb's ``dataimport`` module.
+Using the stores in ``dataimport``
+++++++++++++++++++++++++++++++++++
+``ObjectStore`` is seldom used in real life for importing data, since it is
+only the base store for the other stores and it doesn't perform an actual
+import of the data. Nevertheless, the other three stores, which import data,
+are based on ``ObjectStore`` and provide the same API.
+All three stores ``RQLObjectStore``, ``NoHookRQLObjectStore`` and
+``SQLGenObjectStore`` provide exactly the same API for importing data, that is
+entities and relations, in an SQL database.
+Before using a store, one must import the ``dataimport`` module and then initialize
+the store, with the current ``session`` as a parameter::
+import cubicweb.dataimport as cwdi
+...
+store = cwdi.RQLObjectStore(session)
+Each such store provides three methods for data import:
+#. ``create_entity(Etype, **attributes)``, which allows us to add
+an entity of the Yams type ``Etype`` to the database. This entity's attributes
+are specified in the ``attributes`` dictionary. The method returns the entity
+created in the database. For example, we add two entities,
+a person, of ``Person`` type, and a location, of ``Location`` type::
+person = store.create_entity('Person', name='Toto', age='18', height='190')
+location = store.create_entity('Location', town='Paris', arrondissement='13')
+#. ``relate(subject_eid, r_type, object_eid)``, which allows us to add a relation
+of the Yams type ``r_type`` to the database. The relation's subject is an entity
+whose EID is ``subject_eid``; its object is another entity, whose EID is
+``object_eid``.  For example [#]_::
+store.relate(person.eid(), 'lives_in', location.eid(), **kwargs)
+``kwargs`` is only used by the ``SQLGenObjectStore``'s ``relate`` method and is here
+to allow us to specify the type of the subject of the relation, when the relation is
+defined as inlined in the schema.
+.. [#] The ``eid`` method of an entity defined via ``create_entity`` returns
+the entity identifier as assigned by CubicWeb when creating the entity.
+This only works for entities defined via the stores in the CubicWeb's
+``dataimport`` module.
+The keyword argument that is understood by ``SQLGenObjectStore`` is called
+``subjtype`` and holds the type of the subject entity. For the example considered here,
+this comes to having [#]_::
+store.relate(person.eid(), 'lives_in', location.eid(), subjtype=person.dc_type())
+If ``subjtype`` is not specified, then the store tries to infer the type of the subject.
+However, this doesn't always work, e.g. when there are several possible subject types
+for a given relation type.
+.. [#] The ``dc_type`` method of an entity defined via ``create_entity`` returns
+the type of the entity just created. This only works for entities defined via
+the stores in the CubicWeb's ``dataimport`` module. In the example considered
+here, ``person.dc_type()`` returns ``'Person'``.
+All the other stores but ``SQLGenObjectStore`` ignore the ``kwargs`` parameters.
+#. ``flush()``, which allows us to perform the actual commit into the database, along
+with some cleanup operations. Ideally, this method should be called as often as
+possible, that is after each insertion in the database, so that database sessions
+are kept as atomic as possible. In practice, we usually call this method twice:
+first, after all the entities have been created, second, after all relations have
+been created.
+Note however that before each commit the database insertions
+have to be consistent with the schema. Thus, if, for instance,
+an entity has an attribute defined through a relation (viz.
+a ``SubjectRelation``) with a ``"1"`` or ``"+"`` object
+cardinality, we have to create the entity under discussion,
+the object entity of the relation under discussion, and the
+relation itself, before committing the additions to the database.
+The ``flush`` method is simply called as::
+store.flush().
+Using the ``MassiveObjectStore`` in the ``dataio`` cube
++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+This store, available in the ``dataio`` cube, allows us to
+fully dispense with the CubicWeb import mechanisms and hence
+to interact directly with the database server, via SQL queries.
+Moreover, these queries rely on PostGreSQL's ``COPY FROM`` instruction
+to create several entities in a single query. This brings tremendous
+performance improvements with respect to the RQL-based data insertion
+procedures.
+However, the API of this store is slightly different from the API of
+the stores in CubicWeb's ``dataimport`` module.
+Before using the store, one has to import the ``dataio`` cube's
+``dataimport`` module, then initialize the store by giving it the
+``session`` parameter::
+from cubes.dataio import dataimport as mcwdi
+...
+store = mcwdi.MassiveObjectStore(session)
+The ``MassiveObjectStore`` provides six methods for inserting data
+into the database:
+#. ``init_rtype_table(SubjEtype, r_type, ObjEtype)``, which specifies the
+creation of the tables associated to the relation types in the database.
+Each such table has three column, the type of the subject entity, the
+type of the relation (that is, the name of the attribute in the subject
+entity which is defined via the relation), and the type of the object
+entity. For example::
+store.init_rtype_table('Person', 'lives_in', 'Location')
+Please note that these tables can be created before the entities, since
+they only specify their types, not their unique identifiers.
+#. ``create_entity(Etype, **attributes)``, which allows us to add new entities,
+whose attributes are given in the ``attributes`` dictionary.
+Please note however that, by default, this method does *not* return
+the created entity. The method is called, for example, as in::
+store.create_entity('Person', name='Toto', age='18', height='190',
+uri='http://link/to/person/toto_18_190')
+store.create_entity('Location', town='Paris', arrondissement='13',
+uri='http://link/to/location/paris_13')
+In order to be able to link these entities via the relations when needed,
+we must provide ourselves a means for uniquely identifying the entities.
+In general, this is done via URIs, stored in attributes like ``uri`` or
+``cwuri``. The name of the attribute is irrelevant as long as its value is
+unique for each entity.
+#. ``relate_by_iid(subject_iid, r_type, object_iid)`` allows us to actually
+relate the entities uniquely identified by ``subject_iid`` and
+``object_iid`` via a relation of type ``r_type``. For example::
+store.relate_by_iid('http://link/to/person/toto_18_190',
+'lives_in',
+'http://link/to/location/paris_13')
+Please note that this method does *not* work for inlined relations!
+#. ``convert_relations(SubjEtype, r_type, ObjEtype, subj_iid_attribute,
+obj_iid_attribute)``
+allows us to actually insert
+the relations in the database. At one call of this method, one inserts
+all the relations of type ``rtype`` between entities of given types.
+``subj_iid_attribute`` and ``object_iid_attribute`` are the names
+of the attributes which store the unique identifiers of the entities,
+as assigned by the user. These names can be identical, as long as
+their values are unique. For example, for inserting all relations
+of type ``lives_in`` between ``People`` and ``Location`` entities,
+we write::
+store.convert_relations('Person', 'lives_in', 'Location', 'uri', 'uri')
+#. ``flush()`` performs the actual commit in the database. It only needs
+to be called after ``create_entity`` and ``relate_by_iid`` calls.
+Please note that ``relate_by_iid`` does *not* perform insertions into
+the database, hence calling ``flush()`` for it would have no effect.
+#. ``cleanup()`` performs database cleanups, by removing temporary tables.
+It should only be called at the end of the import.
+.. XXX to add smth on the store's parameter initialization.
+Application to the Diseasome data
++++++++++++++++++++++++++++++++++
+Import setup
+############
+We define an import function, ``diseasome_import``, which does basically four things:
+#. creates and initializes the store to be used, via a line such as::
+store = cwdi.SQLGenObjectStore(session)
+where ``cwdi`` is the imported ``cubicweb.dataimport`` or
+``cubes.dataio.dataimport``.
+#. calls the diseasome parser, that is, the ``entities_from_rdf`` function in the
+``diseasome_parser`` module and iterates on its result, in a line such as::
+for entity, relations in parser.entities_from_rdf(filename, ('gene', 'disease')):
+where ``parser`` is the imported ``diseasome_parser`` module, and ``filename`` is the
+name of the file containing the data (with its path), e.g. ``../data/diseasome_dump.nt``.
+#. creates the entities to be inserted in the database; for Diseasome, there are two
+kinds of entities:
+#. entities defined in the data model, viz. ``Gene`` and ``Disease`` in our case.
+#. entities which are built in CubicWeb / Yams, viz. ``ExternalUri`` which define
+URIs.
+As we are working with RDF data, each entity is defined through a series of URIs. Hence,
+each "relational attribute" [#]_ of an entity is defined via an URI, that is, in CubicWeb
+terms, via an ``ExternalUri`` entity. The entities are created, in the loop presented above,
+as such::
+ent = store.create_entity(etype, **entity)
+where ``etype`` is the appropriate entity type, either ``Gene`` or ``Disease``.
+.. [#] By "relational attribute" we denote an attribute (of an entity) which
+is defined through a relation, e.g. the ``chromosomal_location`` attribute
+of ``Disease`` entities, which is defined through a relation between a
+``Disease`` and an ``ExternalUri``.
+The ``ExternalUri`` entities are as many as URIs in the data file. For them, we define a unique
+attribute, ``uri``, which holds the URI under discussion::
+extu = store.create_entity('ExternalUri', uri="http://path/of/the/uri")
+#. creates the relations between the entities. We have relations between:
+#. entities defined in the schema, e.g. between ``Disease`` and ``Gene``
+entities, such as the ``associated_genes`` relation defined for
+``Disease`` entities.
+#. entities defined in the schema and ``ExternalUri`` entities, such as ``gene_id``.
+The way relations are added to the database depends on the store:
+- for the stores in the CubicWeb ``dataimport`` module, we only use
+``store.relate``, in
+another loop, on the relations (that is, a
+loop inside the preceding one, mentioned at step 2)::
+for rtype, rels in relations.iteritems():
+...
+store.relate(ent.eid(), rtype, extu.eid(), **kwargs)
+where ``kwargs`` is a dictionary designed to accommodate the need for specifying
+the type of the subject entity of the relation, when the relation is inlined and
+``SQLGenObjectStore`` is used. For example::
+...
+store.relate(ent.eid(), 'chromosomal_location', extu.eid(), subjtype='Disease')
+- for the ``MassiveObjectStore`` in the ``dataio`` cube's ``dataimport`` module,
+the relations are created in three steps:
+#. first, a table is created for each relation type, as in::
+...
+store.init_rtype_table(ent.dc_type(), rtype, extu.dc_type())
+which comes down to lines such as::
+store.init_rtype_table('Disease', 'associated_genes', 'Gene')
+store.init_rtype_table('Gene', 'gene_id', 'ExternalUri')
+#. second, the URI of each entity will be used as its identifier, in the
+``relate_by_iid`` method, such as::
+disease_uri = 'http://www4.wiwiss.fu-berlin.de/diseasome/resource/diseases/3'
+gene_uri = '<http://www4.wiwiss.fu-berlin.de/diseasome/resource/genes/HSD3B2'
+store.relate_by_iid(disease_uri, 'associated_genes', gene_uri)
+#. third, the relations for each relation type will be added to the database,
+via the ``convert_relations`` method, such as in::
+store.convert_relations('Disease', 'associated_genes', 'Gene', 'cwuri', 'cwuri')
+and::
+store.convert_relations('Gene', 'hgnc_id', 'ExternalUri', 'cwuri', 'uri')
+where ``cwuri`` and ``uri`` are the attributes which store the URIs of the entities
+defined in the data model, and of the ``ExternalUri`` entities, respectively.
+#. flushes all relations and entities::
+store.flush()
+which performs the actual commit of the inserted entities and relations in the database.
+If the ``MassiveObjectStore`` is used, then a cleanup of temporary SQL tables should be performed
+at the end of the import::
+store.cleanup()
+Timing benchmarks
+#################
+In order to time the import script, we just decorate the import function with the ``timed``
+decorator::
+from logilab.common.decorators import timed
+...
+@timed
+def diseasome_import(session, filename):
+...
+After running the import function as shown in the "Importing the data" section, we obtain two time measurements::
+diseasome_import clock: ... / time: ...
+Here, the meanings of these measurements are [#]_:
+- ``clock`` is the time spent by CubicWeb, on the server side (i.e. hooks and data pre- / post-processing on SQL
+queries),
+- ``time`` is the sum between ``clock`` and the time spent in PostGreSQL.
+.. [#] The meanings of the ``clock`` and ``time`` measurements, when using the ``@timed``
+decorators, were taken from `a blog post on massive data import in CubicWeb`_.
+.. _a blog post on massive data import in CubicWeb: http://www.cubicweb.org/blogentry/2116712
+The import function is put in an import module, named ``diseasome_import`` here. The module is called
+directly from the CubicWeb shell, as follows::
+cubicweb-ctl shell diseasome_instance diseasome_import.py \
+-- -df diseasome_import_file.nt -st StoreName
+The module accepts two arguments:
+- the data file, introduced by ``-df [--datafile]``, and
+- the store, introduced by ``-st [--store]``.
+The timings (in seconds) for different stores are given in the following table, for
+importing 4213 ``Disease`` entities and 3919 ``Gene`` entities with the import module
+just described:
++--------------------------+------------------------+--------------------------------+------------+
+| Store                    | CubicWeb time (clock)  | PostGreSQL time (time - clock) | Total time |
++==========================+========================+================================+============+
+| ``RQLObjectStore``       | 225.98                 | 62.05                          | 288.03     |
++--------------------------+------------------------+--------------------------------+------------+
+| ``NoHookRQLObjectStore`` | 62.73                  | 51.38                          | 114.11     |
++--------------------------+------------------------+--------------------------------+------------+
+| ``SQLGenObjectStore``    | 20.41                  | 11.03                          | 31.44      |
++--------------------------+------------------------+--------------------------------+------------+
+| ``MassiveObjectStore``   | 4.84                   | 6.93                           | 11.77      |
++--------------------------+------------------------+--------------------------------+------------+
+Conclusions
+~~~~~~~~~~~
+In this tutorial we have seen how to import data in a CubicWeb application instance. We have first seen how to
+create a schema, then how to create a parser of the data and a mapping of the data to the schema.
+Finally, we have seen four ways of importing data into CubicWeb.
+Three of those are integrated into CubicWeb, namely the ``RQLObjectStore``, ``NoHookRQLObjectStore`` and
+``SQLGenObjectStore`` stores, which have a common API:
+- ``RQLObjectStore`` is by far the slowest, especially its time spent on the
+CubicWeb side, and so it should be used only for small amounts of
+"sensitive" data (i.e. where security is a concern).
+- ``NoHookRQLObjectStore`` slashes by almost four the time spent on the CubicWeb side,
+but is also quite slow; on the PostGres side it is as slow as the previous store.
+It should be used for data where security is not a concern,
+but consistency (with the data model) is.
+- ``SQLGenObjectStore`` slashes by three the time spent on the CubicWeb side and by five the time
+spent on the PostGreSQL side. It should be used for relatively great amounts of data, where
+security and data consistency are not a concern. Compared to the previous store, it has the
+disadvantage that, for inlined relations, we must specify their subjects' types.
+For really huge amounts of data there is a fourth store, ``MassiveObjectStore``, available
+from the ``dataio`` cube. It provides a blazing performance with respect to all other stores:
+it is almost 25 times faster than ``RQLObjectStore`` and almost three times faster than
+``SQLGenObjectStore``. However, it has a few usage caveats that should be taken into account:
+#. it cannot insert relations defined as inlined in the schema,
+#. no security or consistency check is performed on the data,
+#. its API is slightly different from the other stores.
+Hence, this store should be used when security and data consistency are not a concern,
+and there are no inlined relations in the schema.

changeset 8836	8a57802d40d3
child 8927	885dea8f16a0