Add default values docs (#4371)

Resolves #3520 #3617

Co-authored-by: Andrey Aksenov <38073144+andreyaksenov@users.noreply.github.com>

Author: p7nov

doc/code_snippets/test/default_values/default_functions_test.lua

@@ -0,0 +1,65 @@
+local fio = require('fio')
+local server = require('luatest.server')
+local t = require('luatest')
+local g = t.group()
+g.before_each(function(cg)
+    cg.server = server:new {
+        box_cfg = {},
+        workdir = fio.cwd() .. '/tmp'
+    }
+    cg.server:start()
+end)
+
+g.after_each(function(cg)
+    cg.server:drop()
+    fio.rmtree(cg.server.workdir)
+end)
+
+g.test_default_func = function(cg)
+    cg.server:exec(function()
+
+        -- create_no_arg_function_start
+        box.schema.func.create('current_year', {
+            language = 'Lua',
+            body = "function() return require('datetime').now().year end"
+        })
+        -- create_no_arg_function_end
+
+        -- format_space_default_func_start
+        local books = box.schema.space.create('books')
+        books:format({
+            { name = 'id', type = 'unsigned' },
+            { name = 'isbn', type = 'string' },
+            { name = 'title', type = 'string' },
+            { name = 'year', type = 'unsigned', default_func = 'current_year' }
+        })
+        books:create_index('primary', { parts = { 1 } })
+        -- format_space_default_func_end
+
+        -- insert_ok_start
+        books:insert { 1, '978000', 'Thinking in Java' }
+        -- insert_ok_end
+
+        -- create_arg_function_start
+        box.schema.func.create('randomize', {
+            language = 'Lua',
+            body = "function(limit) return math.random(limit.min, limit.max) end"
+        })
+        -- create_arg_function_end
+
+        -- reformat_space_start
+        books:format({
+            { name = 'id', type = 'unsigned', default_func= 'randomize', default = {min = 0, max = 1000} },
+            { name = 'isbn', type = 'string' },
+            { name = 'title', type = 'string' },
+            { name = 'year', type = 'unsigned', default_func = 'current_year' }
+        })
+        -- reformat_space_end
+
+        books:insert { nil, '978001', 'How to code in Go' }
+
+        -- Tests --
+        t.assert_equals(books:count(), 2)
+        t.assert_equals(books:get(1), { 1, '978000','Thinking in Java', 2024 })
+    end)
+end

doc/code_snippets/test/default_values/explicit_default_test.lua

@@ -0,0 +1,39 @@
+local fio = require('fio')
+local server = require('luatest.server')
+local t = require('luatest')
+local g = t.group()
+g.before_each(function(cg)
+    cg.server = server:new {
+        box_cfg = {},
+        workdir = fio.cwd() .. '/tmp'
+    }
+    cg.server:start()
+end)
+
+g.after_each(function(cg)
+    cg.server:drop()
+    fio.rmtree(cg.server.workdir)
+end)
+
+g.test_default_values = function(cg)
+    cg.server:exec(function()
+        -- configure_space_start
+        local books = box.schema.space.create('books')
+        books:format({
+            { name = 'id', type = 'number' },
+            { name = 'name', type = 'string' },
+            { name = 'year', type = 'number', default = 2024 },
+        })
+        books:create_index('primary', { parts = { 1 } })
+        -- configure_space_end
+
+        -- insert_ok_start
+        books:insert { 1, 'Thinking in Java' }
+        books:insert { 2, 'How to code in Go', nil }
+        -- insert_ok_end
+
+        -- Tests --
+        t.assert_equals(books:count(), 2)
+        t.assert_equals(books:get(1), { 1, 'Thinking in Java', 2024 })
+    end)
+end

doc/concepts/data_model/value_store.rst

@@ -562,6 +562,120 @@ Charts explaining the precise differences from DUCET order are
 in the
 `Common Language Data Repository <https://unicode.org/cldr/charts/30/collation>`_.
 
+
+..  _index-defaults:
+
+Default values
+--------------
+
+*Default values* are assigned to tuple fields automatically if these fields are
+skipped during the tuple :ref:`insert or update <index-box_data-operations>`.
+
+You can specify a default value for a field in the :ref:`space_object:format() <box_space-format>`
+call that defines the space format. Default values apply regardless of the field nullability:
+any tuple in which the field is skipped or set to `nil` receives
+the default value.
+
+Default values can be set in two ways: explicitly or using a function.
+
+..  _index-defaults-explicit:
+
+Explicit default values
+~~~~~~~~~~~~~~~~~~~~~~~
+
+Explicit default values are defined in the ``default`` parameter of the field declaration
+in a :ref:`space_object:format() <box_space-format>` call.
+
+..  literalinclude:: /code_snippets/test/default_values/explicit_default_test.lua
+    :language: lua
+    :start-after: configure_space_start
+    :end-before: configure_space_end
+    :dedent:
+
+To use a default value for a field, skip it or assign `nil`:
+
+..  literalinclude:: /code_snippets/test/default_values/explicit_default_test.lua
+    :language: lua
+    :start-after: insert_ok_start
+    :end-before: insert_ok_end
+    :dedent:
+
+Any Lua object that can be evaluated during the ``space_object.format()`` call
+may be used as a default value, for example:
+
+-   a constant: ``default = 100``
+-   an initialized variable: ``default = default_size``
+-   an expression: ``default = 10 + default_size``
+-   a function return value: ``default = count_default()``
+
+.. important::
+
+    Explicit default values are evaluated **only** when setting the space format.
+    If you use a variable as a default value, its further assignments do not affect the default value.
+
+    To change the default values, call ``space_object:format()`` again.
+
+
+See also the :ref:`space_object:format() <box_space-format>` reference.
+
+..  _index-defaults-functions:
+
+Default functions
+~~~~~~~~~~~~~~~~~
+
+A default value can be defined as a return value of a stored Lua function. To be
+the default, a function must be created with :ref:`box.schema.func.create() <box_schema-func_create>`
+with the function body and return one value of the field's type. It also must not :ref:`yield <app-yields>`.
+
+..  literalinclude:: /code_snippets/test/default_values/default_functions_test.lua
+    :language: lua
+    :start-after: create_no_arg_function_start
+    :end-before: create_no_arg_function_end
+    :dedent:
+
+Default functions are set in the ``default_func`` parameter of the field declaration
+in a ``space_object:format()`` call. To make a function with no arguments the default
+for a field, specify its name:
+
+..  literalinclude:: /code_snippets/test/default_values/default_functions_test.lua
+    :language: lua
+    :start-after: format_space_default_func_start
+    :end-before: format_space_default_func_end
+    :dedent:
+
+A default function can also have one argument.
+
+..  literalinclude:: /code_snippets/test/default_values/default_functions_test.lua
+    :language: lua
+    :start-after: create_arg_function_start
+    :end-before: create_arg_function_end
+    :dedent:
+
+To pass the function argument when setting the default, specify it in the ``default`` parameter
+of the ``space_object:format()`` call:
+
+..  literalinclude:: /code_snippets/test/default_values/default_functions_test.lua
+    :language: lua
+    :start-after: reformat_space_start
+    :end-before: reformat_space_end
+    :dedent:
+
+
+.. note::
+
+    A key difference between a default function (``default_func = 'count_default'``)
+    and a function return value used as a field default value (``default = count_default()``)
+    is the following:
+
+    -   A *default function* is called **every time a default value must be produced**,
+        that is, a tuple is inserted or updated without specifying the field.
+    -   A return value used a field *default value*: the function is called **once**
+        when setting the space format. Then, all tuples receive the result of
+        this exact call if the field is not specified.
+
+See also the :ref:`space_object.format() <box_space-format>` reference.
+
+
 ..  _index-constraints:
 
 Constraints

doc/enterprise/read_views.rst

@@ -21,10 +21,6 @@ Tarantool duplicates only blocks modified after a read view is created.
     Tarantool Enterprise Edition supports read views starting from v2.11.0 and enables the ability
     to work with them using both :ref:`Lua <read_views_lua_api>` and :ref:`C API <read_views_c_api>`.
 
-
-
-
-
 .. _read_views_limitations:
 
 Limitations
@@ -34,8 +30,7 @@ Read views have the following limitations:
 
 - Only the :ref:`memtx <engines-memtx>` engine is supported.
 - Only TREE and HASH :ref:`indexes <index-types>` are supported.
-- Pagination is not supported (the :ref:`after <box_index-select>` option).
-
+- Pagination (the :ref:`after <box_index-select>` option) is supported only for TREE indexes.
 
 
 .. _working_with_read_views:
@@ -73,8 +68,6 @@ After creating a read view, you can see the information about it by calling
 
 To list all the created read views, call the :ref:`box.read_view.list() <reference_lua-box_read_view_list>` function.
 
-
-
 .. _querying_data:
 
 Querying data
@@ -112,6 +105,7 @@ Similarly, you can retrieve data by the specific index.
     ...
 
 
+ADD example with fech_pos and after
 
 .. _closing_read_view:
 
@@ -134,7 +128,6 @@ After the read view is closed,
 its :ref:`status <read_view_object-status>` is set to ``closed``.
 On an attempt to use it, an error is raised.
 
-
 .. _read_views_example:
 
 Example
@@ -202,7 +195,7 @@ as described in :ref:`Using data operations <box_space-operations-detailed-examp
         ...
 
 
-
+??? ADD example with fetch_pos and after
 ..  toctree::
     :maxdepth: 2
     :hidden:

doc/reference/reference_lua/box_space/format.rst

@@ -47,6 +47,10 @@ space_object:format()
           See also: :ref:`key_part.collation <key_part_collation>`.
         * (Optional) The ``constraint`` table specifies the :ref:`constraints <index-constraints>` that the field value must satisfy.
         * (Optional) The ``foreign_key`` table specifies the :ref:`foreign keys <index-box_foreign_keys>` for the field.
+        * (Optional) The ``default`` value specifies the :ref:`explicit default value <index-defaults-explicit>` for the field
+          or the argument of the default function if ``default_func`` is specified.
+        * (Optional) The ``default_func`` string value specifies the name of the field's :ref:`default function <index-defaults-functions>`.
+          To pass the default function's argument, add the ``default`` parameter.
 
         It is not legal for tuples to contain values that have the wrong type.
         The example below will cause an error:

doc/release/3.0.0.rst

@@ -447,7 +447,8 @@ It's possible to revert to the old behavior by toggling the new ``binary_data_de
 Default field values
 ~~~~~~~~~~~~~~~~~~~~
 
-You can now assign the default values for specific fields when defining a :ref:`space format <box_space-format>`.
+You can now assign the :ref:`default values <index-defaults>` for specific fields
+when defining a :ref:`space format <box_space-format>`.
 In this example, the ``isbn`` and ``title`` fields have the specified default values:
 
 ..  code-block:: lua
@@ -490,7 +491,7 @@ Then, assign the function name to ``default_func`` when defining a space format:
         { name = 'year_of_publication', type = 'unsigned', default_func = 'current_year' }
     })
 
-
+Learn more in :ref:`index-defaults`.
 
 .. _3-0-triggers: