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  2. Prometheus
  3. htsql


htsql / doc / roadmap.rst

HTSQL Roadmap

SQL Translator

Rewrite phase

Add a new translation phase rewrite, which optimizes the structure of the code tree. The goal is to help the compiler to generate an optimal term tree.

Optimizations we could apply at this phase:

Unmasking unit spaces
Remove filtering operations from unit spaces when they are already applied by the term to which the unit is attached. Currently, it is done by the compiler, but there is no reason why it couldn't be done directly in the code tree, thus reducing the complexity of the compiler.
Bundling aggregate and scalar units

Aggregate units sharing the same base and plural spaces should be evaluated in the same term node. Similarly, scalar units sharing the same base space should be evaluated in the same term node.

Currently, the compiler takes all aggregate units to be compiled, group them by their base and plural spaces, and then compile the groups. However, that is suboptimal since the compiler could only group units that are scheduled for compilation at the same time, so it misses some potential groupings.

Instead, on the rewrite phase, we will look for aggregates sharing the same base/plual space, make a bundle node, and replace the original unit nodes with links to the bundle node.

A more advanced optimization requires rewriting the operand of an aggregate expression. For instance:


could be rewritten to:


The latter is trivially expressed using a single frame.

Evaluating expressions with literal operands

Perform arithmetic and logical operations directly on the code tree. Examples:

2+2 => 4
3<4 => true()
false()&... => false()

Should be careful since the result of the operation may be backend-dependent.

Milestone: 2.1, 2.2?

Dependencies: none

Difficulty: weeks


Introduce a quotient table: given a table T and a kernel expression k, the quotient table T^k consists of all distinct values of the kernel when it is applied to every row of T. In SQL, a quotient table is expressed as:


There is a natural link from the original table T to the quotient table T^k, that is, the value of kernel expression applied to a row of T determines the link to T^k.

Implementing projections will affect the early phases of the translator: parsing, binding, encoding, and compiling.

Two new operations are to be added: a projection operator and a projection decorator. A projection operator takes a table and a kernel and returns a quotient space. Provisional syntax:

/(T ^ k)
/(T ^ {k1,k2,...})
/T1.(T2 ^ k)

A projection decorator provides short-cut syntax:


is equivalent to:

/(T ^ k){kernel()}

Open questions are:

  • syntax for making a quotient space (T ^ k or quotient(T, k)?);
  • how to refer to the kernel in the quotien context (kernel(), but what about multi-column kernels?);
  • how to follow the link to the original table ((T ^ k).col, but then how to express (T ^ k).avg(sum(col)), where avg() is taken against the quotient space and sum() is taken against the complement space?).

On the code tree level, two new axis spaces are to be added: a quotient space and a complement space. A complement space must conform the space of the original table, though it is not clear how it could be implemented.

The compiler should be updated to handle translation of the quotient and complement spaces into terms.

Milestone: 2.1, 2.2?

Dependencies: rewrite phase?

Difficulty: weeks to months


A segment operator converts a plural expression into a list. Example:


With a scalar base:


Short-cut syntax:


It is a big change that affects both the translator and the presentation stacks.

For the translator, we need to

  • update the grammar to add the prefix / operator (note that it clashes with the division operator) and a short-cut syntax;
  • refactor the query and segment nodes for every phase of the translator to support subsegments;
  • refactor the query plan object to support multi-segments.

For the presentation, we need to update every formatter to support the list data type. Some of the formatters (HTML, TXT) may support several styles for presenting links.

Also, add /* operator, where:


is equivalent to:


Milestone: 2.3, 2.4?

Dependencies: formatting styles? projections?

Difficulty: weeks to months

MySQL, Oracle, Microsoft SQL Server backends

Possibly others. DB2?


First, Oracle, MS SQL do not have Boolean data type; moreover, they do not permit Boolean expressions in the SELECT list. Thus we need to wrap/unwrap every predicate --- non-trivial since it is hard to convert a three-state Boolean value into a non-Boolean expression. That could be done as a part of the reduce phase.

Second, Oracle, MS SQL do not have LIMIT and OFFSET clauses (technically, MS SQL has TOP, which is equivalent to LIMIT). A term node with LIMIT must be rewritten using ROWNUM or ROW_NUMBER. That requires modifying the compiler.


  • add backend modules htsql_mysql, htsql_oracle, htsql_mssql;
  • port the regression schema to each of the backend;
  • add buildbot/virtual machine for each of the backends.

It would be great to have all backends added before other major changes are introduced to the translator.

Milestone: 2.1, 2.2?

Dependencies: split distributions

Difficutly: weeks

Date/Time data types

Currently, HTSQL only supports the DATE data type. More should be added:

  • TIME;


  • not every backend supports every data type;
  • the implementations vary in syntax and semantics.

When a backend does not support a particular date/time type, it could be emulated. For instance, MS SQL Server does not have a DATE data type, but we could represent a date value using the supported DATETIME type. Similarly, an INTERVAL value could be represented as a decimal or a float number; TIME could be represented by a float number in range 0.0 <= t < 1.0.

Support for some of the data types differs considerably across the backends; making them work uniformly in HTSQL may be difficult.

In HTSQL, we add:

  • domains TimeDomain, DateTimeDomain, TimeDeltaDomain;
  • casts time(_), datetime(_), timedelta(_);
  • constructors time(_,_,_), datetime(_,_,_,_,_,_), timedelta(?);
  • respective arithmetic operations and extractors.

Open questions:

  • how to support ... WITH TIMEZONE;
  • how to support INTERVAL FROM ... TO ...;
  • format of time delta literals.

It is very desirable that we add all planned backends before the new data types.

Milestone: 2.2, 2.3?

Dependencies: more backends

Difficulty: weeks


Array/Composite types

For PostgreSQL.

Caching to disk

Also, use CURSOR for the PostgreSQL backend to avoid fetching the whole data set.

Connection pooling


Ad-hoc linking

Including :by, @table.


Using RANK, but what about MySQL and SQLite?


Including id(), identity type and literals.

Meta backend

A native Python backend which evaluates an HTSQL query directly against a Python data structure instead of delegating the query to a SQL server.

Also, includes a meta schema, which gives access to the HTSQL meta data:


Introspecting views

Deduce primary and foreign keys from the definition of a SQL view.

Lookup rules

Make sure that every table and every link is addressable, even when the name is potentially ambiguous. Improve error messages.


Add translation framework.

Users and roles

Native support for database users and roles. Syntax:


Tons of open issues though.

Hard limit

Add LIMIT hard_limit+1 clause to the generated SQL. Report overflows nicely.


Review the semantics of the {} operator; also normalize the grammar of the segment production.



Is it essentially an ORM for Javascript?

Record and Union data types

These do not have SQL representations; used solely for the meta schema.

Stored procedures

Query introspection

UI tools need a way to introspect and modify a query, or even an incomplete query (for instance, to autocomplete).


Perhaps, should be emulated using multi-segments.

Split repositories

  • htsql_core
  • htsql_sqlite
  • htsql_pgsql
  • htsql_tests?

Binary packages

For Debian, Red Hat, Mac OS X, MS Windows.


Implement them as decorator functions? A command interface is then an adapter: Render, Produce, etc applied to the binding tree.


Move the HTRAF demo into a separate repository; convert it to a proper project.

Add support for .htsql proxy.


For accidental DBAs.

Explicit catalog config

To override the implicit configuration by the introspector.

Calculated fields

And custom functions.

Extendable introspection and catalog entities

UI tools may need extra fields for HTSQL entities.

Column headers

Review and fix.

Styling HTML output



Use unicode data type internally.

Also, Python 3 support.

Website and documentation

  • convert Examples to a slideshow;
  • screencast;
  • finish tutorial;
  • insallation instructions;
  • design rationale;
  • language reference;
  • function reference;
  • reference card.