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Negative impact of indices to INSERT, UPDATE and DELETE performance in Firebird SQL

In general, indices are necessary for any serious database, since they are critical to speed up queries with WHERE clause (SELECT, UPDATE. MERGE, DELETE, etc). However, every index has the cost, and the cost is obvious when we compare the speed INSERT/UPDATE/DELETE operations on indexed and non-indexed fields.

In this article, we will demonstrate the impact of the index with a few unique keys to the operations' speed.

Test database

CREATE DATABASE “E:\TESTFIREBIRDINDEX.FDB” USER “SYSDBA” PASSWORD “masterkey”;
CREATE TABLE TABLEIND1 (
    I1            INTEGER NOT NULL PRIMARY KEY,
    NAME          VARCHAR(250),
    MANORWOMAN  SMALLINT
);
CREATE GENERATOR G1;
SET TERM ^ ;

create or alter procedure INS1MLN
returns (
    INSERTED_CNT integer)
as
BEGIN
inserted_cnt = 0;
WHILE (inserted_cnt <1000000) DO
BEGIN
 Insert into tableind1(i1, name, manorwoman) values(gen_id(g1,1), 'TEST name', (:inserted_cnt - (:inserted_cnt/2)*2));
 inserted_cnt=inserted_cnt+1;
END
suspend;
END^
SET TERM ; ^
GRANT INSERT ON TABLEIND1 TO PROCEDURE INS1MLN;
GRANT EXECUTE ON PROCEDURE INS1MLN TO SYSDBA;
COMMIT;

To demonstrate the problem with bad indices let's perform the following operations:

• First, we insert 1 million of records into the table
• Then we update these records
• After that, we delete all records
• And, finally, run SELECT count(*) from the table

set stat on;  /*enable display of statistics in isql*/
select * from ins1mln;
update tableind1 SET MANORWOMAN  = 3;
delete from tableind1;
select count(*) from tableind1;

Let's run it and keep the results for further analysis.

CREATE INDEX TABLEIND1_IDX1 ON TABLEIND1 (MANORWOMAN);

As you can see above in the script, we insert to this column only 0 or 1 integer values. Such index is useless for select queries, and in theory, all database developers should avoid such bad indices (except very special case with an unbalanced distribution of values in the table), but in practice, there are many indices with 2 unique values or even with 1 of them.

Then let's repeat the script with this index and compare results – see the following table:

So, bad index decreases performance by approximately 2 times while inserting or updating. Also, we can see that non-optimal index greatly increases the number of writes and record fetches.

Let’s get statistics for this sample database (with the bad index for MANORWOMAN) and try to find some details. To gather statistics, we run the following command:

gstat -r e:\testfirebirdindex.fdb > e:\teststat.txt

TABLEIND1 (128)
    Primary pointer page: 166, Index root page: 167
    Average record length: 0.00, total records: 1000000
    Average version length: 27.00, total versions: 1000000, max versions: 1
    Data pages: 16130, data page slots: 16130, average fill: 93%
    Fill distribution:
       0 - 19% = 1
      20 - 39% = 0
      40 - 59% = 0
      60 - 79% = 0
      80 - 99% = 16129

    Index RDB$PRIMARY1 (0)
      Depth: 3, leaf buckets: 1463, nodes: 1000000
      Average data length: 1.00, total dup: 0, max dup: 0
      Fill distribution:
           0 - 19% = 0
          20 - 39% = 0
          40 - 59% = 0
          60 - 79% = 1
          80 - 99% = 1462

    Index TABLEIND1_IDX1 (1)
      Depth: 3, leaf buckets: 2873, nodes: 2000000
      Average data length: 0.00, total dup: 1999997, max dup: 999999
      Fill distribution:
           0 - 19% = 0
          20 - 39% = 1
          40 - 59% = 1056
          60 - 79% = 0
          80 - 99% = 1816

To understand the meaning of shown numbers and percentage values, we can use HQbird Database Analyst, which offers visual interpretation of database statistics:

By clicking Reports/View recommendations we can find the appropriate explanation for this index:

Bad indices count: 1.
By `bad` we name indices with many duplicate keys (90% of all keys) and big groups of equal keys (30% of all keys). Big groups of equal keys slowdown garbage collection - MaxEquals here is % of max groups of keys having equal values. Index search for such an index is not efficient. You can drop such indices (if they are not on FK constraints).

Index ( Relation) Duplicates MaxEquals TABLEIND1_IDX1 (TABLEIND1) : 100%, 50%

In production databases often we can see many bad indices, which can greatly affect database performance. In this example we can see table with 13 millions of records which have 7 bad indices, which are (most likely) useless and greatly decrease Firebird performance: 

If HQbird Database Analyst highlights indices as «Useless» (i.e., they have only 1 value), such indices are recommended to be dropped immediately, if possible.

If the index is highlighted as Bad (a few values there), it should be considered for deleting too, but with caution: it is possible that bad index is used in some particular SQL query, which required the specific combination of indices (including bad one) in order to run fast.

Certainly, these SQL queries should be rewritten to use more optimal execution plan without a bad index. In order to find such queries, the audit of SQL plans should be done: the easiest way is to run trace session with HQbird PerfMon and enable SQL plan logging, and then search SQL plans for the specific bad indices.