dbay

𓆤DBay

Table of Contents generated with DocToc

• 𓆤DBay
  • Introduction
  • Documentation
    • Main
      • Using Defaults
      • Automatic Location
      • Randomly Chosen Filename
      • Using Parameters
    • Opening and Closing DBs
      • Opening / Attaching DBs
      • Closing / Detaching DBs
    • Transactions and Context Handlers
    • Query
      • Executing SQL
    • User-Defined Functions (UDFs)
    • Standard Library of SQL Functions (StdLib)
      • List of Functions
      • Use Case for DBay Exceptions and Assertions: Enforcing Invariables
      • Use Case for DBay Variables: Parametrized Views
    • Safe Escaping for SQL Values and Identifiers
      • Purpose
      • Escaping Identifiers, General Values, and List Values
      • Statement Interpolation
    • SQL Statement Generation
      • Insert Statement Generation
      • Insert Statements with a returning Clause
    • Random
  • Note on Package Structure
    • better-sqlite3 an 'Unsaved' Dependency
  • Use npm, Not pnpm
  • To Do

𓆤DBay

DBay is built on better-sqlite3, which is a NodeJS adapter for SQLite. It provides convenient access to in-process, on-file and in-memory relational databases.

DBay is the successor to and a re-write of ICQL-DBA. It is under development and nearing feature-parity with its predecessor while already providing some significant improvements in terms of ease of use and simplicity of implementation.

Introduction

DBay provides

• In-Process,
• In-Memory & File-Based
• Relational Data Processing
• for NodeJS
• with SQLite;
• being based on better-sqlite3,
• it works (almost) exclusively in a synchronous fashion.

Documentation

• Benchmarks

---

Main

Using Defaults

In order to construct (instantiate) a DBay object, you can call the constructor without any arguments:

{ DBay }  = require 'dbay'
db        = new DBay()

The db object will then have two properties db.sqlt1 and db.sqlt2 that are better-sqlite3 connections to the same temporary DB in the 'automatic location'.

Automatic Location

The so-called 'automatic location' is either

• the directory /dev/shm on Linux systems that support SHared Memory (a.k.a a RAM disk)
• the OS's temporary directory as announced by os.tmpdir()

In either case, a file with a random name will be created in that location.

Randomly Chosen Filename

Format dbay-NNNNNNNNNN.sqlite, where N is a digit [0-9].

Using Parameters

You can also call the constructor with a configuration object that may have one or more of the following fields:

• cfg.path (?non-empty text): Specifies which file system path to save the DB to; if the path given is relative, it will be resolved in reference to the current directory (process.cwd()). When not specified, cfg.path will be derived from DBay.C.autolocation and a randomly chosen filename.

• cfg.temporary (?boolean): Specifies whether DB file is to be removed when process exits or db.destry() is called explicitly. cfg.temporary defaults to false if cfg.path is given, and true otherwise (when a random filename is chosen).

---

Opening and Closing DBs

Opening / Attaching DBs

• db.open cfg: Attach a new or existing DB to the db's connections (db.sqlt1, db.sqlt1).

• cfg:

  • schema (non-empty string): Required property that specifies the name under which the newly attached DB's objects can be accessed as; having attached a DB as, say, db.open { schema: 'foo', path: 'path/to/my.db', }, one can then run queries like db "select * from foo.main;" against it. Observe that
    • the DB opened at object creation time (db = new DBay()) always has the implicit name main, and schema temp is reserved for temporary databases.
  • path (string): FS path to existing or to-be-created DB file; for compatibility, this may also be set to one of the special values that indicates a in-memory DB, although that is not recommended.
  • temporary (boolean): Defaults to false when a path is given, and to true otherwise.

• The custom SQLite library that is compiled when installing DBay has its SQLITE_LIMIT_ATTACHED compilation parameter set to the maximum allowed value of 125 (instead of the default 10). This allows developers to assemble a DB application from dozens of smaller pieces when desired.

Closing / Detaching DBs

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---

Transactions and Context Handlers

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---

Query

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Executing SQL

One thing that sets DBay apart from other database adapters is the fact that the object returned from new DBay() is both the representative of the database opened and a callable function. This makes executing statements and running queries very concise. This is an excerpt from the DBay test suite:

{ DBay }            = require H.dbay_path
db                  = new DBay()
db ->
  db SQL"drop table if exists texts;"
  db SQL"create table texts ( nr integer not null primary key, text text );"
  db SQL"insert into texts values ( 3, 'third' );"
  db SQL"insert into texts values ( 1, 'first' );"
  db SQL"insert into texts values ( ?, ? );", [ 2, 'second', ]
  #.......................................................................................................
  T?.throws /cannot start a transaction within a transaction/, ->
    db ->
#.........................................................................................................
T?.throws /UNIQUE constraint failed: texts\.nr/, ->
  db ->
    db SQL"insert into texts values ( 3, 'third' );"
#.........................................................................................................
rows = db SQL"select * from texts order by nr;"
rows = [ rows..., ]
T?.eq rows, [ { nr: 1, text: 'first' }, { nr: 2, text: 'second' }, { nr: 3, text: 'third' } ]

Note In the above SQL has been set to String.raw and has no further effect on the string it precedes; it is just used as a syntax marker (cool because then you can have nested syntax hiliting).

As shown by benchmarks, a crucial factor for getting maximum performance out of using SQLite is strategically placed transactions. SQLite will not ever execute a DB query outside of a transaction; when no transaction has been explicitly opened with begin transaction, the DB engine will precede each query implicitly with (the equivalent of) begin transaction and follow it with either commit or rollback. This means when a thousand insert statements are run, a thousand transactions will be started and committed, leavin performance pretty much in the dust.

To avoid that performance hit, users are advised to always start and commit transactions when doing many consecutive queries. DBay's callable db object makes that easy: just write db -> many; inserts; here; (JS: db( () -> { many; inserts; here; })), i.e. pass a function as the sole argument to db, and DBay will wrap that function with a transaction. In case an error should occur, DBay guarantees to call rollback (in a try ... finally ... clause). Those who like to make things more explicit can also use db.with_transaction ->. Both formats allow to pass in a configuration object with an attribute mode that may be set to one of 'deferred', 'immediate', or 'exclusive', the default being 'deferred'.

Another slight performance hit may be caused by the logic DBay uses to (look up an SQL text in a cache or) prepare a statement and then decide whether to call better-sqlite3's' Database::execute(), Statement::run() or Statement::iterate(); in order to circumvent that extra work, users may choose to fall back on to better-sqlite3 explicitly:

insert = db.prepare SQL"insert into texts values ( ?, ? );" # returns a `better-sqlite3` `Statement` instance
db ->
  insert.run [ 2, 'second', ]

---

User-Defined Functions (UDFs)

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---

Standard Library of SQL Functions (StdLib)

List of Functions

• Strings

  • std_str_reverse()
  • std_str_join()
  • std_str_split()
  • std_str_split_re()
  • std_str_split_first()
  • std_re_matches()

• XXX

  • std_generate_series()

• Output

  • std_echo()
  • std_debug()
  • std_info()
  • std_warn()

• Exceptions and Assertions

  • std_raise( message )—unconditionally throw an error with message given.
  • std_raise_json( facets_json )—unconditionally throw an error with informational properties encoded as a JSON string.
  • std_assert( test, message )—throw an error with message if test is falsy.
  • std_warn_if( test, message )—print an error message if test is truthy.
  • std_warn_unless()—print an error message if test is falsy.

• Variables

  • std_getv()
  • std_variables()

Use Case for DBay Exceptions and Assertions: Enforcing Invariables

• std_assert: ( test, message ) -> throws error if test is false(y)
• std_warn_unless: ( test, message ) -> prints warning if test is false(y)
• often one wants to ensure a given SQL statement returns / affects exactly zero or one rows
• easy to do if some rows are affected, but more difficult when no rows are affected, because a function in the statement won't be called when there are no rows.
• The trick is to ensure that at least one row is computed even when no rows match the query, and the way to do that is to include an aggregate function such as count(*).
• May want to include limit 1 where appropriate.

select
    *,
    std_assert(
      count(*) > 0,
      '^2734-1^ expected one or more rows, got ' || count(*) ) as _message
  from nnt
  where true
    and ( n != 0 );

select
    *,
    std_assert(
      count(*) > 0, -- using `count(*)` will cause the function to be called
                    -- even in case there are no matching rows
      '^2734-2^ expected one or more rows, got ' || count(*) ) as _message
  from nnt
  where true
    and ( n != 0 )
    and ( t = 'nonexistant' ); -- this condition is never fulfilled

Use Case for DBay Variables: Parametrized Views

• An alternative for user-defined table functions where those functions would perform queries against the DB, which is tricky.
• Inside the view definition, use std_getv( name ) to retrieve variable values which must have been set immediately prior to accessing the view.
• Downside is that it's easy to forget to update a given value, so best done from inside a specialized method in your application.

---

Safe Escaping for SQL Values and Identifiers

Purpose

• Facilitate the creation of securely escaped SQL literals.
• In general not thought of as a replacement for the value interpolation offered by DBay::prepare(), DBay::query() and so, except when
  • one wants to parametrize DB object names (e.g. use table or column names like variables),
  • one wants to interpolate an SQL values list, as in select employee from employees where department in ( 'sales', 'HR' );.

Escaping Identifiers, General Values, and List Values

• db.sql.I: ( name ): ->: returns a properly quoted and escaped SQL Identifier.
• db.sql.L: ( x ): ->: returns a properly quoted and escaped SQL Value. Note that booleans (true, false) will be converted to 1 and 0, respectively.
• db.sql.V: ( x ): ->: returns a bracketed SQL list of values (using db.sql.V() for each list element).

Statement Interpolation

db.interpolate( sql, values ): -> accepts a template (a string with placeholder formulas) and a list or object of values. It returns a string with the placeholder formulas replaced with the escaped values.

# using named placeholders
sql     = SQL"select $:col_a, $:col_b where $:col_b in $V:choices"
d       = { col_a: 'foo', col_b: 'bar', choices: [ 1, 2, 3, ], }
result  = db.sql.interpolate sql, d
# > """select "foo", "bar" where "bar" in ( 1, 2, 3 )"""

# using positional placeholders
sql     = SQL"select ?:, ?: where ?: in ?V:"
d       = [ 'foo', 'bar', 'bar', [ 1, 2, 3, ], ]
result  = db.sql.interpolate sql, d
# > """select "foo", "bar" where "bar" in ( 1, 2, 3 )"""

# using an unknown format
sql     = SQL"select ?:, ?X: where ?: in ?V:"
d       = [ 'foo', 'bar', 'bar', [ 1, 2, 3, ], ]
result  = db.sql.interpolate sql, d
# throws "unknown interpolation format 'X'"

---

SQL Statement Generation

DBay offers limited support for the declarative generation of a small number of recurring classes of SQL statements. These facilities are in no way intended to constitute or grow into a full-blown Object-Relational Mapper (ORM); instead, they are meant to make working with relational data less of a repetitive chore.

Insert Statement Generation

To pick one case in point, SQL insert statements when called from a procedural language have a nasty habit of demanding not two, but three copies of a table's column names:

db SQL"""
  create table xy (
    a   integer not null primary key,
    b   text not null,
    c   boolean not null );"""
db SQL"insert into xy ( b, c ) values ( $b, $c )", { b, c, }
#                     ^^^^^^^^        ^^^^^^^^^^   ^^^^^^^^^

As stated above, DBay does not strive to implement full SQL statement generation. Even if one wanted to only generate SQL insert statements, one would still have to implement almost all of SQL, as is evidenced by the screenshot of the SQLite insert Statement Railroad Diagram that will be displayed when clicking/tapping on this paragraph.

Instead, we implement facilities to cover the most frequent use cases and offer opportunities to insert SQL fragments at strategic points.

Often, when an insert statement is being called for, one wants to insert full rows (minus generated columns, for which see below) into tables. This is the default that DBay makes easy: A call to db.prepare_insert() with the insertion target identified with into will return a prepared statement that can then be used as first argument to the db callable:

insert_into_xy = db.prepare_insert { into: 'xy', }
db insert_into_xy, { a, b, c, }

Observe that named parameters (as opposed to positional ones) are used, so values must be passed as an object (as opposed to a list).

In case the actual SQL text of the statement is needed, call db.create_insert() instead:

insert_sql = db.create_insert { into: 'xy', }
# 'insert into "main"."xy" ( "a", "b", "c" ) values ( $a, $b, $c );'

When one or more columns in a table are autoincremented or have a default value, then those columns are often intended not to be set explicitly. What's more, columns with generated values must not be set explicitly. For this reason, db.create_insert() (and, by extension, db.prepare_insert()) will skip generated columns and allow to explicitly specify either included columns (as fields) or else excluded columns (as exclude):

db SQL"""
  create table t1(
    a integer primary key,
    b integer,
    c text,
    d integer generated always as (a*abs(b)) virtual,
    e text generated always as (substr(c,b,b+1)) stored );"""
insert_into_t1 = db.create_insert { into: 't1', }

### Observe `d` and `e` are left out because they're generated, but `a` is present: ###
# 'insert into "main"."t1" ( "a", "b", "c" ) values ( $a, $b, $c );'

### You probably want either this: ###
insert_into_t1 = db.create_insert { into: 't1', fields: [ 'b', 'c', ], }
# 'insert into "main"."t1" ( "b", "c" ) values ( $b, $c );'

### Or this: ###
insert_into_t1 = db.create_insert { into: 't1', exclude: [ 'a', ], }
# 'insert into "main"."t1" ( "b", "c" ) values ( $b, $c );'

There's a subtle yet important semantic difference in how the fields and exclude settings are handled: When fields are explicitly given, the table does not have to exist when generating the SQL; however, when fields is not given, the table must already exist at the time of calling create_insert().

In either case, prepare_insert() can only succeed when all referenced object in an SQL statement have already been created.

The next important thing one often wants in inserts is resolving conflicts. DBay create_insert() supports setting on_conflict to either (1) an arbitrary string that should spell out a syntactically valid SQL on conflict clause, or (2) an object { update: true, } to generate SQL that updates the explicitly or implicitly selected columns. This form has been chosen to leave the door open to future expansions of supported features.

When choosing the first option, observe that whatever string is passed in, create_insert() will prepend 'on conflict ' to it; therefore, to create an insert statement that ignores insert conflicts, and according to the upsert syntax railroad diagram: —

— the right thing to do is to call db.create_insert { into: table_name, on_conflict: 'do nothing', }. Assuming table t1 has been declared as above, calling

db.create_insert { into: 't1', exclude: [ 'a', ], on_conflict: "do nothing", }

will generate the (unformatted but properly escaped) equivalent to:

insert into main.t1 ( b, c )
  values ( $b, $c )
  on conflict do nothing;
  --          |<------>|
  --        inserted string

while calling

db.create_insert { into: 't1', exclude: [ 'a', ], on_conflict: { update: true, }, }

wiil generate the (unformatted but properly escaped) equivalent to:

insert into main.t1 ( b, c )
  values ( $b, $c )
  on conflict do update set  --| conflict resolution clause
    b = excluded.b,          --| mandated by { update: true, }
    c = excluded.c;          --| containing same fields as above

Insert Statements with a returning Clause

It is sometimes handy to have insert statements that return a useful value. Here's a toy example that demonstrates how one can have a table with generated columns:

db SQL"""
  create table xy (
    a   integer not null primary key,
    b   text not null,
    c   text generated always as ( '+' || b || '+' ) );"""
insert_into_xy_sql = db.create_insert { into: 'xy', on_conflict: SQL"do nothing", returning: '*', }
# -> 'insert into "main"."xy" ( "a", "b" ) values ( $a, $b ) on conflict do nothing returning *;'
db.single_row insert_into_xy_sql, { a: 1, b: 'any', } # -> { a: 1, b: 'any', c: '+any+' }
db.single_row insert_into_xy_sql, { a: 2, b: 'duh', } # -> { a: 2, b: 'duh', c: '+duh+' }
db.single_row insert_into_xy_sql, { a: 3, b: 'foo', } # -> { a: 3, b: 'foo', c: '+foo+' }

Generally, the returning clause must be defined by a non-empty string that is valid SQL for the position after returning and the end of the statement. A star * will return the entire row that has been inserted; we here use db.single_row() to eschew the result iterator that would be returned by default.

---

Random

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---

Note on Package Structure

better-sqlite3 an 'Unsaved' Dependency

Since DBay depends on better-sqlite3 with a custom-configured build of the SQLite C engine, it is (for whatever reason) important that better-sqlite3 must not be listed under package.json#dependencies; otherwise, compilation will not work properly. The build script will run npm install better-sqlite3@'^7.4.3' but with an added --no-save flag.

Use npm, Not pnpm

Also, at the time of this writing (2021-09), while the project compiles fine using npm v7.21.1 (on NodeJS v16.9.1 on Linux Mint), but it fails using pnpm v6.14.6 with Unknown options: 'build-from-source', 'sqlite3'. Yarn has not been tried.

Note—These considerations only concern those who wish to fork/clone DBay to work on the code. Those who just want to use DBay as a dependency of their project can both either run npm install dbay or pnpm add dbay, both package managers work fine.

To Do

• [–] port foundational code from hengist &c
• [–] at construction time, allow dbnick when path is given and ram is false
• [–] to solve the table-UDF-with-DB-access conundrum, consider
  • [+] https://github.com/mapnik/mapnik/issues/797, where connection parameters are discussed (see also https://www.sqlite.org/c3ref/open.html); nothing of interested AFAICS
  • [–] mirroring a given DB into a second (RAM or file) location, taking care to replay any goings-on on both instances. This is probably unattractive from a performance POV.
  • [–] using NodeJS worker threads to perform updates; maybe one could even continuously mirror a RAM DB on disk to get a near-synchronous copy, obliviating the necessity to explicitly call db.save(). See https://github.com/JoshuaWise/better-sqlite3/blob/master/docs/threads.md
  • [–] implementing macros so one could write eg select * from foo( x ) as d; to get select * from ( select a, b, c from blah order by 1 ) as d (i.e. inline expansion)
  • [–] Observe that, seemingly, only table-valued UDFs hang while with shared-cache we already can issue selects from inside UDFs, so maybe there's a teeny, fixable difference between how both are implemented that leads to the undesirable behavior
• [–] let users choose between SQLite-only RAM DBs and tmpfs-based in-memory DBs (b/c the latter allow pragma journal_mode = WAL for better concurrent access). Cons include: tmpfs-based RAM DBs necessitate mounting a RAM disk which needs sudo rights, so might as well just instruct users to mount RAM disk, then use that path? Still, it would be preferrable to have some automatic copy-to-durable in place.
• [–] implement context handler for discardable / temporary file
• [+] implement DBay::do() as a method that unifies all of better-sqlite3's Statement::run(), Statement::iterate(), and Database::execute().
• [+] allow to call DBay::do -> ... with a synchronous function with the same semantics as DBay::with_transaction -> ....
• [+] allow to call DBay::do { mode: 'deferred', }, -> ....
• [–] allow to call DBay::do -> ... with an asynchronous function
• [+] make db = new DBay() an instance of Function that, when called, runs DBay::do() Database::execute(). statement = DBay::prepare.insert_into.<table> [ 'field1', 'field2', ..., ]
• [+] change classname(s) from Dbay to DBay to avoid spelling variant proliferation
• [–] implement DBay::open(), DBay::close()
• [–] ensure how cross-schema foreign keys work when re-attaching DBs / schemas one by one
• [–] demote random from a mixin to functions in helpers.
• [–] implement db.truncate() / db.delete(); allow to retrieve SQL.
• [–] implement DBay::insert_into.<table> [ 'field1', 'field2', ..., ], { field1, field2, ..., }; allow to retrieve SQL.
• [–] clarify whether UDFs get called at all when any argument is null b/c it looks like they don't get called which would be unfortunate
• [–] add schematic to clarify terms like database, schema, connection; hilite that UDFs are defined on connections (not schemas or databases as would be the case in e.g. PostgreSQL).
• [–] allow to transparently treat key/value tables as caches
• [+] let db.do() accept prepared statement objects.
• [–] implement escaping of dollar-prefixed SQL placeholders (needed by create_insert()).
• [–] implement
  • [–] db.commit()
  • [–] db.rollback()
• [–] allow to use sets with sql.V()
• [+] make first_row(), all_rows() etc accept statements and strings
• [+] at the moment we use cfg.prefix for (inherently schema-less) UDF names (and require a trailing underscore to be part of the prefix), and cfg.schema for plugin-in-specific DB tables and views; in the future, we should use a single parameter for both (and make the underscore implicit). In addition, it should be possible to choose whether a plugin will create its objects with a prefix (in the same schema as the main DB) or within another schema.