package fun-postgresql

The database connection or file, etc.

A value sent to the database in the place of a query parameter.

A decoder of a single row of the resultset from running a query.

A generic way to write placeholders for different database drivers' prepared statement parameters.

ℹ️ Placeholders are 0-indexed.

The main function through which queries are run is the query function. This function always creates a prepared statement for each partial call to query db sql. This prepared statement can then be called with the actual arguments (if any) and the resultset row decoder:

let add_person =
  query db (sql "insert into people (name, age) values (%a, %a)" placeholder 0 placeholder 1)
let add_person name age = add_person ~args:Arg.[text name; int age] unit

• raises Invalid_argument

  if trying to create multiple prepared statements for the same SQL query in PostgreSQL. To avoid this, just create the prepared statement once only and call it whenever needed, as shown above.

exec_script db sql executes the sql script (possibly made up of multiple statements) in the database db. Note that it ignores any rows returned by any of the statements.

The script must not have a trailing semicolon.

unit indicates that the query doesn't return any meaningful output.

ret decode is a custom return type encoding for a resultset into a sequence of values of the type decoded by decode.

decode constructs a value of the custom type if possible, else raises Failure.

Note that the sequence rows of the resultset is unfolded as it is read from the database. It can only be traversed once, with e.g. List.of_seq or Seq.iter. If traversed multiple times, it will raise Failure.

• raises Invalid_argument

  if any row cannot be decoded.

• raises Failure

  if an unexpected result code is encountered.

Also handles values of all other types. Use this when SQLite can change the exact type of value it returns at runtime, e.g. for very large numbers it can return text.

opt dec col row is the optional value NULL turns to None at column col of the result row.

Helper to construct SQL query strings using placeholders.

migrate db dir applies the SQL migration scripts in dir on the given database db, keeping track of those that have already been applied.

To apply the migrations in the correct order, the migration scripts must be given filenames that are sorted in lexicographical order of the desired migration order, e.g. 0000_0001_init.sql will be applied before 0000_0002_sec.sql, and so on.

Note that this uses exec_script internally, which means the migration scripts must not have trailing semicolons either.

Any files with extensions other than .sql are ignored.

• raises Bad_migration

  an error occurs during applying the migrations.

transaction db f runs f () inside a transaction in the db. If the operation succeeds, it commits the transaction and returns its result. If it fails with an exception, it rolls back the transaction and re-raises the exception.

Thrown if we are expecting at most one result but get more.

only seq is the first and only element of seq. This is a convenience function because all queries return seqs but sometimes we want only a single item, otherwise it should be an error.

Use this in preference to calculating the length of the seq, which would force the entire data structure.

• raises Not_found

  if seq has 0 items.

• raises More_than_one

  if seq has more than 1 item.

optional seq is Some a if a is the first and only element of seq, or None if seq is empty.

• raises More_than_one

  if seq has more than 1 item.