some SQL result is not the same when enable/disable plancache

[lcwangchao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

In a session without plancache

set @@tidb_enable_prepared_plan_cache=0;
set @@time_zone='UTC';
set @@collation_connection='utf8_general_ci';
prepare s from 'select DATE_FORMAT("2020-01-01","%W") = "wednesday"';
execute s; -- output 1
set @@collation_connection='utf8_bin';
execute s; -- output 0

In a session with plancache

set @@tidb_enable_prepared_plan_cache=1;
set @@time_zone='UTC';
set @@collation_connection='utf8_general_ci';
prepare s from 'select DATE_FORMAT("2020-01-01","%W") = "wednesday"';
execute s; -- output 1
set @@collation_connection='utf8_bin';
execute s; -- output 1

2. What did you expect to see? (Required)

The second execute should output a same value not matter tidb_enable_prepared_plan_cache is on or off

3. What did you see instead (Required)

The output of the second execute is not the same for different tidb_enable_prepared_plan_cache value

4. What is your TiDB version? (Required)

master

[lcwangchao]

I think the reason is the expression is computing it's collation using session collation here:

tidb/expression/collation.go

Line 242 in 533de3b

charsetInfo, collation := ctx.GetSessionVars().GetCharsetInfo()

But when the collation of session changes, the cached plan still uses the old value.

[qw4990]

It seems like we should consider connection's charset when hitting the plan cache:

I'll fix it this week.