doc: How to use Postgres variables in test SQL file

[coolaj86]

For testing a dynamic query with variables:

Simple Example

set session my.vars.customer_id = '0192bad7-f177-7578-8b03-700c33509ea1' ;

SELECT * FROM customers WHERE id = current_setting('my.vars.customer_id')::uuid \gx

More Complete Example

set session my.vars.customer_id = '0192bad7-f177-7578-8b03-700c33509ea1' ;
set session my.vars.types = 'email,password,phone' ;
set session my.vars.include_revoked = TRUE ;

SELECT
    "authc"."id" AS "id",
    "authc"."priority" AS "priority",
    "authc"."type" AS "type",
    "authc"."value" AS "value",
    "authc"."verified_at" AS "verified_at",
    "authc"."created_at" AS "created_at",
    "authc"."updated_at" AS "updated_at",
    "authc"."revoked_at" AS "revoked_at",
    CASE
        WHEN "authc"."details" IS NULL THEN NULL
        ELSE my_decrypt( "authc"."details" )
    END AS "details"
FROM
    "authenticators" AS "authc"
WHERE
    "authc"."customer_id" = current_setting('my.vars.customer_id')::uuid
    AND (
        "authc"."type" = ANY(string_to_array(current_setting('my.vars.types')::varchar, ','))
         OR string_to_array(current_setting('my.vars.types')::varchar, ',') IS NULL
        OR array_length(string_to_array(current_setting('my.vars.types')::varchar, ','), 1) = 0
    )
    AND (
        current_setting('my.vars.include_revoked')::bool = TRUE
            OR "revoked_at" IS NULL
    )
ORDER BY
    "authc"."priority" ASC,
    "authc"."value" ASC
\gx