Tag: sql

After 17 years on T-SQL, I at last started working on projects using Postgres. Here are the first things I needed for the transition.

• Do this first: https://wiki.postgresql.org/wiki/First_steps as it answers first questions about connections, databases and login.
• Postgres replaces logins and database users and groups with a single concept: ROLE.
  • Permit a ROLE to login with the With Login clause when you Create or Alter it:
    Create Role MyNameHere With Login.
  • Grant it access to a database with Grant Connect: Grant Connect on Database dbname TO roleName.
  • Make it a group with – well, it already is a group. Add other roles to it with Grant Role1 to Role2. I think this approach works out well both for evolving and managing users & groups.
• No NVarchar is needed, just text. The default installation uses UTF-8. Text is the usual idiom over VarChar(n), allegedly it performs marginally better.
• 'String concatenation is done with ' || ' a double pipe '.
• Date & Time: use timestamp for datetime/datetime2. Read Date/Time Types for more on dates, times and intervals.
• Use semicolon terminators nearly everywhere:
  If ... Then ... ; End If ; T-SQL doesn't need them but Postgres demands them.
• Before version 11, use Function not Procedure (use returns void if there is no return value). From version 11, you can Create Procedure() but note brackets are needed even for no parameters. A Procedure is called with CALL.
• A typical definition:

  Create or Replace Function procedureName( forId int, newName varchar(20) )
  Returns int -- use returns void for no return
  As $$
  Declare
    localvariable int;
    othervariable varchar(10);
  Begin
   Insert into mytable (id,name) values (forId, newName) On Conflict (id) Do Update Set Name=NewName ;
  End $$;
  

• Ooh, did you notice the 'Create or Replace' syntax, and how Postgres has a really good Upsert syntax – Insert On Conflict Update ?
• Postgres does function overloads, so to drop a function you must give the signature: Drop function functionname(int)
• Function and Procedural code is strictly separated from “pure” SQL in a way that T-SQL just doesn't bother with, and is typically written in plpgsql script. Declare variables in one block before Begin. For interactive work you can use an Anonymous Function block starting with DO :

  Select 'This line is plain SQL' ;
  Do $$
  Declare
    anumber integer ;
    astring varchar(100);
  Begin
  If 1=1 Then
   Raise Notice 'This block is plpgsql' ;
  End If;
  End $$ ;
  Select 'This line is SQL again' ;
  

  In fact, Postgres functions are defined in a string. The $$ delimiter is not a special syntax for functions, it is the postgres syntax for a literal string constant in which you don't have to escape any special characters at all. So that's handy for multiline strings and quotes.

• Functions can be defined in other languages than plpgsql. For javascript, for example, see https://github.com/plv8/plv8.
• If you don't need variables other than parameters, or control statements you can write a routine in pure sql by specifying:

  Create or Replace Procedure ProcName(Date onDate)
  Language Sql
  As $$
  Select * from LatestNews where PublishTimeStamp::Date = onDate
  $$;
  

• The double colon :: is the cast/conversion operator.
• Whereas the T-SQLer does everything in T-SQL, other database systems use non-sql commands for common tasks. For working at a command-line, learn about psql meta-commands. Start with \c to change database, \l to list databases, \d to list relations (ie tables), and \? to list other meta commands.
• Variables: are only available inside plpgsql (or other language) code, not in plain SQL. Just use a plain identifier myvariablename with no @ symbol or other decoration. BUT as a consequence you must avoid variable names in a query that are the same as a column name in the same query. BUT BUT in Ado.Net Commands with Parameters, still use the @parametername syntax as you would for SQL Server
• Postgres has probably stayed ahead of of T-SQL in keeping close to the SQL standard syntax, though they both become more, not less, standards-compliant with each version. The docs go into detail on deviations. But this means that much of your code for databases, schemas, users, tables, views, etc can be translated fairly quickly.
• Replace Identity with the SQL Standard GENERATED BY DEFAULT AS IDENTITY for Postgres 10 onwards. (You will see Serial in older versions). For more complex options, Postgres uses SQL Sequences.
• The Nuget package for Ado.Net, including for .Net core, is Npgsql
• Scan these
  • Postgres SQL language
  • Postgres DataTypes
  • Postgres Server Admin

Converting T-SQL to Postgres

Here's my initial search/replace list for converting existing code:

Authorization

Usually you will connect to postgres specifying a username. The psql.exe commandline tool will default it to your OS login username. You can avoid passwords in scripts that use psql by putting them in the pgpass.conf file).

If you want to set up integrated security, the limitation is that for computers not in a Domain it only works on localhost. The instructions at Postgres using Integrated Security on Windows take about 5 minutes for the localhost case, and include a link to the extra steps for the Domain case.

GUI Admin

Your replacement for SQL Server Manager is the pgAdmin GUI which gets you nicely off the ground with a live monitoring dashboard.

Command Line and Scripting

psql -h localhost -d myDB -c 'Select Current_User'

runs the quoted command. Note -h for “host”, not -s for “server”. Use psql --help to see more options. You can also use the pipe:

echo 'Select Version(), Current_Database();
Select Current_TimeStamp' | psql -h localhost

For large data dumps, pg_dump and pg_restore are designed to generate consistent backups of entire databases or selected tables —data or DDL or both—without blocking other users.

There are more command line utilities