16 Easy Interaction

Sometimes you just want to poke around manually, interacting with a database connection by typing in exploratory commands, and the external command-line program psql(1) does not provide enough insight or customizability (or perhaps is not available). This chapter presents an alternative method for running such a REPL (read-eval-print loop).

To get started, load the postgres-gxrepl module:

(use-modules (database postgres-gxrepl))

This provides the procedure gxrepl. The “gx” stands for “guile extensible”, which is not the case at the moment, but we have great and humble plans for this module...

Procedure: gxrepl conn

Run a read-eval-print loop, talking to database conn. conn may be a string naming a database, a string with var=val options suitable for passing to pg-connectdb, or a connection object that satisfies pg-connection?.

The repl accepts two kinds of commands:

• SQL statements such as CREATE TABLE or SELECT are executed using pg-exec.
• Comma commands are short commands beginning with a comma (the most important being ‘,help’) that do various meta-repl or prepackaged operations.

Sending an EOF exits the repl.

Most comma commands are relatively self-explanatory, with guidance from ,help. The rest of this chapter delves into some of the more arcane commands.

Primarily, the fixed-part select is an exploratory type of interaction where you can fix certain parts of a normal select query so that you can concentrate on varying the rest.

For example:

,fix #:from icbmcoords
Use the table ‘icbmcoords’ (with columns ‘one’, ‘two’ and ‘importance’).

,fsel one two #:where (< 9 importance)
Select some tuples.

,fix #:cols ("latitude" . one) ("longitude" . two)
More descriptive titles.

,fix #:where (< 9 importance) (> 3 importance)
We are scatterbrained, which helps ameliorate the evilness.

,fix #:where/combiner or
Uh oh, wits recovered, we once again are a menace.

For comparison, here is the last example again, recast into SQL:

SELECT one AS "latitude", two AS "longitude"
  FROM icbmcoords
 WHERE ((9 < importance) OR (3 > importance));