feat: add an executable for expanding slash commands to SQL

[murrayju]

I wanted an executable that takes a psql slash command (like \dt), and expands it into the corresponding SQL without executing it (we will use the SQL for execution elsewhere). This can be accomplished via psql itself like so:

psql postgres://user:pw@localhost:5432/test -q --set=ECHO_HIDDEN=noexec -c '\d'

The problem is, this requires a valid connection string, and must establish a successful connection before outputting the SQL.

What I've done here is take the entrypoint code for psql, and strip it down to the bare minimum that we need here. It creates a fake/empty db connection object, and avoids actually connecting it. I've also simplified the parameters, to just take a single argument (the slash command). Invoking this looks like:

slash2sql '\dt'

To test

brew install icu4c
export PKG_CONFIG_PATH="/opt/homebrew/Cellar/icu4c@77/77.1/lib/pkgconfig:$PKG_CONFIG_PATH" && ./configure
make
DYLD_LIBRARY_PATH=./src/interfaces/libpq ./src/bin/psql/slash2sql '\d'

Can also install, to avoid the lib path issue

sudo make install
/usr/local/pgsql/bin/slash2sql '\d'

Example output

/usr/local/pgsql/bin/slash2sql '\d'
/******** QUERY *********/
SELECT n.nspname as "Schema",
  c.relname as "Name",
  CASE c.relkind WHEN 'r' THEN 'table' WHEN 'v' THEN 'view' WHEN 'm' THEN 'materialized view' WHEN 'i' THEN 'index' WHEN 'S' THEN 'sequence' WHEN 't' THEN 'TOAST table' WHEN 'f' THEN 'foreign table' WHEN 'p' THEN 'partitioned table' WHEN 'I' THEN 'partitioned index' END as "Type",
  pg_catalog.pg_get_userbyid(c.relowner) as "Owner"
FROM pg_catalog.pg_class c
     LEFT JOIN pg_catalog.pg_namespace n ON n.oid = c.relnamespace
WHERE c.relkind IN ('r','p','v','m','S','f','')
      AND n.nspname <> 'pg_catalog'
      AND n.nspname !~ '^pg_toast'
      AND n.nspname <> 'information_schema'
  AND pg_catalog.pg_table_is_visible(c.oid)
ORDER BY 1,2;
/************************/