PC-12	Fri Mar 18 18:52:53 2016
	Database: err_db
	Index 16 is corrupt (record 0 have missing entries) in table T (192)

CREATE TABLE T (
    ID                  INTEGER NOT NULL,
    REF_ID              INTEGER,
    NAME1               VARCHAR(1000),
    NAME2               VARCHAR(60),
    COMMENTS            VARCHAR(255),
    DELETED             SMALLINT DEFAULT 0 NOT NULL,
    CREATEDATE          DATE,
    CREATOR             VARCHAR(31),
    GUID                CHAR(36) NOT NULL
);

ALTER TABLE T ADD CONSTRAINT PK_T PRIMARY KEY (ID);
ALTER TABLE T ADD CONSTRAINT UNQ_T UNIQUE (REF_ID);

CREATE DOMAIN D_ID AS INTEGER NOT NULL;

CREATE DOMAIN D_GUID AS VARCHAR(36) CHARACTER SET WIN1251 COLLATE WIN1251;

CREATE TABLE EMPLOYES (
    ID    D_ID NOT NULL,
    GUID  D_GUID NOT NULL
);

ALTER TABLE EMPLOYES ADD CONSTRAINT PK_EMPLOYES PRIMARY KEY (ID);

CREATE INDEX EMPLOYES_IDX_GUID ON EMPLOYES (GUID);

select ID, count(*)
  from EMPLOYES
 group by ID
having count(*) > 1

ID      COUNT
187112	2

SQL> show version;
ISQL Version: WI-V2.5.5.26952 Firebird 2.5
Server version:
Firebird/x86/Windows NT (access method), version "WI-V2.5.5.26952 Firebird 2.5"
Firebird/x86/Windows NT (remote server), version "WI-V2.5.5.26952 Firebird 2.5/XNet (IT-03)/P12"
Firebird/x86/Windows NT (remote interface), version "WI-V2.5.5.26952 Firebird 2.5/XNet (IT-03)/P12"
on disk structure version 11.2
SQL>

select ID + 0, count(*)
  from EMPLOYES
 group by 1
having count(*) > 1

Constraint Name        On Field        Index Name        Index Sorting
PK_EMPLOYES            ID              RDB$PRIMARY16     Ascending

select *
  from EMPLOYES
 where ID = 187112

Plan
PLAN (EMPLOYES INDEX (RDB$PRIMARY16))

Adapted Plan
PLAN (EMPLOYES INDEX (PK_EMPLOYES))

select *
  from EMPLOYES
 where ID + 0 = 187112

Plan
PLAN (EMPLOYES NATURAL)