Using SQL Databases from APL Dyalog other J Merrill Analytical Software Corp jamesmerrill usa net Overview • About my APL and SQL experience • T

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Using SQL Databases from APL Dyalog other J Merrill Analytical Software Corp jamesmerrill usa net Overview • About my APL and SQL experience • T Powered By Docstoc
					Using SQL Databases from
   APL (Dyalog & other)

           J. Merrill
   Analytical Software Corp.
    jamesmerrill@usa.net
                    Overview
• About my APL and SQL experience
• This talk emphasizes information about SQL
  – Concepts you must know
  – Weaknesses you need to understand
  – Ideas about SQL database design
• Why so much about SQL?
  – Could a few people learn the details about SQL, and
    build tools to shield other developers from the issues?
• Why you should not hide SQL from APLers
  – The very successful project that was a failure
• APL talking to SQL is not hard to do (any more)
           My Background
• I started working with APL professionally
  in 1975, as a user of STSC timesharing.
• While working at STSC, I took a course in
  relational database technology and SQL
  from a representative of a small company
  called Relational Software Inc. Later they
  changed their name to Oracle – I‟ve
  worked with SQL for longer than Oracle
  Corporation has existed!
          SQL Work at STSC
• I automated connection of an Oracle database of
  microcomputer software sales data to the
  existing (APL-based) accounting system.
• I designed a portable interface between STSC‟s
  APL*Plus and SQL on these platforms:
  – VMS talking to RDB
  – VMS talking to Oracle
  – Unix talking to Oracle
  and implemented the APL in those interfaces.
 STSC, Manugistics, and Later
• I was a software development consultant
  for many years with both STSC and (after
  the name change) Manugistics.
• Client projects included migrating an APL-
  based database of foreign exchange
  trading data to Microsoft SQL Server.
• Since founding Analytical Software Corp.
  I‟ve worked extensively with clients using
  SQL Server from APL+Win.
    SQL Concepts You Must Know
•   SQL has only four data manipulation verbs
•   RDBMSs use Client / Server techniques
•   SQL has the concept of null
•   All data is in a table (like an APL matrix)
•   SQL columns are named, not numbered
•   SQL columns are “strongly typed”
•   Relationships are data not structure
DML: select insert update delete
• SQL‟s select is extraordinarily versatile
• SQL‟s insert and update normally operate
  on a single row at a time and therefore are
  quite primitive
• If insert and update work on multiple rows,
  not a single row, they become very
  powerful (as powerful as select)
• SQL‟s delete is usually simple but can use
  some powerful select techniques
     SQL Has DML and DDL
• DML means data manipulation language
• SQL‟s four DML verbs are select, insert,
  update, delete
• For defining the database structure
  (schema), SQL has DDL
• DDL means data definition language
• DDL is less standardized between
  products
   SQL select is Very Powerful
• select supports extremely complex statements
  that can access data from multiple tables,
  potentially joining them using every available
  join technique in a single query
• Much of select‟s power is based on the ability to
  define derived tables within a statement, and
  use those tables as if they held real data – but
  no data is actually stored to run the query
  – The syntax is intended let you express intent without
    specifying a particular processing sequence
• Powerful does not mean simple or intuitive
insert & update: SQL‟s Weak Spot
• When data is in memory (in an application)
  – insert statements allow you to add one row,
    giving values for each column in the new row
  – update statements typically reference a single
    row by its primary key (though it can change
    many columns in one update)
• Applying a group of changes based on
  user input typically involves issuing many
  one-row-each insert or update statements
   Multi-Row insert and update
• insert supports adding the result of any
  select to an existing table
• update similarly can update many rows of
  one table from the contents of another
  table, providing the tables can be joined
  (usually on the primary key)
  – The syntax for this is not completely standard
    and some products may not support this
• Get the data on the server if possible!
 SQL delete Does What It Says
• delete can use the full power of select to
  determine what row or rows of a table
  should be deleted
• When deleting rows based on a list of
  primary key values, implementations that
  limit the length of a SQL statement (or an
  individual clause within the statement) can
  require multiple delete statements to
  delete a large number of rows
    SQL Concepts You Must Know
•   SQL has only four data manipulation verbs
•   RDBMSs use Client / Server techniques
•   SQL has the concept of null
•   All data is in a table (like an APL matrix)
•   SQL columns are named, not numbered
•   SQL columns are “strongly typed”
•   Relationships are data not structure
     Client / Server Concepts
• Clients send requests to the server and it
  responds with the results
• Only the DBMS server touches the data
• It‟s good to have the server do things like
  compute totals so that results are smaller
• The number of back-and-forth interactions
  is potentially more significant than the
  amount of data being transferred
  SQL is the Language for Server
    Requests and Responses
• Your program sends SQL statements (only) to
  the server and handles the responses
• Some tools hide the fact that everything is done
  with SQL, by generating SQL for you
• SQL is very powerful for reading data (select)
  and deleting data (delete)
• SQL is quite stupidly simple for writing data in
  memory into a table (insert, update)
• The over-the-wire protocol is complex, but is
  hidden by (most) client libraries
One Program Touches the Data
• A single coordinating program, the DBMS
  server, performs all access (reading and
  writing) to data files holding the SQL data
• Access to data is controlled by the server
• The server process can be run on a very
  powerful (expensive) computer
• Conflicting requests from different users
  can be handled more easily
   Let the Server Do the Work
• The server should be a better computer
  than the one running your program
• It is optimal if the server can get the job
  done without sending a lot of data over the
  wire to your program
• Downside: the more you get the server to
  do the work (by sending SQL), the more
  you have to worry that things will need to
  change if you switch to another DBMS
      Avoid “Chatty” Programs
• Each interaction with the server takes time for
  the back-and-forth protocol, in addition to
  whatever actual data is passed
• Rather than sending a series of steps to the
  server one after the other, it‟s better to send one
  larger request
• SQL‟s simplistic insert & update is an issue
• Downside: the more you “bunch up” your
  requests, the more difficult it is to handle errors
    SQL Concepts You Must Know
•   SQL has only four data manipulation verbs
•   RDBMSs use Client / Server techniques
•   SQL has the concept of null
•   All data is in a table (like an APL matrix)
•   SQL columns are named, not numbered
•   SQL columns are “strongly typed”
•   Relationships are data not structure
     SQL‟s Concept Called null
• Intended to represent a “missing” or “unknown”
  value, rather than an “empty” value
• If a value is null, it is not equal to any particular
  value, nor is it not-equal to any particular value.
  Suppose A is an arbitrary SQL expression:
      int_column = 5
      char_column = „hello‟
  One would think that the result of
      A or (not A)
  has to be true but in SQL that‟s not the case (!).
null Causes “three valued logic”
• APLers think we understand booleans, but then
  along comes SQL‟s notion of null that changes
  the rules we‟ve learned
• When nulls are involved, the result of a boolean
  expression is one of true, false, or null
• The result of any expression is null if any value
  involved in the expression is null
• The SQL function coalesce returns the first not-
  null parameter value passed:
    coalesce(column, value_if_null)
    Use Null Values Sparingly
• Allowing nulls sometimes makes sense
  – Definitely makes sense for date or time
  – Makes sense for numeric data when 0 is not
    an appropriate “no information” replacement
     • Don‟t use a “missing value code” in the database,
       even if you use one in your APL data
  – Any distinction between NULL and „‟ (an
    empty string) is hard to see, justify, or explain
    to non-programmers (or programmers!).
• Sometimes it‟s not your decision
    SQL Concepts You Must Know
•   SQL has only four data manipulation verbs
•   RDBMSs use Client / Server techniques
•   SQL has the concept of null
•   All data is in a table (like an APL matrix)
•   SQL columns are named, not numbered
•   SQL columns are “strongly typed”
•   Relationships are data not structure
     All SQL Data is in Tables
• There is no other persistent data structure
• SQL table values are conceptual scalars
  – SQL can hold strings (enclosed char vectors)
  – Otherwise, no nested data (in standard SQL)
• If APL had scalar strings, a SQL table
  would be just like a simple APL matrix
• SQL locates rows using data values only
  – No notion like row number in an APL matrix
              SQL Columns
• Columns are named, not numbered
• Each column has a (scalar) data type
• Most data types can support the special
  SQL value null
  – Handling null values correctly can be difficult
           Column Names
• One difference between products is in
  what column names they allow
• Some allow spaces in names, but that
  makes it necessary to “quote” those
  column names (so SQL can parse
  statements using them)
• Standard quoting techniques vary (!)
• Advice: use simple, not-very-long names
• It‟s not always your choice
    SQL Concepts You Must Know
•   SQL has only four data manipulation verbs
•   RDBMSs use Client / Server techniques
•   SQL has the concept of null
•   All data is in a table (like an APL matrix)
•   SQL columns are named, not numbered
•   SQL columns are “strongly typed”
•   Relationships are data not structure
        Column Type Catgeories
•   Numbers
•   Strings
•   Dates and Times
•   Binary
•   Time Intervals
•   Boolean
                 Numbers
• Integers of various sizes (1, 2, 4, 8)
  – Usually no “unsigned integer”
• 4- or 8-byte floating point
• Fixed width and number of decimals
• Money sometimes available
  – 64-bit integer with 4 implied decimals
                   Strings
•   Fixed length with maximum size
•   Variable length with maximum size
•   “Unlimited size” often available
•   Multi-byte data (Unicode) often available
            Dates and Times
• One or more of date, time, datetime
• Concept of NULL is very useful here
• Dates are stored as Julian date values
  – Many support negative Julian dates
• Time values are stored as fraction of a day
  – 0.25 is 6am, 0.5 is noon, 0.75 is 6pm
• Datetime values stored as (day# + timefraction)
• If only datetime is available,
  – time can are represented as being “on day 0”
  – date can be represented as being “at midnight”
            Time Intervals
• Supposedly standard
• Not universally available (SQL Server)
• Can be represented with a date or
  datetime value holding the result of
  subtracting one date or datetime value
  from another
                 Binary
• Fixed length with a maximum size
• Variable length with a maximum size
• “Unlimited size” often available
                     Boolean
• A single bit (!)
  – Not “fixed or variable length with a maximum
    size” like string and binary data
• Sometimes boolean columns support null
  (so there are 3 states)
• My advice: use a one-wide character
  column instead
Time for a Break ?
    SQL Concepts You Must Know
•   SQL has only four data manipulation verbs
•   RDBMSs use Client / Server techniques
•   SQL has the concept of null
•   All data is in a table (like an APL matrix)
•   SQL columns are named, not numbered
•   SQL columns are “strongly typed”
•   Relationships are data not structure
How Relationships Work in SQL
• In APL, you can choose to represent
  related structures by nesting
• In SQL, there are only tables and
  there is no possibility of nesting
• Relationships are stored by having a
  column in one table store a value
  identifying a related row in another
  table (or null if there is no related row)
            SQL Table Design
• All columns in a table should represent
  information about one entity (or concept)
  – row = data about one instance
  – column = same data re many instances
     • The term “field” is deprecated (but widely used)
• Each table needs a “primary key” column whose
  value can be stored in any other table that has a
  relationship with rows in this table
• SQL-generated primary key values allow
  relationships to be represented easily and can
  prevent issues that arise when users think they
  want to change key values
         SQL Relationships
•   One to many (parent - child)
•   Many to one (lookup)
•   Hierarchy
•   Many to many
 Relationships – one to many
• Each (one) invoice can have any number
  (many) of line items
  – LineItem table has an InvoiceID column
    holding Invoice table primary key (PK) value
  – “No line items for invoice N” is easily
    represented by there being no rows in the
    LineItem table with InvoiceID = N
• An APL application might use nesting,
  storing a vector or matrix of lineitem data
  within each Invoice data structure
One to Many = Parent - Child
• The “one” table (Invoice) is parent
• The “many” table (LineItem) is child
• Column holding primary key (PK) of
  another table is called a foreign key
  (FK) to (or into) that table
• Foreign keys are critical to competent
  SQL database design
 Relationships – many to one
• There can be any number (many) of employees
  in each (one) department
  – Employee table has a column holding Department
    table PK value (read as “FK to Department”)
  – “No employees in department N” is again easy
• Not thought of as parent / child; you don‟t think
  of departments as “owning” employees the way
  invoices “own” line items
• An APL application could (but wouldn‟t) use
  nesting, storing a vector of EmployeeID values
  (but not Employee data) for each department
  Relationships – hierarchy
• Each employee has a manager; the
  manager is also an employee
• Employee table has a ManagerID
  column holding FK to the same
  (Employee) table
• Top of hierarchy is represented by
  null ManagerID value (no manager)
• APL could use nesting, but I haven‟t
 Relationships – many to many
• Requires a third table to hold two many-to-
  one relationships to the other two tables
• ClassStudent table records that
  – each class can have any number of students
    enrolled in it
  – each student enrolls in any number of classes
• You can‟t use nesting for this (without
  double-storing the information), so APL
  applications usually do it the same way
   More SQL Design Thoughts
• Advice: use your product‟s “identity” or
  “sequence” feature to create PK values
• When in doubt, define another table
• Examples of extra tables that add flexibility
• Storing historical data (not just the current
  value, but changes over time)
    Advice: Use DBMS-assigned
      Values as Primary Keys
• Most implementations support having the DBMS
  assign the value of a particular column to be 1+
  the previous value, during insert of a new row
• Columns defined this way make excellent PKs
  that are efficient and easily used as FKs
• Columns that are user-visible “primary keys”
  (e.g. Product Code, Region Code, Department
  Code) become attributes
  – Changing them no longer causes pain and trouble
    When In (Any) Doubt, Define
          Another Table
• When you see a numeric suffix on a
  column name, that‟s an indication that
  another table should have been built
• FK columns in an Operation table named
  Doctor1 Doctor2 Doctor3 Doctor4 would
  be a poor design
  – What if more than 4 doctors involved?
  – Better to have a child table with any number
    (even zero) of doctors for an operation
 More Tables Are Often Better
• Some cases where using more tables
  results in a more flexible system:
  – Address table where both Customer and
    Employee tables have AddressID column
    • Address table has column AddressType (FK to
      AddressType table with descriptions Billing,
      Shipping, Home, Office…)
  – Name table where Patient table has NameID
    • Column Primary in Name table marks the current
      (primary) name to use for this person
                Historical Data
• Data that changes over time where history
  needs to be maintained can be in a child table
  with a ValidUntil column, and possibly also a
  ValidSince column
  – The currently active child table row is marked by a
    null ValidUntil value
  – Find the active child table row on date X:
     (X >= coalesce( ValidSince, X )) ^
     (X <= coalesce( ValidUntil, X ))
  – Code needs to ensure that timespans don‟t overlap
  – Can store ID of active child table row in parent
                     SQL Joins
• Because data values are used to define
  relationships, SQL needs to be able to combine
  (join) tables based on the defined FK-to-table
  relations – and it can do so quite flexibly
• Each SQL statement must re-specify the join
  condition(s); tools to generate SQL code can
  make this easier to get right
• SQL supports different kinds of joins:
  – Natural join
  – Left (or right) join
  – Cross join
Time for Lunch ?
   SQL Join is What in APL?
• Similar to:
  A,B[B[;pk]A[;fkToB];]
• Natural join
   – values of A[;fkToB] that would cause
     INDEX ERROR in APL instead remove those
     rows from A (!)
   – if there are multiple rows that match in B, rows of A
     are repeated to match
• Left join: adds a row of null values to B to avoid
  losing rows of A with unmatched values:

  A,(Bnull)[B[;pk]A[;fk
  ToB];]
• Cross join: like APL outer product (usually this
  Relationships vs. Join Types
• Parent-child relationship = left join
• Lookup table = left join
• Natural join removes unmatched rows –
  make sure that‟s what you want
• Many-to-many should normally be two left
  joins from the middle table, because it‟s
  two combined one-to-many relationships
• Hierarchies are non-trivial to handle
    Given the Issues, Why Use SQL?
•   Standard outside the APL world
•   Superb support for transactions
•   Security is built-in
•   Advanced database features would be
    very difficult to provide otherwise
 SQL Databases are Standard
• Wide availability of expertise
• Backup and recovery are solved problems
• Other parts of an application can be
  developed by others or using other tools
• Many non-APL tools can work with them
  – Logical and physical data modeling
  – Reporting
  – Performance analysis
      Database Transactions
• ACID characteristics
  – Atomicity, Consistency, Isolation, Durability
• All-or-nothing data updates, ensuring data
  consistency even when there are errors
  during updates
• Each user‟s work is not affected by the
  work of other users
• Server or application software crashes
  leave data intact after “a restart
            Security Built In
• DBMS server controls all access to data
  – application can be the only way to data; or
  – can use Windows identity as SQL identity
• Access can be removed from the
  underlying tables, perhaps granting select
  access via views that use user identity
• Corporations are accustomed to securing
  their databases, but not their (APL) files
      Advanced DBMS Features
•   Flexible high-performance indexing
•   Views (including “materialized” views)
•   Sub-queries
•   Stored procedures
•   Triggers
•   XML data handling
•   Partitioned tables
•   Materialized views
          Database Indexes
• Auxiliary data structures to speed access
  to data, like the index of a book
• Can include multiple columns
• A “unique index” prevents duplicates
• A “clustered index” changes the physical
  arrangement of rows
  – Clustering a child table by the parent key
    places all child rows adjacent on disk
            Database Views
• Almost any SQL select statement can be
  saved as a view
• Views act like virtual tables
  – There is no data in a view, only in the table or
    tables referenced by the view
  – Views can be targeted by select statements
    exactly as if they were physical tables
  – In some limited cases, views can be updated
  – Users can be granted access to a view rather
    than to the table(s) named in the view
            SQL Subqueries
• SQL statements can define derived tables
  that exist only for that select
  – Like a view that exists for one statement
• Who has not purchased product X?
  select … from Customer where CustomerID not in
    (select distinct inv.CustomerID
     from Invoice as inv join LineItem as li
     on inv.InvoiceID = li.InvoiceID
     where li.Product = X)
Stored Procedures and Triggers
• Programs that run within the database
  – Stored Procedures (SPs) can be called from
    client programs, or by other SPs
  – Triggers are a special type of SP that fire
    (run) on any of insert, update, delete
  – Can be used to validate or audit
• Usually written a product-specific
  proprietary language
  – Writing in Java or .Net is becoming possible
       Support for XML Data
• Products are vying for the best support
• XML data can of course be stored as
  variable-length character data, but that
  doesn‟t require any special support
• Examples of XML support
  – Load data or create virtual tables from XML
  – Use XPath syntax to locate data within the
    XML stored in a column, and use that data to
    select rows
         Partitioned Tables
• Dividing a logical table, one defined by a
  union view, into separate physical tables
• The purpose is to divide the workload of
  accessing (or updating) the table between
  multiple servers
• Only appropriate for huge tables, or ones
  that become a bottleneck in a transaction
  processing system
           Materialized Views
• Some views are better off being computed once
  and stored, rather than being virtual (and thus
  re-computed each time they‟re accessed)
• If SQL can know how to update the view when
  the underlying data changes, this can speed up
  access to the view at the cost of storing the
  materialized view and making needed updates
• The views that are eligible to be materialized are
  fairly limited in most products today
A Quick Look at Implementation
• Usually multiple tables in each physical file
    – flexible assignment of tables to files
•   Each table‟s data is a group of pages
•   A page of data contains multiple rows
•   Pages of a table are linked together
•   Index data may be in file with table data
•   Indexes reference rows by page and
    position within the page
   Which database to choose?
• It‟s often not your choice
• If you can choose, what matters most to you?
  –   cost
  –   reliability
  –   availability of knowledgeable people
  –   performance
• If performance is critical, test lesser-known
  products; some are amazingly fast for simpler
  usage scenarios yet quite robust and stable
         Can You Choose?
• In corporate development, the data likely
  already exists in a particular database
• Only if you are building a new system (or a
  new product) are you given the opportunity
  to select the database platform
• Many customers care only that it‟s “one of
  the big ones” (Oracle, SQL Server, DB2)
• To customers, the formula is often that the
  unknown is risky, and risk is bad
      What‟s Most Important?
• Licensing costs vary dramatically
  – Pure open source is “free” for a reason
     • no support unless you pay
  – The “big three” cost an arm and a leg
     • but your company may already license it
  – Lesser-known products can be a great value
• If it‟s mission-critical, you want “big 3”
  – but it‟s still your problem (and will be said to
    be your fault) if the database fails
      What‟s Most Important?
• Sometimes you‟ll need to find expertise
  – Oracle is notorious for needing a guru
  – If any part of the application is built outside
    APL, you‟ll find people more easily if you go
    with one of the big boys
• Performance is rarely known to be critical
  – Starting out, you may have no idea if the
    DBMS‟s performance will be a problem
  – Except in extreme cases, you can handle it
      Lesser-known Products
• I‟m somewhat biased towards them
  – Sometimes support is unbelievably great
  – They‟re swimming against the tide – help
    show the world that they deserve to exist
• Using APL is already “weird” and “risky”
  – If your test results impress you as much as
    they‟ve impressed me, why not use what you
    think is the best tool, despite the “risk” of an
    unknown tool?
    APL-based or APL-aware?
• If you find a system that seems to fit well,
  and is either based on APL or has some
  special support for APL callers, why would
  you reject it automatically?
• But you might not really be “using a SQL
  RDBMS” in all such those cases, and it
  depends on why you‟re leaning towards
  SQL in the first place
Time for a Break ?
    APL Can Speak to SQL Easily
• SQAPL for Dyalog APL (uses ODBC)
    – Also available for Dyalog Unix (not free, nor are the
      best ODBC drivers for Unix)
       • DataDirect apparently has the best Unix ODBC drivers; they
         are recommended by Dyalog (others work as well)
    – Same API for APL+Win is called APL+Link
•   ADO.Net – Windows database API #6
•   ADO (original), or OLE DB, via COM/OCX
•   Write directly to some API
•   Use a home-grown DLL, OCX, or AuxProc
              SQAPL Wins
• It‟s very hard to prove that statement in
  every environment, with every workload
• Why it‟s better
  – It‟s the only interface that understands how
    APL works and inherently supports its arrays
  – Other interfaces require major work to hide
    one-row-at-a-time programming models
• Version 5 has a new mechanism that can
  provide a significant performance
  improvement (10x better has been
  reported) with a relatively small change
    Other Choices Are Inferior
• Not APL oriented
• Very “loopy” with “scalar thinking” APIs
• Only recently have APIs (in .Net) reached parity
  with SQAPL techniques from 5+ years ago!
• Why bang your head against the wall when
  SQAPL comes free with Dyalog APL?
• SQAPL is a proven, tested, robust solution
  – Put in something about the stupidity of trying to re-
    invent a tough, well-worn, smooth, very round wheel
             SQAPL Sets
• APL developers think of changes to data
  happening all at once, when an array
  value is stored (e.g. in a file component)
• Storing an APL array as SQL data
  frequently involves multiple SQL
  operations, because each SQL operation
  changes only one row at a time
• SQAPL Sets are an abstraction to make
  working with SQL data more APL-like
        Basics of Using Sets
• If you plan to modify data, start by reading
  data using one of the “set” tools; the result
  is a “set handle” and a data matrix
• Modify the matrix by adding and/or
  deleting rows, and/or updating values
• Call “set update” to store the data, passing
  the set handle and the new matrix
        How Do Sets Work?
• The retrieved data is stored, associated
  with the “set handle” that is returned to you
• When you call “set update”, APL is used to
  compare the old and new “tables” and
  make the changes by generating the
  needed SQL statements and running them
• Sets are an extraordinarily simple but
  powerful idea; take advantage of them
      SQAPL‟s Current “Sets”
  Implementation is Not Complete
• Should use SQL transactions
  – So the new version of the in-memory array
    will be successfully stored in the database, or
    the database will be unchanged
• Could support DBMS-assigned “identity”
  values
  – As I‟ve said, I think these are very useful
  – Returning the identity values back to APL is
    very important (to avoid the need to re-query)
 More Possible Enhancements
• Fix a few bugs
  – SQL handles are not closed ASAP
  – Occasional inconsistent error handling (some
    now fixed)
• Could easily support multi-column keys
  (but I prefer to use identity values as keys)
• Could support optimistic concurrency
• Could support automatic updates to
  related (child) tables
      Database-Specific Tools
• SQAPL provides the tools to talk to various SQL
  databases. (Remember that they only
  understand SQL statements.)
• A desirable tool would be one to improve the
  performance of Set Update by passing data to
  be updated in fewer steps, perhaps as XML.
• With SQL Server in particular, sending XML data
  for insert and update can let them operate in
  bulk, rather than one row at a time. Impressive
  performance gains would be quite likely.
   Improving SQAPL‟s API
Working with an APL+Win client, I did
major design and implementation work on
an object-oriented layer built on the
SQAPL code. This included making most
of the enhancements discussed above,
adding major features for calling stored
procedures, creating and modifying tables,
and much more. I hope to have a similar
opportunity in Dyalog APL.
        Performance Issues
• Most of these are SQL / DBMS issues, not
  issues of the APL interface to SQL
  – LAN vs WAN makes a huge difference
    (latency)
  – Consider using a “middle tier” in WAN case
• The nature of these issues varies wildly
  from one DBMS to another
  – The same SQL statement may be very fast in
    one implementation, very slow in another
   Figuring Out What‟s Wrong
• APL is the wrong tool for analyzing
  performance problems
  – APL can‟t see what the DBMS is doing
  – APL can‟t see the system-level slowdowns
  – Non-APL tools exist for no other purpose
• If the problem is “too much back and
  forth”, can you find a way to “blast” the
  data across in fewer steps?
       Metadata Based Tools
• Most APL applications are based on, or at least
  use, metadata – data about the data being
  manipulated. APLers use these techniques
  naturally. It surprises me how infrequently other
  programmers in other languages seem to do so.
• Tools that know your database structure can be
  used for many things. For example, if the table
  hierarchy is available as metadata, a routine can
  be written to produce the SQL where clause to
  join an arbitrary list of tables.
              Other Tools
• Knowing the datatype of database table
  columns can simplify creating UI screens.
• Knowing the relationship between tables
  can let (tools called by) model code
  retrieve related data automatically.
• Automating changes to the database
  schema given metadata is possible – I
  know, I‟ve done it. (It‟s not trivial!)
           Lessons Learned
• Always (except that one should never say
  always) use identity values as PKs
  – Unless you can prove that you have one of
    the (very rare, in my opinion) cases where
    there‟s a real reason not to
  – GUID PKs are an alternative to identity
    values, but …
    • they are comparatively “fat” vs. integers
    • the problems with identity values are overstated
    • the “any location” benefit is overstated
             More Lessons
• Always write APL that writes SQL
  – Build tools to help you do so
• Don‟t hide SQL from APL developers
• Table names should be singular (my
  opinion); not all people agree
• Try not to base your design on the ability
  of your (current) DBMS to handle it well
     Unproven (by me) Ideas
• These are thoughts that haven‟t been fully
  implemented (at least not by me in APL),
  so be aware of that before basing your
  development on them.
• They represents some potential “best
  practices” that borrow significantly from
  work done by others, mostly in other
  languages.
 Model Definition is Often Hazy
• The line between UI, data model, and
  database can be unclear.
• Data manipulated by the UI of a form is
  most easily stored in attributes of the form.
• Many programs have a “UI data model”
  that‟s separate from the model of the data
  as stored persistently (whether or not it‟s
  stored in a SQL database).
Problems Caused by UI Models
• Changes must be coordinated
  – When model data is stored in the UI, making changes
    to either the UI or the data definition forces them to
    change at the same time.
• UI validation code works from the UI model
  – Code for event-handling in the UI naturally does BR
    (business rule) validation using UI-stored data.
• Validation logic gets repeated
  – The model must enforce BRs to prevent bad data
    from being stored, so logic gets repeated.
      Use Model Code in the UI
• When a model‟s data is simple – and the power of APL
  to manipulate data structures makes that common when
  writing in APL – it can be tempting to do the manipulation
  directly.
• User interface code should avoid that, instead invoking
  methods of the model even though they may be trivial.
• Consider a web app – server-side code can‟t rely on
  data being correct when submitted, so it must do
  validation.
• Code for browser-side validation either repeats the logic
  or consults the server. Ideally, the latter is done
  asynchronously (using an Ajax-style model).
    UI / Data Model Separation
• It is clear that the user interface code is best
  kept separate from (a client of) data models.
• This results in models that are easier to test, and
  easier to re-use in other contexts.
• When the UI needs to implement a Save action,
  it can be tempting to use your app‟s data storage
  APIs directly in the code that gathers the data
  from where it‟s stored in the UI.
• Better would be to call a model‟s Save method.
  UI Forms Reference a Model
• UI forms should have a reference to a
  model object.
• The model should be able to hold data
  about multiple data instances (database
  table rows) – this is APL, after all.
• The model can have documented events
  to which the UI can subscribe.
• UI data should be passed to the model as
  soon as possible.
 Unified Memory / Database Model
• The model should know about the state of the
  data it holds relative to the database.
  – Is it in the database (vs. new data)?
     • If it is, has it changed since then?
  – Is it currently valid (ready to be saved)?
     • If not, what error message data is there?
• One good source of design ideas is Scott
  Ambler‟s ActiveRecord model. It is only slightly
  array-oriented, so it is not ideal unchanged.
          Writing Models
Write model methods on an as-needed
basis. APL programmers often work to
create tools that have much more
functionality than is needed today. When
writing models, put required functionality
into lower-level tools (creating them as
needed) and keep only high-level logic,
and calls to the lower-level tools, in the
models. Maximize the work not done.
 A Big Thank You to Morten
I would not be here if Morten had not
asked me to deliver this talk. I had not
seen Dyalog APL in more than 20 years,
having used only APL*PLUS (now APL+)
for all that time. I appreciate seeing where
Dyalog APL is today, and I‟m always
happy to visit Denmark. I‟ve enjoyed
what‟s felt like a warm welcome from this
community. I thank both you and Morten
for your generosity.