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From liuyu000 <...@git.apache.org>
Subject [GitHub] incubator-trafodion pull request #1229: [Trafodion-2734] Create *Trafodion L...
Date Fri, 15 Sep 2017 02:43:58 GMT
Github user liuyu000 commented on a diff in the pull request:

    https://github.com/apache/incubator-trafodion/pull/1229#discussion_r139053292
  
    --- Diff: docs/lob_guide/src/asciidoc/_chapters/work_with_lob.adoc ---
    @@ -0,0 +1,738 @@
    +////
    +/**
    +* @@@ START COPYRIGHT @@@
    +*
    +* Licensed to the Apache Software Foundation (ASF) under one
    +* or more contributor license agreements. See the NOTICE file
    +* distributed with this work for additional information
    +* regarding copyright ownership. The ASF licenses this file
    +* to you under the Apache License, Version 2.0 (the
    +* "License"); you may not use this file except in compliance
    +* with the License. You may obtain a copy of the License at
    +*
    +*   http://www.apache.org/licenses/LICENSE-2.0
    +*
    +* Unless required by applicable law or agreed to in writing,
    +* software distributed under the License is distributed on an
    +* "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
    +* KIND, either express or implied. See the License for the
    +* specific language governing permissions and limitations
    +* under the License.
    +*
    +* @@@ END COPYRIGHT @@@
    +*/
    +////
    +
    +[#work with lob]
    += Work with LOB
    +
    +[#create a sql table with lob columns]
    +== Create a SQL Table with LOB Columns
    +
    +When creating a SQL table with LOB columns, following relevant tables and files are created
as well:
    +
    +* One LOB MD table.
    +* Two dependent descriptor tables.
    +* HDFS data file (locates at /user/trafodion/lobs) for each column.
    +
    +[#syntax]
    +== Syntax
    +
    +```
    +CREATE TABLE table-name (lob-column-spec[, lob-column-spec]…)
    +```
    +
    +```
    +lob-column-spec is:
    +column {lob type}
    +
    +lob type is:
    +BLOB | CLOB [({numeric literal} [unit])] [STORAGE 'storage literal']
    +
    +unit is:
    +empty | 
    +K     | 
    +M     | 
    +G 
    +```
    +
    +[#semantics]
    +=== Semantics
    +
    --- End diff --
    
    1. STORAGE clause 
    
    Syntax:
    ...
    lob-type is:
    BLOB | CLOB [({numeric literal} [unit])] [STORAGE 'storage literal']
    ...
    
    So the STORAGE clause is optional, it's ok if you don't specify it (the attachment
    ![create a sql table with lob columns](https://user-images.githubusercontent.com/20532956/30423142-8cfd079a-9974-11e7-9eb9-151183271ee5.jpg)
    ).
    
    2.  limit on LOB size
    I've asked Sandhya the some question, the answer from her:
    
    Minimum size:
    We allow for an “empty” LOB so technically that’s a  0 length lob. 
    We can insert using empty_blob(), empty_clob()  syntax.
    
    Maxmum size:
    Currently this is set to 10GB and it's controlled by the CQD LOB_MAX_SIZE. 
    But this can be changed depending on the customers needs to whatever they want. 
    There isn’t a real limit as such so we could go to 8 terabytes if needed but the user
will have to configure enough HDFS disks to be able to support such a large LOB and several
of them potentially. 


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