Question

I have a Silverlight app that makes multiple (often concurrent) asynchronous calls to an Oracle database. The largest database table stores around 5 million records. Below is a summary of how the Silverlight app works, followed by my question.

1. The user sets query criteria to select a particular group of records, usually 500 to 5000 records in a group.

2. An asynchronous WCF call is made to the database to retrieve the values in four fields (latitude, longitude, heading, and time offset) over the selected group of records (meaning the call returns anywhere from 2k to 20k floating point numbers. These values are used to plot points on a map in the browser.

3. From here, the user can choose to graph the values in one or more of an additional twenty or so fields associated with the initial group of records. The user clicks on a field name, and another async WCF call is made to retrieve the field values.

My question is this: does it make sense in this case to store the records selected in step one in a temp table (or materialized view) in order to speed up and simplify the data access in step three?

If so, can anyone give me a hint regarding a good way to maintain the browser-to-temp-table link for a user’s session?

Right now, I am just re-querying the 5 million points each time the user selects a new field to graph–which works until the user selects three or more fields at once. This causes the async calls to timeout before they can return.

Answer 1

We can do this using a CONTEXT. This is a namespace in session memory which we can use to store values. Oracle comes with a default namespace, ‘USERENV’, but we can define our own. The context has to be created by a user with the CREATE ANY CONTEXT privilege; this is usually a DBA. The statement references a PACKAGE which sets and gets values in the namespace, but this package does not have to exist in order for the statement to succeed:

SQL> create context user_ctx using apc.ctx_pkg
  2  /

Context created.

SQL>

Now let’s create the package:

SQL> create or replace package ctx_pkg
  2  as
  3      procedure set_user_id(p_userid in varchar2);
  4      function get_user_id return varchar2;
  5      procedure clear_user_id;
  6  end ctx_pkg;
  7  /

Package created.

SQL>

There are three methods, to set, get and unset a value in the namespace. Note that we can use one namespace to hold different valiables. I am just using this package to set one variable (USER_ID) in the USER_CTX namespace.

SQL> create or replace package body ctx_pkg
  2  as
  3      procedure set_user_id(p_userid in varchar2)
  4      is
  5      begin
  6         DBMS_SESSION.SET_CONTEXT(
  7          namespace  => 'USER_CTX',
  8          attribute  => 'USER_ID',
  9          value      => p_userid);
 10      end set_user_id;
 11
 12      function get_user_id return varchar2
 13      is
 14      begin
 15          return sys_context('USER_CTX', 'USER_ID');
 16      end get_user_id;
 17
 18      procedure clear_user_id
 19      is
 20      begin
 21         DBMS_SESSION.CLEAR_CONTEXT(
 22          namespace  => 'USER_CTX',
 23          attribute  => 'USER_ID');
 24      end clear_user_id;
 25
 26  end ctx_pkg;
 27  /

Package body created.

SQL>

So, how does this solve anything? Here is a table for the temporary storage of data. I’m going to add a column which will hold a token to identify the user. When we populate the table the value for this column will be provided by CTX_PKG.GET_USER_ID():

SQL> create table temp_23 as select * from big_table
  2  where 1=0
  3  /

Table created.

SQL> alter table temp_23 add (user_id varchar2(30))
  2  /

Table altered.

SQL> create unique index t23_pk on temp_23(user_id, id)
  2  /

Index created.

SQL>

… and over that table I create a view:…

create or replace view v_23 as
select 
         id
        , col1
        , col2
        , col3
        , col4
from temp_23
where user_id = ctx_pkg.get_user_id
/

Now, when I want to store some data in the table I need to set the context with a value with uniquely identifies my user.

SQL> exec ctx_pkg.set_user_id('APC')

PL/SQL procedure successfully completed.

SQL>

This statement populates the temporary table with twenty random rows:

SQL> insert into temp_23
  2  select * from
  3      ( select b.*, ctx_pkg.get_user_id
  4        from big_table b
  5        order by dbms_random.random )
  6  where rownum <= 20
  7  /

20 rows created.

SQL>

I can retrieve those rows by querying the view. But when I change my USER_ID and run the same query I cannot see them any more:

SQL> select * from v_23
  2  /

        ID       COL1 COL2                           COL3            COL4
---------- ---------- ------------------------------ --------- ----------
    277834       1880 GV_$MAP_EXT_ELEMENT            15-OCT-07       4081
    304540      36227 /375c3e3_TCPChannelReaper      15-OCT-07         36
   1111897      17944 /8334094a_CGCast               15-OCT-07         17
   1364675      42323 java/security/PublicKey        15-OCT-07         42
   1555115       3379 ALL_TYPE_VERSIONS              15-OCT-07          3
   2073178       3355 ALL_TYPE_METHODS               15-OCT-07          3
   2286361      68816 NV                             15-OCT-07         68
   2513770      59414 /5c3965c8_DicomUidDoc          15-OCT-07         59
   2560277      66973 MGMT_MNTR_CA                   15-OCT-07         66
   2700309      45890 /6cc68a64_TrustManagerSSLSocke 15-OCT-07         45
   2749978       1852 V_$SQLSTATS                    15-OCT-07       6395
   2829080      24832 /6bcb6225_TypesTypePair        15-OCT-07         24
   3205157      55063 SYS_NTsxSe84BlRX2HiXujasKy/w== 15-OCT-07         55
   3236186      23830 /de0b4d45_BaseExecutableMember 15-OCT-07         23
   3276764      31296 /a729f2c6_SunJCE_n             15-OCT-07         31
   3447961      60129 HHGROUP                        15-OCT-07         60
   3517106      38204 java/awt/im/spi/InputMethod    15-OCT-07         38
   3723931      30332 /32a30e8e_EventRequestManagerI 15-OCT-07         30
   3877332      53700 EXF$XPVARCLST                  15-OCT-07         53
   4630976      21193 oracle/net/nl/NetStrings       15-OCT-07         21

20 rows selected.

SQL> exec ctx_pkg.set_user_id('FOX_IN_SOCKS')

PL/SQL procedure successfully completed.

SQL> select * from v_23
  2  /

no rows selected

SQL>

---

So, the challenges are:

1. to establish a token which you can use automatically to uniquely identify a user
2. to find a hook in your connecting code which can set the context each time the user gets a session
3. just as importantly, to find a hook in your dis-connecting code which can unset the context each time the user leaves a session

Also, remember to clear out the table once the user has finished with it.