[PHOENIX-4164] APPROX_COUNT_DISTINCT becomes imprecise at 20m unique values.

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Invalid
• Assignee: Unassigned
• Reporter: Lars Hofhansl
• Created: 06/Sep/17 05:06
• Updated: 09/Sep/17 05:51
• Resolved: 09/Sep/17 05:29

Description

0: jdbc:phoenix:localhost> select count(*) from test;
+-----------+
| COUNT(1)  |
+-----------+
| 26931816  |
+-----------+
1 row selected (14.604 seconds)
0: jdbc:phoenix:localhost> select approx_count_distinct(v1) from test;
+----------------------------+
| APPROX_COUNT_DISTINCT(V1)  |
+----------------------------+
| 17221394                   |
+----------------------------+
1 row selected (21.619 seconds)

The table is generated from random numbers, and the cardinality of v1 is close to the number of rows.
(I cannot run a COUNT(DISTINCT(v1)), as it uses up all memory on my machine and eventually kills the regionserver - that's another story and another jira)

aertoria

Issue Links

• relates to: PHOENIX-4160 research for a proper hash size set for APPROX_COUNT_DISTINCT