Metadata tables

[dpordomingo]

WIP notice

• more tests,

PR Description

I tried to follow these concept from mysql docs

INFORMATION_SCHEMA is a database within each MySQL instance, the place that stores information about all the other databases that the MySQL server maintains.
The INFORMATION_SCHEMA database contains several read-only tables. They are actually views, not base tables

Advantages of SELECT syntax:

• It conforms to Codd's rules, because all access is done on tables. (https://en.wikipedia.org/wiki/Codd's_12_rules#Rules)
• You can use the familiar syntax of the SELECT statement, and only need to learn some table and column names.
• You can filter, sort, concatenate, and transform the results from INFORMATION_SCHEMA queries into whatever format your application needs.

If this PR is merged, it could be done the following:

SELECT * FROM SCHEMATA;
SELECT schema_name FROM SCHEMATA where schema_name="gitql";

SELECT * FROM TABLES;
SELECT table_name FROM TABLES WHERE table_schema="gitql";

SELECT * FROM COLUMNS;
SELECT column_name, data_type, is_nullable, column_key, column_default FROM COLUMNS WHERE table_name="commits";

Design:

The metadata –new– package, includes the implementation of the new INFORMATION_SCHEMA database, that contains the three metadata tables: SCHEMATA, TABLES and COLUMNS, that embeds mem.Database and mem.Table respectively.
The –new– three tables behaves like a view so they doesn't store data nor are writable.

Other changes, required by the main development:

• Databases should be added through sql.Engine::AddDatabase(db sql.Database) error method,
• sql.Catalog is now an Interface and operations over its handled databases should be done through its exposed methods:
  • Database(name string) (Database, error)
  • AddDatabase(db Database) error
  • NEW: Dbs() Databases
  • NEW: Table(dbName string, tableName string) (Table, error)
• tables are fetched from the Database through the sql.Database::Table(string) (Table, error) method instead of accessing the database tables directly,
• mem.NewCIDatabase(name string) constructor will build a mem/Database that will store the tables in a case insensitive way,
• mem.Table embeds a new mem.underlayingData that handles the storage, row iteration...,
• sql.Node implements fmt.Stringer in order to be debugging friendly,
• it is implemented three new iterators:
  • DBIterator, iterates over the sql.Database handled by the given sql.Catalog,
  • TableIterator, iterates over the sql.Table stored by the given, for sql.Database,
  • ColumnIterator, iterates over the sql.Column defined by the sql.Schema of the passed sql.Table,

[codecov-io]

Codecov Report

Merging #17 into master will decrease coverage by 0.24%.
The diff coverage is 54.59%.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
- Coverage    53.3%   53.06%   -0.25%     
==========================================
  Files          34       41       +7     
  Lines        1724     2039     +315     
==========================================
+ Hits          919     1082     +163     
- Misses        743      885     +142     
- Partials       62       72      +10

Impacted Files	Coverage Δ
sql/functionregistry.go	92.59% <ø> (ø)	⬆️
sql/plan/show_tables.go	57.14% <0%> (-3.47%)	⬇️
sql/plan/cross_join.go	60.65% <0%> (-2.06%)	⬇️
sql/plan/project.go	58.82% <0%> (-12.61%)	⬇️
sql/plan/group_by.go	70.83% <0%> (-8.61%)	⬇️
sql/unresolved_database.go	0% <0%> (ø)	⬆️
sql/core.go	0% <0%> (ø)
sql/plan/limit.go	54.83% <0%> (-3.79%)	⬇️
sql/plan/values.go	0% <0%> (ø)	⬆️
sql/row.go	50% <0%> (-3.13%)	⬇️
... and 25 more

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[smola]

assert.NoError if the return type is error.

[smola]

Ok. Makes sense. If we want to change the name of a table we could probably add a wrapper TableAlias in the future.

[smola]

Why there is mem and memory package?

[smola]

I think it's better to make this type private and expose only the RowIter interface.

[smola]

Rename to Catalog