Paweł Wiejkut developer,logistyk i pseudo elektronik :)
All of you probably at least once in your life try to insert some records to a database table, but is anyone of you ever try to understand how Oracle actually saves the entries in the database? Last time I was making a small experiment and I want to share my results with you.
Step one, let’s build a small table and insert a few records:
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CREATE TABLE tab1 (c1 VARCHAR(3), c2 number(3)) |
Now, let’s insert some data to table:
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INSERT INTO tab1 VALUES('AAA',111); INSERT INTO tab1 VALUES('AAA',222); INSERT INTO tab1 VALUES('AAA',333); INSERT INTO tab1 VALUES('AAA',444); INSERT INTO tab1 VALUES('BBB',111); INSERT INTO tab1 VALUES('BBB',222); INSERT INTO tab1 VALUES('BBB',333); INSERT INTO tab1 VALUES('BBB',444); INSERT INTO tab1 VALUES('CCC',111); INSERT INTO tab1 VALUES('CCC',222); INSERT INTO tab1 VALUES('CCC',333); INSERT INTO tab1 VALUES('CCC',444); |
After that my table content looks like below:
Let’s start some fun and check how my dump from Oracle DB looks like. First of all, I need to find the block where the table is stored:
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SELECT segment_name,header_file, header_block+1 FROM dba_segments WHERE segment_name = 'TAB1'; |
Create a dump from the selected block:
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alter system dump datafile 1 block 102161; |
Now I have to run the console, ssh into the remote system and check last files in Oracle directory:
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cd /u01/app/oracle/diag/rdbms/orcl12c/orcl12c/trace find $1 -type f -exec stat --format '%Y :%y %n' "{}" \; | sort -nr | cut -d: -f2- | head # cat orcl12c_ora_10640.trc |
Section of my dump looks like below:
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block_row_dump: tab 0, row 0, @0x1f95 tl: 11 fb: --H-FL-- lb: 0x1 cc: 2 col 0: [ 3] 41 41 41 col 1: [ 3] c2 02 0c tab 0, row 1, @0x1f8a tl: 11 fb: --H-FL-- lb: 0x1 cc: 2 col 0: [ 3] 41 41 41 col 1: [ 3] c2 03 17 tab 0, row 2, @0x1f7f tl: 11 fb: --H-FL-- lb: 0x1 cc: 2 col 0: [ 3] 41 41 41 col 1: [ 3] c2 04 22 tab 0, row 3, @0x1f74 tl: 11 fb: --H-FL-- lb: 0x1 cc: 2 col 0: [ 3] 41 41 41 col 1: [ 3] c2 05 2d tab 0, row 4, @0x1f69 tl: 11 fb: --H-FL-- lb: 0x1 cc: 2 col 0: [ 3] 42 42 42 col 1: [ 3] c2 02 0c tab 0, row 5, @0x1f5e tl: 11 fb: --H-FL-- lb: 0x1 cc: 2 col 0: [ 3] 42 42 42 col 1: [ 3] c2 03 17 tab 0, row 6, @0x1f53 tl: 11 fb: --H-FL-- lb: 0x1 cc: 2 col 0: [ 3] 42 42 42 col 1: [ 3] c2 04 22 tab 0, row 7, @0x1f48 tl: 11 fb: --H-FL-- lb: 0x1 cc: 2 col 0: [ 3] 42 42 42 col 1: [ 3] c2 05 2d tab 0, row 8, @0x1f3d tl: 11 fb: --H-FL-- lb: 0x1 cc: 2 col 0: [ 3] 43 43 43 col 1: [ 3] c2 02 0c tab 0, row 9, @0x1f32 tl: 11 fb: --H-FL-- lb: 0x1 cc: 2 col 0: [ 3] 43 43 43 col 1: [ 3] c2 03 17 tab 0, row 10, @0x1f27 tl: 11 fb: --H-FL-- lb: 0x1 cc: 2 col 0: [ 3] 43 43 43 col 1: [ 3] c2 04 22 tab 0, row 11, @0x1f1c tl: 11 fb: --H-FL-- lb: 0x1 cc: 2 col 0: [ 3] 43 43 43 col 1: [ 3] c2 05 2d end_of_block_dump |
If we look closer at block_row_dump, you will see some repeatability. Let’s check what is the meaning of lines with c2 at the start.
To achieve this, we can just execute SQL statement below:
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var n number; exec dbms_stats.convert_raw_value('c2020c',:n); print |
and a result of this statement is:
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111 |
Value of raw ‚c2 03 17’ is : 222. You can check all these values and everything will look like our inserts on the beginning. To be more precise we are actually on „row” level of database storage:
That’s all for today, this is a basic knowledge related to database data save in tables. This knowledge is a key to understand how to works indexing or compression on the database.
