INTRODUCTION

This package contains two User-Defined Load functions, ODBCSource() and ODBCLoader(), that can be used to:

• load data directly from a remote database
• run queries against remote databases from Vertica (for example to join local Vertica-managed tables with MySQL, PostgreSQL, etc.)

Data retrieved from external databases is neither converted into an intermediate file formats nor landed to disk; the data is fetched over the network via ODBC and copied directly into Vertica as it is received. When used with Vertica versions 10.1 and above predicate pushdown and column filtering is applied to the remote database data extraction process.

INSTALLING / UNINSTALLING

Preprequisites

In order to install the ODBCLoader package you need to install on all nodes of your Vertica cluster:

• an ODBC Driver Manager. This library has been tested with unixODBC. You need to install the development libraries (for example: yum install unixODBC-devel)
• the ODBC Drivers to interface the remote databases
• Perl Compatible Regular Expression library (yum install pcre-devel pcre-cpp)

In order to compile the ODBCLoader you also have to setup a development environment as defined in the standard documentation (Setting Up a Development Environment).

Building and installing the library

This operation has to be executed as dbadmin from one of the node of the cluster (Vertica will propagate the libraries across the cluster):

$ make
$ make install

Please check/modify ddl/install.sql to build the library "FENCED" or "UNFENCED"

Uninstalling the library

To remove the ODBCLoader library from your cluster:

$ make uninstall

Usage

The ODBCLoader can be used to load data from external databases or to query non-Vertica databases through external tables.

Data loading

The general syntax to load data from external databases via ODBCLoader is:

COPY myschema.myverticatable 
    WITH SOURCE ODBCSource() PARSER ODBCLoader(
        connect='DSN=some_odbc_dsn;<other connection parameters>',  
        query='select * from remote_table',
        [rowset=<number>]
    )
;

where rowset is an optional parameter to define the number of rows fetched from the remote database in each SQLFetch() call (default = 100). Increasing this parameter can improve the performance but will also increase memory usage.

This will cause Vertica to connect to the remote database identified by the given "connect" string and execute the given query. It will then fetch the results of the query and load them into the table myschema.myverticatable.

myverticatable must have the same number of columns as remote_table. The column types must also match up, or the ODBC driver for the remote database must be able to cast the column types to the Vertica types. If necessary, you can always explicitly cast on the remote side by modifying the query, or on the local side with a Vertica COPY expression.

The query argument to ODBCLoader can be any valid SQL query, or any other statement that the remote database will recognize and that will cause it to return a rowset.

The connect argument to ODBCLoader can be any valid ODBC connect string. It is common to configure /etc/odbc.ini and /etc/odbcinst.ini with all the necessary information, then simply reference the DSN listing in /etc/odbc.ini in each query. For help configuring these files, or for more information on valid 'connect' strings, please see the documentation that came with the ODBC driver for the remote database product that you are connecting to, as the format of the string is specified by the driver.

Federated queries

As we said we can use the ODBCLoader to run federated queries against other databases (for example to join Vertica tables with MySQL tables) taking advantage of both predicate pushdown and column pruning in order to move from the external database to Vertica only the data really needed.

To use this feature we Use Vertica External Tables as a gateway. Let's use an example to illustrate the process. External Tables, differently from Vertica-managed tables, leave the data outside Vertica In the following example we define an External Table in Vertica (public.epeople) retrieving data from the ODBC Data Source "pmf" through the query SELECT * FROM public.people:

CREATE  EXTERNAL  TABLE  public.epeople(
    id INTEGER,
    name VARCHAR(20)
) AS  COPY  WITH
    SOURCE  ODBCSource()
    PARSER  ODBCLoader(
        connect='DSN=pmf',
        query='SELECT * FROM public.people'
) ;

When you define the External Table nothing is retrieved from the external database; we just save - in Vertica - the information needed to extract the data from the external source.

Now, if we run, in Vertica, a query like this:

SELECT * FROM public.epeople WHERE id > 100;

The ODBCLoader will rewrite the original query defined in the previous External Table definition as follow:

SELECT id, name FROM public.people WHERE id > 100;

As you can see this query will pushdown the predicate to the external database.

The other important feature to limit the amount of data being moved from the external database to Vertica is columns pruning. This means we extract from the external database only the columns needed to run the query in Vertica. As we have to retrieve all columns defined in the External Table, the ones not needed in the Vertica query will be replaced by NULL. So, for example, if we run, in Vertica:

SELECT id FROM public.epeople WHERE id > 100;

the following query will be executed against the external database:

SELECT id, NULL FROM public.people WHERE id > 100;

Optional configuration switches

The ODBCLoader accept, in the External Table definition, the following, optional, configuration switches.

Amount of rows retrieved during each SQLFetch() iteration from the external database. The default value for this parameter is 100 and you can alter it in the CREATE EXTERNAL TABLE statement by defining a different rowset. For example:

CREATE  EXTERNAL  TABLE  public.epeople(
    id INTEGER,
    name VARCHAR(20)
) AS  COPY  WITH
    SOURCE  ODBCSource()
    PARSER  ODBCLoader(
        connect='DSN=pmf',
        query='SELECT * FROM public.people',
        rowset = 500
) ;

Please consider that increasing this parameter will also increase the memory consumption of the ODBCLoader.

You can switch predicate pushdown on/off with the optional boolean parameter src_rfilter. The default value is true (meaning we perform predicate pushdown). You can set src_rfilter either in the CREATE EXTERNAL TABLE statement or using a SESSION PARAMETER as follows:

ALTER SESSION SET UDPARAMETER FOR ODBCLoaderLib src_rfilter = false ;

You can switch columns filtering on/off with the optional boolean parameter src_cfilter. The default value is true (meaning we perform predicate pushdown). You can set src_cfilter either in the CREATE EXTERNAL TABLE statement or using a SESSION PARAMETER as follows:

ALTER SESSION SET UDPARAMETER FOR ODBCLoaderLib src_cfilter = false ;

You can overwrite the query defined in the original CREATE EXTERNAL TABLE statement using the session parameter override_query as follows:

ALTER SESSION SET UDPARAMETER FOR ODBCLoaderLib override_query = '
    SELECT * FROM people WHERE id % 2
'

The default length for session parameters is 2kB so, if your override_query is longer you might need to increase the session parameter max length before setting override_query. For example:

ALTER SESSION SET MaxSessionUDParameterSize = 16384 ;
ALTER SESSION SET UDPARAMETER FOR ODBCLoaderLib override_query = '
     SELECT 00 AS id, name, gender,bdate FROM mmf.mypeople UNION ALL 
     SELECT 01 AS id, name, gender,bdate FROM mmf.mypeople UNION ALL 
     SELECT 02 AS id, name, gender,bdate FROM mmf.mypeople UNION ALL
     SELECT 03 AS id, name, gender,bdate FROM mmf.mypeople UNION ALL
     SELECT 04 AS id, name, gender,bdate FROM mmf.mypeople UNION ALL
... long query here ...
'

Please note: session parameters are - of course - session-scoped. Have a look to the standard Vertica documentation to learn how to set/clear/check User-Defined Session Parameters.

Pushed-down predicates conversion

When pushing predicates down to the external database, ODBCLoader performs the following changes:

1. Vertica specific data type casting ::<data type> will be removed
2. ~~ will be converted to LIKE
3. ANY(ARRAY) will be converted to IN(). For example ID = ANY(ARRAY[1,2,3]) will be converted to ID IN(1,2,3)

Database Specific Notes

ORACLE. All integers in Vertica are 64-bit integers. Oracle doesn't support 64-bit integers; their ODBC driver can't even cast to them on request. This code contains a quirk/workaround for Oracle that retrieves integers as C strings and re-parses them. However, the quirk doesn't reliably detect Oracle database servers right now. You can force Oracle with an obvious modification to the setQuirksMode() function in ODBCLoader.cpp. If you know of a more-reliable way to detect Oracle, or a better workaround, patches welcome :-)

MYSQL. The MySQL ODBC driver comes in both a thread-safe and thread-unsafe build and configuration. The thread-unsafe version is KNOWN TO CRASH VERTICA if used in multiple COPY statements concurrently! (Vertica is, after all, highly multithreaded). Linux distributions aren't consistently careful to package thread-safe defaults. So if you're connecting to MySQL, be very careful to set up a thread-safe configuration.

VERTICA. If you have to COPY data from one Vertica cluster to another use the Vertica's built-in IMPORT/EXPORT capabilities which are dramaticaly faster

DEBUGGING

ODBC layer tracing

The simpler way to check how the ODBCLoader rewrite the query sent to the external database is to enable ODBC traces in odbcinst.ini. For example:

[ODBC]
Trace=on
Tracefile=/tmp/uodbc.trc

And then grep the SQL from the trace file:

$ tail -f /tmp/uodbc.trc | grep 'SQL = '

Please remember to switch ODBC traces off at the end of your debug session because they will slowdown everything and create huge log files...

ODBC_Loader DEBUG

If the ODBC tracing was not enough you can (re)compie this library with LOADER_DEBUG flag set to 1 as shown here:

$ rm -rf build && make install LOADER_DEBUG=1

this will print extra messages in the Vertica log files (either UDxLogs/UDxFencedProcesses.log or vertica.log depending if the library was "FENCED" or "UNFENCED"). Caution: don't do this in production because it will flood your logs with debug messages and slowdown everything.

PCRE Missing symbols

The following error has been reported, during the deloyment phase, on a few Linux Distributions:

undefined symbol: _ZNK7pcrecpp2RE13GlobalReplaceERKNS_11StringPieceEPSs

To fix this issue you might want to...

STEP 1: get rid of the standard pcre packages: Remove pcre-devel and pcre-cpp packages (if installed) using the appropriate package management commands. For example:

# yum remove pcre-devel pcre-cpp

STEP 2: install PCRE from sources:

# tar xzvf pcre-8.45.tar.gz 
# cd pcre-8.45
# ./configure CXXFLAGS='-std=c++11 -D_GLIBCXX_USE_CXX11_ABI=0'
# make && make install

STEP 3: update you ld.so config and recreate its cache:

# echo "/usr/local/lib" > /etc/ld.so.conf.d/local.conf && rm /etc/ld.so.cache && ldconfig

Sample ODBC Configurations

The following two configuration files odbc.ini and odbcinst.ini have been used to define two data sources: pmf to connect to PostgreSQL and mmf to connect to MySQL:

$ cat /etc/odbc.ini
[ODBC Data Sources]
PSQLODBC  = PostgreSQL ODBC
MYODBC  = MySQL ODBC

[pmf]
Description  = PostgreSQL mftest2
Driver = PSQLODBC
Trace  = No
TraceFile  = sql.log
Database = pmf
Servername = mftest2
UserName =
Password =
Port = 5432
SSLmode  = allow
ReadOnly = 0
Protocol = 7.4-1
FakeOidIndex = 0
ShowOidColumn  = 0
RowVersioning  = 0
ShowSystemTables = 0
ConnSettings =
Fetch  = 1000
Socket = 4096
UnknownSizes = 0
MaxVarcharSize = 1024
MaxLongVarcharSize = 8190
Debug  = 0
CommLog  = 0
Optimizer  = 0
Ksqo = 0
UseDeclareFetch  = 0
TextAsLongVarchar  = 1
UnknownsAsLongVarchar  = 0
BoolsAsChar  = 1
Parse  = 0
CancelAsFreeStmt = 0
ExtraSysTablePrefixes  = dd_
LFConversion = 0
UpdatableCursors = 0
DisallowPremature  = 0
TrueIsMinus1 = 0
BI = 0
ByteaAsLongVarBinary = 0
LowerCaseIdentifier  = 0
GssAuthUseGSS  = 0
XaOpt  = 1
UseServerSidePrepare = 0

[mmf]
Description  = MySQL mftest2
Driver = MYODBC
SERVER = mftest2
PORT = 3306
SQL-MODE = 'ANSI_QUOTES'

$ cat /etc/odbcinst.ini
[ODBC]
Trace=off
Tracefile=/tmp/uodbc.trc

[PSQLODBC]
Description=PostgreSQL ODBC Driver
Driver64=/usr/lib64/psqlodbcw.so
UsageCount=1

[MYODBC]
Driver=/usr/lib64/libmyodbc8w.so
UsageCount=1

[MySQL ODBC 8.0 ANSI Driver]
Driver=/usr/lib64/libmyodbc8a.so
UsageCount=1

License

Apache 2.0 License, please see LICENSE for details.