The document discusses database indexing and summarizes the results of a short quiz about indexing techniques. The quiz contains 5 questions that test knowledge of indexing technologies like index column order, indexing date fields, and indexing fields with wildcards. Taking the time to properly learn and apply indexing is important for optimizing database performance, but indexing is often neglected. The presenter is an expert on database performance tuning who provides training and writes on the topic.

1. Unterschätzte Details der
Datenbank-Indizierung
(und deren Auswirkung)
Vienna System Architects Meetup
2013-12-02
© 2013 by Markus Winand

2. Was weiß man über Indizierung?
Ergebnisse des “3-Minuten Tests”
http://use-the-index-luke.com/de/3-minuten-test
5 Fragen: Jede prüft das Wissen zu einer
bestimmen Indizierungs-Technik ab.

3. 3-Minute Quiz: Indexing Skills
Q1: Good or Bad?
(Function use)
CREATE
INDEX
tbl_idx
ON
tbl
(date_column);
SELECT
text,
date_column
FROM
tbl
WHERE
TO_CHAR(date_column,
'YYYY')
=
'2013';

4. 3-Minute Quiz: Indexing Skills
...WHERE
TO_CHAR(date_column,
'YYYY')
=
'2013';
Seq
Scan
on
tbl
(rows=365)
Rows
Removed
by
Filter:
49635
Total
runtime:
118.796
ms
...WHERE
date_column
BETWEEN
'2013-­‐01-­‐01'
AND
'2013-­‐12-­‐31'
Index
Scan
using
tbl_idx
on
tbl
(rows=365)
Total
runtime:
0.430
ms
(Above: simplified PostgreSQL execution plans when selecting 365 rows out of 50000)

5. 3-Minute Quiz: Indexing Skills
Q2: Good or Bad? (Indexed Top-N, no IOS)
CREATE
INDEX
tbl_idx
ON
tbl
(a,
date_col);
SELECT
id,
a,
date_col
FROM
tbl
WHERE
a
=
?
ORDER
BY
date_col
DESC
LIMIT
1;
Understandable
controversy!

6. 3-Minute Quiz: Indexing Skills
It is already the most optimal solution
(w/o index-only scan).
Limit
(rows=1)
-­‐>
Index
Scan
Backward
using
tbl_idx
on
tbl
(rows=1)
Index
Cond:
(a
=
123::numeric)
Total
runtime:
0.053
ms
As fast as a primary key lookup because it
can never return more than one row.

7. 3-Minute Quiz: Indexing Skills
Q3: Good or Bad?
(Column order)
CREATE INDEX tbl_idx ON tbl (a, b);
SELECT id, a, b FROM tbl
WHERE a = ? AND b = ?;
SELECT id, a, b FROM tbl
WHERE b = ?;

8. 3-Minute Quiz: Indexing Skills
As-is only one query can use the index (a,b):
...WHERE a = ? AND b = ?;
Bitmap
Heap
Scan
on
tbl
(rows=6)
-­‐>
Bitmap
Index
Scan
on
tbl_idx
(rows=6)
Index
Cond:
((a
=
123)
AND
(b
=
1))
Total
runtime:
0.055
ms
...WHERE b = ?;
Seq
Scan
on
tbl
(rows=5142)
Rows
Removed
by
Filter:
44858
Total
runtime:
29.849
ms

9. 3-Minute Quiz: Indexing Skills
Change the index to (b, a) so both can use it:
...WHERE a = ? AND b = ?;
Bitmap
Heap
Scan
on
tbl
(rows=6)
-­‐>
Bitmap
Index
Scan
on
tbl_idx
(rows=6)
Index
Cond:
((a
=
123)
AND
(b
=
1))
Total
runtime:
0.056
ms
...WHERE b = ?;
Bitmap
Heap
Scan
on
tbl
(rows=5142)
-­‐>
Bitmap
Index
Scan
on
tbl_idx
(rows=5142)
Index
Cond:
(b
=
1::numeric)
Total
runtime:
6.932
ms

10. 3-Minute Quiz: Indexing Skills
Q4: Good or Bad?
CREATE
ON
SELECT
FROM
WHERE
INDEX tbl_idx
tbl (text);
id, text
tbl
text LIKE '%TERM%';
(Indexing LIKE)

11. 3-Minute Quiz: Indexing Skills
B-Tree indexes don’t support prefix wildcards.
Seq
Scan
on
tbl
(rows=0)
Rows
Removed
by
Filter:
50000
Total
runtime:
23.494
ms
Using a special purpose index (e.g. in PostgreSQL):
CREATE
INDEX
tbl_tgrm
ON
tbl
USING
gin
(text
gin_trgm_ops);
Bitmap Heap Scan on tbl (rows=0)
Rows Removed by Index Recheck: 2
-> Bitmap Index
Scan on tbl_tgrm (rows=2)
Index Cond: ((text)::text ~~ '%TERM%'::text)
Total runtime: 0.114
ms

12. 3-Minute Quiz: Indexing Skills
Q5: Good or Bad?
(equality vs. ranges)
CREATE INDEX tbl_idx
ON tbl (date_col, state);
SELECT id, date_col, state FROM tbl
WHERE date_col >= TO_DATE(‘2008-12-02’)
AND state = 'X';

13. 3-Minute Quiz: Indexing Skills
For the curious, the data distribution:
SELECT
count(*)
FROM
tbl
WHERE
date_column
>=
TO_DATE(‘2008-­‐12-­‐02’);
-­‐-­‐-­‐>
1826
SELECT
count(*)
FROM
tbl
WHERE
state
=
'X';
-­‐-­‐-­‐>
10000
SELECT
count(*)
FROM
tbl
WHERE
date_column
>=
TO_DATE(‘2008-­‐12-­‐02’)
AND
state
=
'X';
-­‐-­‐-­‐>
365

14. 3-Minute Quiz: Indexing Skills
CREATE INDEX tbl_idx
ON tbl (date_col, state);
Index
Scan
using
tbl_idx
on
tbl
(rows=365)
Total
runtime:
0.893
ms
CREATE INDEX tbl_idx
ON tbl (state, date_col);
Index
Scan using
tbl_idx
on
tbl
(rows=365)
Total
runtime: 0.530
ms

15. Indexes: The Neglected All-Rounder
Everybody knows indexing is
important for performance,
yet nobody takes the time to
learn and apply is properly.

16. About Markus Winand
Tuning developers for
high SQL performance
Training & co (one-man show):
winand.at
Author of:
SQL Performance Explained
Geeky blog:
use-the-index-luke.com